Tech Questions asked 1000's of times by everybody who ever raced a mini road racer

Last Updated: 4/16/2001

The first place you should start is this: then

The following document is a list of tips that have been developed by the collective knowledge of the YSR racers from around the US.  The information here is a collection of email tips that have been put together into meaningful sections and formatted to be remotely readable. A lot of this information is just opinion and should be taken as such. 

If you have anything to add or find that some tip doesn't work and it really ticks you off, please let me know.  Email me your tips at:

Index of tips:

  1. Pipes
  2. Gear box oil
  3. Premix Oil
  4. Gas
  5. Body work
  6. Bars
  7. Piston Clearances and recommendations
  8. Suspension tips 
  9. Fork Braces
  10. Gearing suggestions
  11. Gear Ratio Chart
  12. Gearing for specific tracks
  13. Tires
  14. Tire Pressures
  15. Places which do engine work 
  16. Brake in/Break in


  1. Tuning/Jetting
  2. Current Carbs
  3. Muratic Acid 
  4. What causes a seizure
  5. Reed Blocks
  6. Reed Stops
  7. Reeds
  8. Evaluating plugs
  9. Plug Recommendations
  10. Tool recommendations 
  11. Setting up a Stock YSR 50
  12. What is a stroker?
  13. Aprilia RS50 Info
  14. Honda NS50 Info
  15. XR 100 info



Exhaust pipes (still in manufacturing 4/1/2001):

Who Manufacturer Comments
Ron West Tommy Crawford Great top end pipe, has most overall hp. Very small powerband ( 9-12k) Contact to order one. Pipes are made by hand and come uncoated. Expect to wait several months for one.
Jason Weltch Tommy Crawford There are 3 types of pipe that TC makes: 
Stage 1: For a mildly built motor (ie: no porting, etc) 
Stage 2: Moderately built motor (Supersport to Superbike/60gp) 
Stage 3: Only for use with "stroked" engines (use of GT80 crank) with porting. 
Be sure to ask for the thicker steel when ordering, as the "normal" thin stuff tends to break underneath the right footpeg due to vibration or rider heels.
Ron West Calamari: Twin Stinger Most bottom to mid (7k-11k powerband). Very easy to ride fast with.
Ron West Calamari: Single Stinger Never used one, but I've been told it is comparable to crawford.
Ron West Pro Flo Good power range. Less top end than Crawford, but more range of delivery. Contact Has improved with iteration. Only weakness is pipe chroming that makes the metal brittle over time.  
Jason Weltch Pro Flo Pro-Flo pipe comes with a threaded piece to allow use of an EGT sensor.
    80 CC pipes
Ron West Banke Pipe made for CR 80 engine. The standard mold fits both RS and MH chassis. Absolutely great power from the bottom to the top.
Ron West Tommy Crawford I've been told they are similar to the 50 design: total HP, lacks range
Ron West Calamari They have a range of both down pipes and normal mx-styled pipes that will fit in a YSR chassis.
Ron West Moriwaki CR80 pipe available. Contact:

Gear box oil

Ronw:  The correct amount for a YSR is 600cc. My YSR's use Castrol 20w50 in the transmission and it works well.
Ron w On my 80's I use Dumande tech

Premix Oil:

Who Ratio Description
Ian 32:1 Red Line premix oil 
Leonard Mellgren 18:1 YSR Yamalube 2R mixed 18:1 with regular unleaded pump gas using stock jetting.
Leonard Mellgren 30:1 Water Cooled Engines  (NS50R and CR80)  Castrol A747 mixed 30:1
Jason Weltch 28:1 If you're really nervous about lubrication breakdown, use Castrol A747 at 28:1. This is the absolute BEST two-stroke oil you can buy, and the price tag reflects that fact. Yamaha recommends this oil for use on their TZ250 GP bikes as well as the current production YZ125. Remember the oil in the fuel lubricates and cools not only the piston to cylinder, but also every moving part on the crankshaft, including main bearings. The oil also improves the ring seal on the cylinder. This equals horsepower.
Ron West 32:1 Yamalube. The only time I have had problems was when the gas did not mixed correctly.


Pete  Neilson Well, this is almost as dangerous a subject as religion or politics, but I can't help adding my 2 cents. It is my understanding that an engine will actually make more power with the lowest octane gas that will not pre-ignite or detonate. This is because lower octane fuel burns more quickly than higher octane. Higher octane rating equals a resistance to burning, hence the lesser chance of pre-ignition or detonation. By the way, octane rating is a comparison to the characteristics of the fuel "octane" and 100 is as high as it goes, anything above that is a theoretical #. The above does not take into account the new oxygenated fuels like Elf or Nutec, which add power chemically. In my opinion a stock YSR should operate just fine on regular gas. You should avoid any gas with alcohol though. Modified engines or MX 80s could probably use more octane. I use 50/50 mix of 100LL Avgas and regular unleaded in my RM80 and it works great. 100LL Avgas is very similar to 106-108 race gas and is available at any airport for a little over $2 a gallon. I also run about 25:1 oil mix, because I have seen evidence that more oil = more power. Be aware that more oil requires richer jetting because it is taking up some of the space in the jet that was fuel.
Leonard Mellgren You are right on the point. Low compression engines run best on low octane fuel. Engines went to liquid cooling to keep the engine at a temperature that produces max power. The YSR engine is air cooled and runs at temperatures higher than this. At streets of willow there is a long uphill at the end of the front straight. With the correct gearing a YSR can pull the hill in 5th gear at the beginning of the race. By the end of the race, the engine temp has resulted in a loss of power that is noticeable sometimes requiring a downshift to 4th going up the hill. The more oil the cooler the engine runs. I run a ratio of 18:1 in my air cooled engines. In the water cooled engines I use 30:1.
Ian  I've used a lot of fuel in a lot of bikes over the years, and across the board whatever the application I've gotten noticibly better performance from race formulations, besides, they look cool and smell better than pump gas! I run 108 octane VP Red at 32:1 with Red Line premix oil and I'm very happy with it.
Ron West I run a mix of 2.5 gallons of 108 race gas with 2.5 gallons of 92 pump gas.  But in the stocker, we only use the 92 pump gas. Also, I do not use race gas in the XR 100. It makes the bike run hot and the performance suffers.
Ted Foster

