Here is a good tip for Carburetor troubleshooting.

[OBrian-DK]

Found it on http://www.thumperfaq.com/jetting.htm and thought it should be posted here too.


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Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly. Remember the throttle position, not the RPMs, determine which circuit is controlling the mixture. Ideally, you would select the main jet first, then the needle and clip position, and then the pilot circuit. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, MJ is the likely problem. While jetting the carburetor, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting. Clean the air filter and warm the bike up.

Then using this diagram to locate the problem.

Circuit

Components

Throttle Position

Idle / Pilot

Pilot Screw, PAJ, PJ

0 - 1/4

Needle

Throttle Valve, Needle, Needle Jet

1/4 - 3/4

Main

MAJ, Main Jet

1/2 - WOT

[md86]

I use this one all the time .....

http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

However , these things don't take into account worn components so much . You assume that everything is in decent shape starting out .

[Dan Dubeau]

I use this one all the time .....

http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

However , these things don't take into account worn components so much . You assume that everything is in decent shape starting out .

And don't forget the plugs. It's ALWAYS the plugs

[md86]

Well yeah , duh . The plugs will tell you if it's rich or lean , too . ACtually , that's how the used to do it in "the old days" . Plug chop . Put in new plugs , go WOT and flip the ignition off and coast to a stop . Pull plugs , and that'll tell you if your main jets are right . You can kinda do the same thing at idle . I'm still working on that one , but it seems a promising method .....