Yamaha kt100 carbs

ttownwideglider

New member
If your talking about a stock yamaha just run the wb3a that came with it. .045 fulcrum with no gasket, 10 psi pop off . Tune low jet 3/4 turn ,high jet 1 1/4 then tune high jet at wide open throttle on track and watch cylinder head temp 425f max gas fuel on new style head.
 

racer11x

Member
New style can be run up to 500 but not for long. I do have a friend that has a son that ran Micros in a KT class and always ran at 500. They never stuck it but did eventually burn a hole in the piston.
 
New style can be run up to 500 but not for long. I do have a friend that has a son that ran Micros in a KT class and always ran at 500. They never stuck it but did eventually burn a hole in the piston.

I run old style Yamaha and have gotten that hot a few times, normally run in the 430-450 range. Difference is I run a metal treatment called ProTec (1oz per gallon) at breakin with Maxima Castor 927 and I`ll throw in a little after running the engine for a while. In earlier days of racing I used a Scott Pruett oil blend but have ALWAYS used the ProTec and can honestly say that I have NEVER stuck an engine. Learned about ProTec from a article in WKA magazine in mid 80`s by E.C.Birt in which he started an engine then answered the phone, talked for 20 minutes, came back and realized engine had no oil. Tore engine down and everything was perfect cause it had been treated with ProTec. He sold me on the stuff.
 

Don K

Member
I believe you leave the gasket in place when setting the fulcrum arm height. Set to .045 for starters.

There are a lot of theory regarding wb3a. This is what worked for us successfully. 10-12 psi pop off, .045 w/gasket, rubber pumper, and high speed needle 1/8 to 1/4 max. We never went more than 3/16. If using a modern pipe and jack shaft clutch setup set the stall at 10.8k to 11k. Start the engine with low set to 3/4. Hit the track at 1&1/4 and pace laps at 1&1/4. Start the race at 1&1/2 and open 1/4 every two laps till you get to max 2 or 2&1/4. Go back down to 1&1/4 for yellows and restart at 1&1/2. Do it all over again. This works for pipes and cans. Only stall changes.

If your carb is clean and you use this method your temps will hover around 370-390 degrees with an old style head and 420-430 for the new style heads. Your plug will read an almost white with a visible yellow ring half way down the insulator. Read the plug every time off the track.

We started teaching kids this as soon as they got to JR2 and some kids that payed attention started when they were in their last year of JR1.

DK
 

Don K

Member
I agree with Stggerdog about sticking pistons. We lost only one after hundreds of races only because a crank seal leaked. I was always amazed how durable the KT was.

DK
 
I too ran that PRO-TEC treatment for yrs. , got it from a 4-stroke guy in Tenn. loved the smell and the anti-wear part, but it sure made welding a bear.
 

ttownwideglider

New member
Hey Mike the wb3a will run a modified Yamaha very well. The performance gain from carburation change will be very minimal unless its a big bore and has a lot of port work done. The Tilly is a better carb and a little more tunable but does the gain justify the cost, tillys aren't cheap. The measurement on the fulcrum is without the gasket at .045. Depending on your set up the jet settings will vary but what I gave you is a good place to start.
 

alvin l nunley

Site Supporter
If your talking about a stock yamaha just run the wb3a that came with it. .045 fulcrum with no gasket, 10 psi pop off . Tune low jet 3/4 turn ,high jet 1 1/4 then tune high jet at wide open throttle on track and watch cylinder head temp 425f max gas fuel on new style head.
Those are sure different numbers from what I use to run. I'm thinking maybe you just got them backwards.
 
For gas the WB3A works real well, for an open class, bore the venturi to about 1.00, bore the throttle bore to handle a 1.100 or 1.125 shutter then shape the venturi with a forming tool to a small radius. After that cut from the venturi to the throttle bore on an angle that will zero at the throttle bore. I would also work the front side some cutting it on a shallow angle up to the venturi, building an air filter adapter, a cup style not a velocity stack to match with as big a radius as you can where it meets the face of the carb. Add a high flow butterfly and throttle shaft assembly an now you have something that will get the job done for an open. Merry Christmas , Jon
 

Yeti Racing

New member
It's an open class but can only run gas. Can you really run them that hot?

The OP finally clarifies/states here after his opening post "open class" so... IMHO For stock class - Ditto in part as wideglider states "The performance gain from carburetor change will be very minimal unless it has port work done." Cast & billet carbe's will be close in max hp, BUT the appropriate tuned billet or reworked carbe's will have a snappier & smoother throttle response, easier to adjust, & less finicky (you get what u pay for, usually). IMHO On the other hand, For Open class ported engine stock bore or big bore or stroked - Again with the appropriate size carbe for the engine & type of racing (i.e. long, short, momentum, shoot/gun track, j/s or DD, Gross weight of fully loaded kart/driver 345# vs 515#'s) properly tuned the carbe choice will be significant. Quality of Port job & your pocketbook depth will dictate a whole lot more than a carbe choice.
 
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