Body work 

Who Where Description
Ted Foster K-Mart You forgot the least expensive option of all,....the K-mart plastic trash can cut into front fairing, tail section and front fender!!!
Ron West Team Calamari I like the predator flat tail. It gives ,ahem, larger guys like me room to move around on the back. Not the prettiest, but it crashes well and is functional.

The upper: I really like the shark nose. my 2nd choice would be the new predator upper. One of my bikes has the prototype of the new design, but I don't know if it ever went into production. Either one of these uppers crash very well. 

For the lowers I like the look of the works lower, but I can tell you it didn't crash well and the fit was poor. You're best off with either the stock stuff or stock replica (or none at all).

Ron West The bodywork is what it costs: cheap. The fit is less than optimal. For me (being a lard ass) the gsxr tail is too constricting to when transitioning from side to side. Maybe there have been changes in design since I bought it a year and a half ago, but the stuff is too thin, doesn't fit right and falls apart in a crash.
Tim Caroll His Blurb: " Current price has been set at $100.00 per piece. Quality is better than the rest. Craftsmanship is great with many options to make the bodywork the way you want it. Layer thicknesses are available in many choices from thin and light to heavy but hard to bust. Colored gelcoats are available only limited by your imagination, including glitter and carbon fiber, in colors too. Custom will obviously cost more. If you would like to see the quality or fit and feel come to any EARA race and you will be surrounded with my works. There are FRR tails in all YSR clubs nationwide. We even surpassed the competition to make the first "fakey" tank, much like the ones found on FZR 400/600s and Moriwake 80cc GP bikes. You can't make products without knowing their useful background. We rode our YSRs to several championships in the stock classes, the 80cc class and even the Rookie of the year award was won twice by FRR. Once by Mike Carroll and then by his wife Jodi as the first female winner of this honor."
Ron West The R7 bodywork was going to be distributed by TCR for cheetah, but there were serious production delays and 6 months worth of missed deadlines. There were a lot of people that got screwed on this, Taro being the local representative of this club


Who Where Description
Jason Weltch JLF makes some nice one piece clip-ons. These can be bought from Wings.
Ron West I highly recommend the bars that Pro-flo sells. They are fully adjustable for length (they can be set quite a bit longer than stock) and the bar portion is replaceable should it ever bend. Plus they come in cool colors... :)

Piston Information:

Can someone please give me some insight into the pros and cons of the yamaha cast pistons vs. the available wiseco forged pistons. apart from the obvious bore size differences, here's what i've been told:

1) obviously, the wiseco piston requires greater clearance. - (0.005" vs. around 0.0015"). 
2) if the clearance is too tight, seizure will result. 
3) the greater clearance results in lower compression. - i don't buy this one, as at operating temp, i believe the clearance of a properly set up forged piston equals that of a properly set up cast piston -- correct me if i'm wrong. 
4) the port timing of the currently available wiseco pistons is not correct, resulting in a performance loss that offsets any benefit of increased bore size. 
5) bottom line, i've been told to stay away from the wiseco piston for a mod 50 YSR (Steve Korol)

Who Answer:
Bret Cason  The Yamaha pistons break-in faster and don't grow as much. The current Wiseco pistons have an incorrect geometry which is why they need .005 clearance. I don't recall the term but Susan at Calamari could give you the details. Normally the clearance should be .003
Vinny Evans  Wiseco's work great for us! Just needs .0035 clearance, have the cylinder "precision honed" with a 63 surface finish (RMS) (32 finish for a Yamaha piston) and you'll have no problems. In the stock class with the CMRRA, we are "now" allowed to use up to 41 mm Wiseco!(4th over stock) Its great! Nobody is blowing up down here. All I can say is they work for us! My .02$........
Leonard Mellgren In Japan a 60.3cc big bore kit is available for the YSR50 motor. The kit includes a new cylinder, piston, rings, piston pin and gasket kit. The Kit uses a 44mm piston. Price aprox. $460.00. Replacement pistons $80.00. Leonard
Roland James:  Here is what I have gleaned from several years of experience with Wiseco and stock Yamaha pistons: Wiseco pistons sieze easily during the first hour of use, I have recently started to use KALGARD piston kote on the skirts, and ceramic coated the crown. Using .003" clearance works fine for me... Piston height is exactly the same as stock ysr 50, meaning that port timing is also exactly the same. I generally relieve the crown at the transfers aprox 1.3mm to increase the transfer timing, as well I generally add two additional 10mm transfer holes above the stock ones, lightening the piston and keeping it slightly cooler. Generally I run one ring to reduce friction. Basically there are no other choices, unless you want to modify a KX60 piston (44mm, piston ring retaining pin in wrong location..real BITCH to relocate) or run at a smaller bore size (stock piston). I have one of the Daytona 60 cc cylinders, the pistons are complete crap, I wouldn't use one on a bet..
Jason Weltch If the bore is 40 to 40.5mm, stick with Yamaha. Have had great success running these at approximately .002" (two-thousandths) clearance. Recently tried a 40mm Wiseco on a stocker and the power was noticeably down compared to the other motors I've built with Yamaha pistons (same rider). Will replace the top end with a Yamaha piston and let everyone know if it makes a difference. Beyond 40.5mm there is no other choice than to use Wiseco (unless you can mix and match another piston to make it work). Have had great success running Wiseco pistons at .0035" - .004" clearance. I am generally much more cautious while breaking Wiseco pistons in.

