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Jetting a 20mm carb

3949 Views 4 Replies 3 Participants Last post by  OLDSCHOOL
Ive got a 20mm carb on my bone stock 110. Everything is stock other than the carb and a K&N filter on it. Right now it has i believe an 89 main and a 35 pilot. Its hard as hell to crank the way its jetted now and acts weird with adjustments. Like if i turn the pilot screw out 1 turn from full in it wont idle unless i turn the idle way up, and if i turn it out it idles way high, but is hard to crank that way. i think its running lean because i leaned the needle jet out some and that made it run worse. Is anyone running the same carb on a stock 110 and if so what kind of jetting are you using? And how about a baseline on the pilot screw?
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Usually the mixture screw is adjusted to best idle when warm. Its possible that you need a size larger pilot jet. But id clean that carb 1st, crap in air passages or the idle circuit/jets
Yea ill see about adjusting the pilot again tommorow. The carb is extremely clean though. I think its lean because when i go to rev it all the way it doesnt rev it just makes that empty sounding "wuhhh" noise and will cut off if i dont let off the gas. Revs fine if i get on it slow though. Started doing that after i leaned out the needle by moving the clip up one
Carburettor tuning guide - Pit Bike Club Hope this helps. Oh and jet each area of jetting separately, ie pilot, needle and main. Also checking the plug is the best way to determine lean/richness, it tells you exactly whats happening during combustion.
Dude what Ive been finding lately is that the Ignition systems have been crapping out on the china bikes (Coil or Rev Box) on any given bike and causing (Carburetor type issues)

Your jet sizes sounds like they are on or close to being correct, so before you start going into the carb see if you can swap out a Rev Box and coil with a buddys bike.

Only reason being your Issue sounds like the same ive been running across with a bunch of different China bikes that all run the same type ignitions.

Process of elimination

If you go the carburetor route and jet throught the spectrum and still have issues...check the Ignition system!!!

here is my Pit Bike guide to Jetting

1) GET A PLUG READING: You need to run the bike in as tall of a gear as you can. and then kill the engine with the throttle wide open. Dont idle the engine, now remove the spark plug and look at the INSULATOR, the white porcelain part. as a rule it should look like coffee and cream( TAN ) If its Dark brown or sooty black your motor is running RICH. A gray ashy or white reading means its running LEAN. A plug reading only gives you an overall view of how the motor is running.

Do this with a new plug and make sure your bike is warm first.

2) THE MAIN JET: The carbs main jet affects how the carb works from half to full throttle After you have made your test run with the stock jetting. Install a main jet that is two sizes richer it will have a higher number than you removed. do your test run again, pay attention to how the bike feels. If your bike is reluctant to REV out like before, then you are too rich. Now go back and install the next smaller main jet keep doing this until you feel a noticable improvement in power once you have the main jet set go on to your pilot jet.

3) PILOT JET: The pilot jet has the greatest influence from idle to 1/4 throttle to test the pilot jet that is two sizes richer (just like the main jet) now go out and test it pay attention to how its feels when you pull out of the slow turns. If your bike is too rich it will feel sluggish and unresponcive untill you are past 1/4 throttle A perfectly tuned pilot jet will help your bike accelerate off the line and out of corners. Repete this test until you get one that feels the best.

4) THE AIR SCREW: How to adjust it, with a warm engine bring the bike up to a steady throttle setting (about 1800 rpms) get a friend to hold the throttle steady......Once the engine is running at fast idle. Turn the air screw in clockwise until the engine rpm drops. Then slowly turn the air screw out until engine rpms pickup.....Stop turning the air screw once the RPMs reaches its peak. Peak rpms is when the engine runs its cleanest and fastest. The most common air screw adjustment is from 1/2 a turn to 2 turns out. If your air screw is more than 2 turns out it is a warning that you should switch to the next LEANEST PILOT JET....If the engine has peaked before turning a 1/2 turn out. Then switch to the next largest Pilot jet.....

5) NEEDLE JET: Your carbs needle affects throttle settings from 1/4 to 3/4 turns lucky you dont have to replace the needle to adjust. Start out with the clip in the middle setting make a couple of runs, then change the clip down two positions. This is the quickest and fastest improvement you can make to the bikes jetting. Dont be afraid to move the clip up..LEANER or down..RICHER. You spend most of your time riding in this range.

Try all clip positions on the needle see what runs the best.... If you find your clip position at the very top or bottom. then you need to go back to your main jet and pilot jet and make a different setting...

When all is done your clip should be in one of the three middle notches.
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