Proper main jet after upgrade  By  nate7504 (Read 612 times)

I've read stock is typically 105 or 108. my original main jet was unmarked.
and caused it to die when the throttle was even slowly opened when cold, when warm it backfired through carb and/or occasionally died getting off idle and rpm went up and down due to the lean surgeing at part throttle with the rear wheel off the ground as it struggled to rev up. ( I didn't even try riding it since it ran so bad and I know from experience the effects of lean mixtures are even worse under load. )
I went to 112.5 with all else stock. 110 was a hair lean, 112.5 was a hair rich, 115 was sloppy rich. After gutting my muffler to remove the restriction it acts a hair lean getting off idle 'till it warms up. ( I haven't had time to play with the carb since de-restricting my muffler. sparklug readings looks right after a high speed run and after letting it idle 15 minutes, no idle fouling that'll make it hard to restart after extended idling before shutdown, so I'm pretty close )
My suggestion would be to get a 112.5, 115, and 117.5 and try the 115 first. then the others. if the 112.5 or 117.5 is better , go buy the next size in the direction you went and try that. once you figure out where too rich and too lean is, install the one in the middle. If two in the middle, go with the richer one.
I also went to a 38 idle jet which made the mixture screw work like it should.
2 turns out is was best, 1/2 turn less caused rpm drop due to too lean, 1/2 turn more caused rpm drop due to too rich.
( before changing it, I could open the idle mixture screw 'till it fell out without ever getting too rich at idle which made it impossible to find the best spot to set it. )

The rule of thumb is 2 sizes bigger on the main jet for a free flow filter and if a performance exhaust is added bump up the main jet by 2 more sizes . Just about all the 150s come with a 24mm carb and a #35 pilot jet is really all you need. With a #35 the adjustment falls in between 2-3 turns out depending on if it is winter or summer.

If your adjustment on the Fuel Ratio Mixture are way off from 2-3 turns in or out Check your valves they most likely need to be adjusted and your trying to compensate for the bad running with the Fuel Mixture when its the valves that need attention.
Alleyoop

Yea, you way to rich on the main, just for example my motor has a 62mm with a 60mm stroker crank and it runs like a scared ape with a 122.5 main a 125 it runs rich, have a #38 for th pilot. It runs with a 120 but I play on the save side for the top end.

You are killing your power on the high end with such a big main jet.
Alleyoop

Just to throw up my stats on the subject for comparison...

Tank 150cc with high performance parts - 115 main jet was too much, 109 main jet works great.

gy6girl,

What are the specs on your scoot? mine are in my sig, but now I am down to a 120 main, and it runs alot better. IIRC, my A/F screw is around 1.5 - 1.75 turns out, and she runs real good. Even in this super-blustery cold 28F weather, I got it up to 68 mph. However I stopped when my fingers and knuckles became completely numb. lol

I also no longer get any after-fire in the exhaust, which is a huge sign that I was running rich to begin with.

Afterfire occurs when un-combusted fuel ends up in the exhaust and is detonated in the header rather than in the combustion chamber. This occurs when the a/f mixture is too rich.