Also, if your particular organization allows it, use the top piston ring only. Yamaha pistons use different rings between the two, while Wiseco uses identical rings. If you've got a Yamaha piston, discard the bottom ring. If you've got Wiseco, save the bottom ring and use it for a spare/maintenance at a later date

Suspension tips 

Ron West Front Forks:  Pull off the front wheel and forks. Check the seals on the forks. Are they leaking? If they are, replace them. It's a 10 minute job and the best way to get your suspension working. Remove the caps from the forks. Turn them upside down and drain the fluid. You may have to pump them to get all the fluid out. I bet for most folks the fluid is black. Then pull the springs out (remember which way it went in and which spring goes in which fork tube). Compress the forks. Fill with your favorite weight of fork oil up to 6 inches below the top of the fork (Jason, can you confirm this measurement?). I cut a zip tie at exactly 6 inches and hold in down in the tube while filling with oil. We use 15 or 20 weight fork oil because all of us are lard butts. some folks have suggested using 10w40 but this didn't work well for me when I tried it last year.

Rear Shock: The fox shock is simply the best. The works shock simply doesn't work as good. I won't ride a race bike without it.

Jason Weltch Yeh, thats everything that I do, 'cept pump the forks after you add oil, and then wait a bit and return to check the height. I usually use 30wt oil (PJ-1 makes 30wt fork oil) for this, but 10w-40 will work just fine.
Jason Weltch Fox Shock: Out of the box, better than a stock YSR shock, however its still just a spring, damping is nearly nonexistent. Notice if you turn the clicker for compression damping it makes virtually no difference? If you can order a shock straight from Fox Factory, be sure to tell them you want about 10 times more damping than their regular run of shocks! Otherwise send it off to a Race-Tech or other good suspension guru. 

The Fox Shock also has about 1/2" more overall length than the stock shock. This means more rear ride height which quickens the steering and helps the front wheel feel more "planted"..

Works Shock: Used one only once, but have friends who used them a lot. It's even worse than the Fox, and has no adjustment for damping OR preload. They want your old shock when you order a Works, because they want to use the spring off of it! (This is why they are so much cheaper than Fox). Maybe someone needs to tell them that there is a difference between spring rate and preload.

Ron West Rumor: There is another bike out there in the ether that has the same fork tube dimensions as a YSR, but has adjustments for damping and rebound.  I am not sure of what bike this is, but if you find it, please let me know.

Fork Braces:

What are they and why do you care?

Who What did they say?
Truman Grandy But the interesting thing (well, to me -- you're on your own to find something interesting) was his statement that "YSR lower triple clamps are all different sizes," by which I suppose he means that either: 1) production tolerances were rather large, 2) tolerances (specs) are tight, but there was crappy QA 3) multiple suppliers and/or production runs were using different specs/tolerances 4) some or all of the above.
Susan The YSR forks are manufactured by Yamaha to +-20% tolerances (and loosely described as "tolerances")
Seth LaForge Hmm, that must be why I've had such an impossible time getting my Team Calamari brace installed without binding. I suppose a possible solution would be to make/buy a replacement lower triple clamp. Has anyone heard of doing this? I don't have my YSR hand to look at right now, so I don't really know this, but it seems like it couldn't be that tricky of a piece...
Susan Elements that cause stiction after fork brace installation: 
1. Incorrect installation 
2. The exception would be if your forks, inners or outers, and/or t-stem is bent. Call Calamari to diagnose the problem.
Ron West Nope, nobody has made a replacement bottom clamp. The hard part about it is fitting the steering stem and bearings. I think the reason no one has made them is that is not cost effective to produce.

The key to the calamari fork brace is to make sure the spacer is the correct size. Too much or too little and you will have binding.

The forks on the YSR are small and weak enough in combination with a weak bottom clamp design that allow for the forks to flex laterally. Plus, there is a +/- 20% allowable tolerance for this bike (which is a HUGE %). Imagine if your fuel line varying in size 20% where it attaches to the tank--guarantees a leak! 

This flex is usually only felt under racing conditions where the front end of the bike is severely loaded (Remember, these ysr's were built for moderate street riding with 130lb riders) The reaction of the forks is to twist causing the front wheel to turn abnormally and the twisting reduces the amount of up and down work (aka stiction) the forks can do. There are 3 ways to fix this problem: 

1. Larger diameter fork tubes (no other bike I know of uses this small of diameter fork tube). 
2. More effective upper and bottom triple clamp (no one makes an aftermarket bottom clamp that I know of). 

