Poor performance after installing 72cc kit.

[kiwibaotianist]

I have recently installed the scooter assassins 72cc kit on my baotain, while it starts and idles fine, it will not performe at all when full throttle is applied.

Spark is fine, compression is fine, valve clearances are within spec.

The bike runs fine up to about 1/2 throttle but above that, it begins to splutter and die. Checking the spark plug revealed white residue indicative of an excessively lean mixture however my attempts to adjust the mixture screw yeilded no results.

I think I need to increase the size of the main jet again (I am already running a #90) however I would like to know if anyone else has experianced this problem and if so, what they did to fix it.

Thanks.

[px166bajaj]

Raising the jet size is necessary befor your lean mixture overheats and melts your piston.

It sounds as though your kit is making demands that your carb cant deliver. In my experience (with Vespas) increasing the cc with a kit will merely increase torque, so you'll find once its running, your bike will be better at climbing hills and running two up. This is exactly what I did when I stuck a sidecar on my Bajaj. It's no faster but it pulls like a train.

Most auto pilots get the best out of a kit by changing the carb as well as the kit. This will allow the engine to rev its nuts off. To harness the extra power and convert it into speed, an upgear kit will be required to raise the gearing of the final drive.

[kiwibaotianist]

Thanks. I will try increasing the jet size yet again and start looking for a bigger carb.

[blubayou]

Sounds like a carb problem.

My 72cc is running a more or less standard carb / airbox with the standard 0.85 jet, 0.90 was too rich.

I still have a flat spot at 3/4 throttle which I can't dial out, but pulls well at full throttle. Messing with the accelerator pump at the moment.

Was all OK before the big bore?

Let us know how you get on.

[kiwibaotianist]

I have since removed the cockroach from the carburator, the bike runs much better now.

I sitll have a few issues however, the bike has a tendency to stall if the throttle is opened to fast and I am geting backfires into the carburettor at various speeds and throttle settings.

I think I need to find a bigger carburettor, although I am open to sugestion for other things I could try.

[blubayou]

Excellent! You are probably running too rich, hence the backfires. Run the bike round the block on an open intake, if it's better try a 0.85 ish jet size.

A bigger or better carb would be a good idea, but all those that seem easy to fit have a vertical throttle cable entry which fouls the seat tub. I seem to remember Chknugget used a C70 honda carb, not sure how he solved the clearance problems.

Anyone else?

[spock]

Basic rules when changing carb jets, if you change the main jet apply this rule.
Main jet up 2 sizes, change the pilot jet up one.
The pilot jet is the one next to the main jet, if you go bigger than 4 sizes on the main jet then you should replace the emulsion tube {spray tube} as well as the needle to a richer profile.
Up one size emulsion, down to a thinner profile needle one size.

[jose]

Play with your air filter.. either too much or too little air is entering.. Or the back door route, play with the air/fuel mixture.
I removed my air box.. witch allowed for a stairght air draw from the 25mm hose.. I had installed a starndard airfilter.. it allowed too much air.. top speed was now 40 from 58MPH.
Covered the air filter with a plastic bag.. 1/2 of it, top speed was now 50, covered 3/4 of the air filter and top speed is 55..but in all the handling.. broke the neck off the cheap air filter I had laying around.. so off to the parts store for a 25mm (little) air filter.

Good luck

Jose'