3. Brace the fork legs together 

The fork brace is designed to provide support for both fork legs simultaneously. There is no way for either fork tube to twist as both forks are working as a single unit. One common problem in fork braces is that stiction may occur due to the incorrect amount of pressure being applied when the brace is installed. This incorrect setting either attempts to bow the forks outward or inward. This prevents the internal workings (springs and the upper part of the fork) of the forks from sliding downwards in a straight up and down motion.

Seth LaForge You can especially see it when braking hard. Since there's only a brake rotor on one side, there's an uneven force on the forks. I was really shocked the first time I looked down while braking hard and saw that my handlebars were at least 10 degrees out of line while the bike was going straight
Jason Weltch Another way, which I've only seen done once before (by me) is to fashion a way for the fork to "clamp" the axle, preventing the forks from rotating around it. It would work better with a larger axle shaft, but this would be difficult to do without major modification to the wheel itself. Example: Look at any modern sportbike. They don't use a fork brace because the forks have pinch bolts around the axle. These axles are usually 3/4 to 1-1/2" diameter, giving the fork more area to bite. We successfully used this design (I used to have pictures, I'll have to dig around for them) on our YSR for a season, without the help of a brace. We tested a Wing's fork Brace and it >DID< help, but our setup without the brace was still much more effective than than nothing at all..
Ted Foster Stiction, is it a word? 

It is to bike suspension guys. It's an abbreviation of "static friction". It is the amount of force required to begin the fork moving, which is more than the force required to keep it moving. How much more depends on how bad your forks are. This can happen to rear suspension as well, but it gets less press than the front. Ideally you won't have any at all, then your bike will soak up bumps like a Jaguar with active suspension. Fork seals and dust boots will create some, that's unavoidable. Chunky fluid, scored and bent fork tubes, chewed anti-friction surfaces, misaligned forks clamped in the triple clamp, and braces putting tension on the fork alignment are all sources of additional stiction that can be controlled.

Who makes fork braces?

Manufacturer: Where Who Comments
Team Calamari Ron West The standard in fork braces. Priced reasonably. 1 piece design (front to back). Ensure fork spacers are installed correctly to avoid stiction. Works well when installed correctly.
Bob Wing Ron West Excellent quality, multi piece design. Highly adjustable. Works well, but expensive
Mini Moto Madness Ron West Multi piece design. I've personally never used one. Not as elegant as Wings design, but cheaper. 


Gearing suggestions:

Who What
Jason Weltch Gearing:  no real suggestions for gearing, as it varies with horsepower, rider ability and track, HOWEVER...   I recommend the use of a Regina Gold #420 chain. It's the proper size but lighter, the link plates are visibly smaller. Besides, it's GOLD! That makes it cooler! :) If you're really looking to lose weight in the chain dept, you can run a #415 chain (this is standard on 125 roadracers) the only problem is that sprockets will need to be custom made.
Ron West

Any almost any track, anywhere start with 12x46.  The ratio is the 2nd number divided by the 1st.  So a 12x46 is a 3.8 ratio. check out the chart below for a quick guide to the ratios.

Rose at calamari recommends staying away from the 11 front. The reason being is that this sprocket tends to damage the chain quickly due to the number of revolutions. Notice that an 11x44 is the same ratio as a 12x48. 

Gearing by Track:

Who Track  Rider Level Bike Type Front Rear
Ron West Pat's Acres (Canby, OR) Novice YSR/ Stock 50 12 48
Ron West Pat's Acres Novice YSR/ Mod 50 12 46 or 47
Ron West Pat's Acres Expert YSR/ Mod 50 12 46
Ron West Pat's Acres Expert NS 50 13 48
Ron West Pat's Acres Expert CR 80 13 47 or 48
 Club: CMMRA    
Leonard MellgrenAdams Kart Track, RiversideExpertStock YSR501243
Leonard MellgrenAmago Raceway, Palomar MountainExpertStock YSR501244
Leonard MellgrenWillow Springs Kart Track (Counter Clockwise)ExpertStock YSR501246
Leonard MellgrenWillow Springs Kart Track (Clockwise)ExpertStock YSR501245
Leonard MellgrenWillow Spring Streets TrackExpertStock YSR501443
  Club: EARA        
Darrin Gauvin Monrovia Amateur Supersport YSR 50 11 47 or 48
Darrin Gauvin Batavia (NY..CanAm) Amateur Supersport YSR 50 12 48
Darrin Gauvin Ohio Amateur Supersport YSR 50 12 48
  Club : CIA        
Tony Tice MarshallTown Expert YSR/YZ426F 14 or 15 33
Paul  MarshallTown Expert YSR/YZ250 14 or 15 33
Tony Tice MarshallTown Expert YSR Mod 50 14 44
Tony Tice MarshallTown Expert YSR Superbike 14  53
Tony Tice MarshallTown Expert 80's 14 40
Tony Tice MarshallTown Expert 125's 14 36

Gear Ratio Chart:

  36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
113.27 3.363.453.543.633.723.813.9044.
1233. 4.334.414.5
13 2.77 2.85 2.92 3.00 3.08 3.15 3.23 3.31 3.38 3.46 3.54 3.62 3.69 3.77 3.85 3.92 4.00 4.08 4.15
14 2.57 2.64 2.71 2.79 2.86 2.93 3.00 3.07 3.14 3.21 3.29 3.36 3.43 3.50 3.57 3.64 3.71 3.79 3.86
15 2.4 2.47 2.53 2.60 2.67 2.73 2.80 2.87 2.93 3.00 3.07 3.13 3.20 3.27 3.33 3.40 3.47 3.53 3.60
16 2.25 2.31 2.38 2.44 2.50 2.56 2.63 2.69 2.75 2.81 2.88 2.94 3.00 3.06 3.13 3.19 3.25 3.31 3.38
17 2.12 2.18 2.24 2.29 2.35 2.41 2.47 2.53 2.59 2.65 2.71 2.76 2.82 2.88 2.94 3.00 3.06 3.12 3.18
18 2 2.06 2.11 2.17 2.22 2.28 2.33 2.39 2.44 2.50 2.56 2.61 2.67 2.72 2.78 2.83 2.89 2.94 3.00
19 1.89 1.95 2.00 2.05 2.11 2.16 2.21 2.26 2.32 2.37 2.42 2.47 2.53 2.58 2.63 2.68 2.74 2.79 2.84
20 1.80 1.85 1.90 1.95 2.00 2.05 2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70
21 1.71 1.76 1.81 1.86 1.90 1.95 2.00 2.05 2.10 2.14 2.19 2.24 2.29 2.33 2.38 2.43 2.48 2.52 2.57


Who Brand/Description  Comments
Ron West Stock Throw them away. Pretend you never had them. 
Ron West IRC 740/IRC 750 Awesome tires I can't say enough good things about them. Stick in all weather conditions + temps. The 750s are a bit more predicatable when they are going to slide than the 740s. Get up to riding temp real quick
Ron West Dunlop TT91 Darrin loves em, I won't ride them. They work well in the cold, but they get slicker than snot when heated up. When they went away on me, there was no warning

Tire Pressures:

Who Brand/Description  Comments
Leonard Mellgren Dunlop TT90 15 psi to 18psi
Leonard Mellgren Dunlop TT91 25 psi to 28 psi

Places which do engine work 

Shop Name Address/Contact Info Contact Person
Team Calamari Susan/Trish/Rose
Vinny's Performance Vince Evans
Pro Flo John
Wings R&D Bob
Motorcycle Works of Renton 425-226-2928
485 Rainer South Renton, WA 98055-2444
Pat Dowd
motomorphic 707-763-4295. 331 Petaluma Blvd N Petaluma, CA 94952 Jim Davis
TBT Racing 206-71-tuner Travis


 Brake in/Break in

Bret Cason During break-in, there is no amount of oil ratio that will keep your motor from seizing. The piston must work harden to the point where it doesn't expand faster than the cylinder. I heat cycle my new motors ~10 times before I start to "get on it". Here is the schedule I use:

1st) 30 seconds idle 2nd) 2.5 minutes idle 3rd) 2.5 minutes idle 4th) 5 minutes idle 5th) 5 minutes idle 6th) 20 minutes fast idle 7th) 20 minute relaxed ride varied throttle 8th) 20 minute relaxed ride varied throttle 9th) 20 minute ride varied throttle brief wide open 10th) 20 minute ride varied throttle brief wide open Between each cycle you must let the engine completely cool down, otherwise your wasting your time.

Team Calamari  
Vinny Evans About the heat cycle thing...... I warm the bike up for about 5 minutes with varying rpm's, let it cool, retorque the head and go racing! No Voodoo! No rolling the bones! But you have to understand that the cylinders are processed differently than most shops.


Ron W So, what jet size would folks recommend for a 50 with two-stinger pipe and 20mm carb with air filter? I'm thinking around 135 off the top of my head...

180 seems just about right for that carb. I would say leave it as it is for the moment and run a few laps in practice then do a plug chop (before the last turn, put a leg out , pull in the clutch and hit the kill switch). Then check the plug. Either carb will work fine. The 20mm will give a bit more torque, while the 24mm will give you a bit more top end hp.

Ron W  With the choke on, the engine races to redline, and with the choke off (after warming up), the engine totally bogs at low RPMs. When it bogs, if I go full throttle it takes a few seconds before the engine starts to run again. From my previous experience with riding with the choke on (oops), this seems like it must be too rich, but I'm not sure. I'm running a 180 jet and the clip in the middle position, on a 24mm flat-slide carb. Ron recommended a 175 jet rather than 180, but I wouldn't expect one size to have this dramatic an effect...

Answer: What it sounds like is either you've got an air leak Bingo! And a really big one - I'd forgotten to hook up the vacuum chamber thing that goes off the intake manifold. *sigh* With that hooked up, it works just great, and I've been heat-cycling my engine. It runs like stink with the 60cc cyclinder! Too bad it's raining now...

Leonard You may want to look at your low speed jet. It controls the fuel mixture at small throttle openings.

Case 1 Lean: If the bike runs better just off idle with the choke on you might want to try a larger low speed jet. 

Case 2 Rich: When getting back on the throttle going through a corner, if the power hesitates and then hits hard try a smaller low speed jet. 

Case Correct: When getting back on the throttle going through a corner, the power will come on smoothly. From your comments, it sounds like the slow speed jet is too small. You might want increase the size.

Jason Weltch Small jetting tip:  To check to see if your pilot jet and air screw are adjusted right.. Warm up the motor and let it idle as low as possible while still running smooth. From idle, whack the throttle wide open then let it close completely. Listen to the motor. If it bogs right when you whack the throttle open, then revs up, turn the air screw in 1/2 turn then try again. Do this until it revs up crisply. After the motor revs up, listen to it revving back down. If the revs drop quickly, and the motor starts to bog, and/or die, then you're too rich on the air screw, back it out 1/4 turn at a time. If after you let the throttle off the motor tends to run on and on while revving down very slowly, you're too lean and need to turn the air screw in 1/4 turn at a time. You want the revs to come up from idle quickly and smoothly, then drop back down to idle the same way. If you turn the air screw all the way in and it still needs to go further, then you will need a larger pilot jet. The opposite is also true: if you are backing the screw out so far that it darn near falls out, you will need a smaller pilot jet. Stock and Supersport bikes should rarely ever need to deviate from the stock pilot of 17.5! On cold or extremely cool days, you may benefit by putting a #20 pilot in. Aftermarket carbs can be tuned the same way, but I'm not familiar with what sizes should necessary. NOTE: This is all done in the pits, you ARE NOT RIDING the bike to check this!
  Basics of carb tuning:
  Carb tuning, by Eric Gorr:

Current Carbs:

Name Size Comments:
Ron West 20 Works well. Has good torque, sacrifices the very top end for more rideability.
Ron West 22 Doesn't work with stock unmodified reed cage. the problem is this carb does not provide enough velocity in the midrange. Cage needs to be converted from 4 petals to 2 (cut divider out of reed cage, uses had cut reeds from carbon tech reed materials). Fill in with JB weld or equivalent to keep up speed of charged air/gas. Also needs different needle with larger taper in the mid range.
Ron West 24 Flat slide Works well. Sacrifices bottom end pull for top end power. 
Ron West 28mm flat slide Simply uncivilized. We do have this on one of our bikes and it works (albeit with a 300 main jet).  Power band is a light switch. While the bike is fast, but is extremely hard to ride.
    80 Carbs
Ron West 28mm Flat slide When trying to find the right size carb for an 80, do what the kart guys do: go to your local suzuki dealer and ask for the carb directly off the RM 80.  This has the best setup available. If not, you are going to be playing with needles and jets forever.  My experience is limited to the CR motors, not sure if this applies to YZ/RM/KX.
Ron West Keihn 28mm PWK The stock needle would cause a hesitation and noticeable power loss in the midrange. Bob Wing does have the correct needle for you.  My experience is limited to the CR motors, not sure if this is the case on YZ/RM/KX.


Muratic Acid 

It's used to remove aluminum reside from seized cylinders

Who  What
Jason Weltch Any Pool maintenance supply place should have some.. 
Ian Carcosa It's good for etching rusty gas tanks, too.
Harvey Rotor Any hardware store should have it. I've used it to clean my fireplace bricks. It works well.It is also known as cement cleaner

What causes a seizure:

Ron... 4 corner seizures aren't usually caused by timing problems. This isn't to say that it DIDN'T cause it, but I would look elsewhere first, at least if your timing was set close to correct. We ran about 1.6mm of advance on our 60, and occasionally ran up to 1.8mm without any problems. Keep in mind this was on a "stroked" YSR, so if yours isn't a stroker, then you'll probably use a bit less advance. Timing problems can be attributed to detonation (the piston crown gets a hole, or the crown along with the cylinder head look sandblasted) or sometimes overheating. What was your piston to cylinder clearance? Wiseco or Yamaha piston? What was your premix ratio? >DID YOU LET THE BIKE WARM UP before wringing its neck?<

A seizure is simply a piston that expands beyond the space allowed within the cylinder.  

Typical causes (in this order):

  • Lean jetting
  • Air leak (see lean jetting)
  • Bad gas/oil mix
  • Inadequate engine warm up
  • Wrong bore size
  • Timing setup wrong

Guide to reading a seized piston: This will give you an idea of what to fix to keep it from happening again.

Reed Blocks:

Stay away from the mosbarger setup. It will screw up the jetting enough on your YSR that you will never get it right again.

Reed Stops:

One more question. The standard reed block has curved metal bits screwed on to prevent the reed opening more than a certain amount. I'm noticing that Charles' reed block has these metal thingies, but the reed block from my bike doesn't. Are they good? Bad? Indifferent? Should I remove them to save the 0.5 oz? :)

Those are reed stops. What they do is prevent the reed from opening too far while at full throttle. And you do need them.

Jason Weltch: Help control the amount of opening by the reeds. Helps prevent reed flutter at high RPMs. Should not be opened more than the stock amount (see service manual on measurement procedure and bending limit)


Who Type manufacturer Comments/benefits
Ron West Fiberglass Boysen Seem to work well. Pretty much the YSR standard
Ron West Carbon Fiber Carbon-Tech I use this reed on my NS (order the one made for the NSR) and it works well. This was the only reed available at the time so I am making do with it. 

Evaluating plugs 

What is a "plug chop" and how does one chop a plug?

A plug chop is a final reading of the plug at as close to full throttle as possible.  To get a plug drop, run a few laps on the bike. As you are going down the final straight away, pull in the clutch and hit the kill switch.  Coast in to the pits and pull the plug out.  This should be done in top gear if at all possible

Info about reading plugs:

How to read the coloring patterns on a  plug:

Plug Recommendations:

Who What Plug Comments:
Ron W BR10ES We use this on our fully done up CR80 motors. We are not changing plugs from this one ever again!
Ron W BR9IEX We tried this on our fully done up CR80 motors. It's a new iridium plug from NGK. DON'T USE IT!!!!!!  It runs too hot. The insulator burned up on Darrin's bike and caused my bike to have a minor seizure.
Ron W BR9HVX For any YSR that is ready to race
Jason Weltch NGK B8HVX Plug Recommendations:  Stockers should use an NGK B8HVX.  
The lower the number of the plug, the hotter it is. 
Stock motors do not benefit from cooler sparkplugs unless its 110 degrees on the track. 

*note: (Ron West) according to Rose at Calamari, this plug is no longer in production with NGK

Jason Weltch B10HVX Modified motors create more heat, and therefore will benefit from the cooler sparkplug. Our fully pumped up 60 used B10HVX's

Setting up a Stock 50:

Who Tip
Ron West 1. Remove lighting coil (if allowed in your club)
2. Remove oil pump and replace with cover
3. Replace crankcase seals
4. Clean that pipe every 5 or 6 races (stock pipes get tremendous build up of unburned premix oil)
5. Get piston clearances set right (.003). Use Yamaha pistons if at all possible
6. Get a BR9HVX plug (or an 8 if you can find one...)
7. Replace stock reeds with Boysen or Carbon Tech (see reeds above)
8. Suspension tips: Get the right side fork spring, then see suspension tips above (get a Fox shock if rules allow it)
9. Reduce speedo drag on front wheel. Either remove the housing and replace with aftermarket or at least take the gears out of it
10. Change rings every 5 races.
11. Get a fork brace 
12. See crankcase oil tips above
13. Brakes are more than adequate stock. Bleed any air from them and leave them alone.

What is a stroker?

A stroker is an engine that utilizes a different crank to achieve more displacement with the same engine.  The stroke is the full up and down measurement of the piston (a YSR has a stock stroke of 39.2mm). The most typical crank for this in a YSR is a GT80.  One tell tale sign of a stroker is a spacer plate between the bottom end and the cylinder.  This plate is used to keep the timing of the ports and the spacing to head in the same position as if the bike had a shorter stroke.  Before you protest someone as having a stroker, you may want to ask them first.  There are some bikes out there utilizing spacer plates to change the port timing without changing the stroke.

The practical cc limit for a un-stroked YSR is 64cc, while a stroker can get upwards of 68cc.  While the strokers can put out more power, they are also seem to be very finicky for tuning. Much more attention must be paid to removing heat from the engine (correct jetting and plug heat range) with a stroker or it will melt itself down.

Tool recommendations :

Ian loaned me (and gave instruction on) his MotionProŠ chain breaker. It is a nicely made tool, like the bicycle chain breakers with which I'm familiar, but hella stout. Anybody know of a pliers-style chain breaker? I've seen 'em for bicycle chain, and they seem easier and quicker to use. But maybe for heavier chain (even YSR chain ;-) they don't give enough leverage...

Aprilia RS 50

A special thanks goes to Sandra Whitney for this information. She is currently racing a Yellow 2000 model RS50 in the Golden State Mini Road Racing Association.

Who Part Supplier Comments
Sandra Exhaust Giannelli exhaust ($207.00)

All purchased from Taffspeed in the UK. Contact information for them is:

Terry Frankland,Taffspeed Racing
Link: Prices - Aprilia RS50 Motorcycle
Tel: 01633 840450  (Int. 0044 1633 840450)
Fax: 01633 246175  (Int. 0044 1633 246175)

Sandra Carb 24mm Malossi carb kit with new intake manifold with 6 main jets ($127.50)
Sandra Reeds carbon fiber reeds ($22.00)
Sandra Tires Bridgestone slicks ($240.00). Stock tire sizes are: 
front: 90/80 ZR 17"
rear: 110/80 ZR 17"

Front rim is 2.5x17, rear is 3.0x17"

Bare Bones Machine  in CO.

Link: Bridgestone Tires from Bare Bones Machine

Sandra Gearing 52 tooth Rear I am running a 52 tooth rear sprocket (stock is 47) and longer chain ($100.00) from Sprocket Specialists


Sandra Race Prep I removed the entire wiring harness, battery, oil pump and oil tank. I had a plug manufactured to block off the oil pump drive. I am guessing that I lost around 30-40 lbs after removing all of the unnecessary parts. The starter motor is going to go before the next races and I will have a plate machined to cover the hole in the engine.
Sandra Carb Setup I bought a 24mm flatslide Mikuni, but am having slight difficulties getting a new intake manifold and throttle cable.  Top Performance makes the parts, but they are in short supply currently
Sandra Porting Motomorphic in Petaluma did a bore job and some minor porting. Contact info:  
Sandta Porting Work: Motomorphic:Owned and operated by Jim Davis, an avid stock-class YSR racer. 707-763-4295.  331 Petaluma Blvd N Petaluma, CA 94952
Sandra More R&D I will be testing a 13 tooth counter sprocket next week. I am able to out-accelerate the Superbike YSR's and NS50's on the straightaway, but only by a hair. The new counter sprocket should give me exactly the edge I am looking for top-end wise. Anyone who wants to race an Aprilia is more than welcome to contact me directly.

NS 50 Information:

I want to point out that Calamari has done a great job describing how they setup their NS on their web site. This link will take you right there. My information is based on a conversion of a NS50F (1990 US model) into a race bike. 

Who Part Supplier Comments
Ron West Parts list Any Honda shop.


This is the most common parts that any NS will need. All of these parts are available at your local honda dealer. Most of these parts are not kept in stock and will take a couple of days to get.
Ron West

Piston list

* it is important to note: in most clubs you are limited to 52cc for a watercooled bike.  The .5 piston will put you at something like 52.003cc

Any honda shop As you can see there are 5 pistons available for the NS.  The only problem is that your dealer won't see the .25 or .5 oversized pistons on the microfiche, it's only on the computer.

The ring is the same for A,B,C pistons

Ron West Carb Mikuni 24mm Flatslide The same carb that is the standard on the YSR superbikes works well on the NS
Ron West Carb Boot   There are 2 that fit without any complication: 1. The stock boot for the 24mm can be drilled to fit or a CR80 boot will fit bolt up perfectly.  If you use the CR80 boot and the 24mm carb, ensure you are not getting an air leak where the carb meets the boot.
Ron West Pipe Tommy Crawford This is a great pipe for the NS. It fits well and makes good power. There are plenty of pipes that can be imported from Japan, but they are quite expensive.
Ron West Reeds Carbon Tech Ask for the reeds from the NSR50 (they are the same)
Ron West Clip ons Pro-Flo I thought the stock bars were too high so I ordered a set of 30mm clip ons from pro-flo to put the bars below the top clamp.  Keep in mind that the forks are 29mm and there are no bars created this size. We shimmed the clamps 1mm and the bars worked out great.
Ron West Race Setup   1. Remove the kickstarter gear and oil pump. btw: the oil pump is a PIA to remove, the bolt holding it in will strip. I went to my local autoparts store and found freeze plugs to fill the holes. Then I used JB Weld around the plugs to ensure they never leak and never come out. 
Ron West     2. Remove the lighting coil (the huge spun coil on the bottom of the stator)
Ron West Electrical Setup   Most important thing to remember: all the green wires need to be grounded. There is a single green wire hidden in the wire harness that comes from the stator that will keep the bike from running if it isn't grounded.
Ron West Front Forks   The front forks need the stock fluid transfer ports welded closed, then new smaller ports drilled. This will greatly improve the damping on the front end.
Ron West Rear Shock Works Works makes a shock for this bike, it's just listed as the wrong year.  Don't let them tell you they don't have a spring for this shock, they do. You just have to find the one guy there knows about it.  Also, the rubber cone bumper that comes installed on the shock is between 1/4" and 1/2" too long. Trim the fat side of it for more travel on the shock without bottoming out.

NS50 Parts List

Part Name Honda Part  #
HEAD GASKET 12251-GE2-013
CLUTCH DISK (fraction) 22201-GF6-000
CLUTCH SPRINGS 22401-166-000
SHIFT SHAFT 24610-168-000
SHIFT FORKS 24210-GT5-000
SHIFT SHAFT SEAL 91209-612-003
CRANK SEAL 91205-166-004
CRANK SEAL 91206-GE2-611
BEARING 96100-6201000
PISTON PIN 13111-166-000

NS 50 Piston List

Piston Name: Honda Part Number:
0.25  13102-GE2-305
0.5  13103-GE2-305


XR 100 Information

Who Part Supplier Comments
Ron West XR 100 Richmond Motorsports Did you know that an XR100 costs $2300 (list price) at your local Honda dealer?  Did you also know that the same bike costs $2300 Canadian?  You figure in the exchange rate and that same bikes only costs around $1600 USD. There is no import duty on this bike either.  It takes an extra 10 minutes at the border to complete the paperwork.
Ron West
Tim Barkley
Jerry Douglas
Tire Front Size Rear size
Metzler ME33 3.25X19 54H 110/90H-16
Dunlop k591 Elite 100/90H-19 100/90H-16
Dunlop K180 100/90H-19  
*Bridgestone BT45F 100/90H-19 110/90H-16

it is important to note that the rear wheel on the XR is not wide enough to have tires made for it.  The workaround to this is to order a 16" front tire and use it on the rear. You must remember to mount it backwards.

* Most recommended tire

Ron West 17" wheels   If you are interested in lacing up 17" wheels on stock XR hubs, the widest rim you can use is a 2.75". My bike has 2.75" rims on both the front and rear. I run dunlop rain tires on my 17" wheels and they last about 6 race weekends (including my home training sessions).
Ron West Rear Suspension Works Take that whimpy stock rear shock off your bike and put it in the garbage.  Get yourself a Works rear shock (much less $$$ than the YSR fox shock!!!) and spring.
Ron West Exhaust   Take the baffle off of your XR.  Now, remove the restrictive spark arrestor and then put the baffle back in.  You will get your engine to breath more, without blasting everyone behind you.
Ron West Carb Settings   Stock XR's are purposely setup slightly lean from the factory. Take the snorkle off the airbox, cut another hole in the top-front section of the airbox that is approx the same size as the snorkle area.  Now put a 100 main jet in your bike and it will run like a champ.
Ron West Winter Maint.   Between race seasons it is important to freshen up your bike. Replace the valves, valve seals and valve springs every year, with new stock parts.  The parts should be less than $60 and will do much to preserve the lifespan and performance of your bike.
Ron West Front Suspension   To get the front end of your XR to handle take apart the forks, weld shut the bottom 4 transfer ports on the damping rod.  Redrill the holes with a 1/16" drill bit.  Also, there are 2 small ports near the top of the rod.  Weld closed the bottom of the 2 holes. Redrill this hole about 1/2 way down the rod (makes suspension more progressive). Put the forks back together, compress the forks without the springs in it. Fill with 10w oil until 100mm below the top of the fork.