Re: Noobish Tomos Question

Joe Woods /
This single post is part of a larger thread. Start from the top or view this post in context.

Main jet size affects mixture near Wide Open Throttle (WOT). The correct main jet depends on a lot of things that are most often unique to you.. your riding style.. your bike.. it's state of tune.. your environment.

Unless your spark plug shows definate signs that it's overheating at WOT, the main jet that's in there now is delivering enough fuel. If you go to a larger main jet, the engine will lose power at WOT and may be slugish at smaller throttle openings as well. MPG will suffer as extra fuel is wasted.

So before going larger, find out if the jet that's in there is adequate with a top-speed plug chop.


Find a long, quiet, flat road.

Thoroughly warm up the engine.

Remove your spark plug and install a brand new spark plug.

Go top speed for about 3/4 mile and 'kill' the engine with the kill or ignition switch .. Coast to a stop.

Remove the new spark plug, reinstall the old plug and drive home.

Examine the test-plug under good lighting and strong magnification.


If you see black soot on the plug and the plug otherwise looks brand new, you're not running lean. This black soot is unburned fuel. Going to a larger jet will leave even more soot.

But if the corner edges of the plug's electrodes are no longer sharp (rounded like they have melted) or if the porcelain insulator is grey or has tiny aluminum or black spots on it, or if the insulator has a rough surface or a grey color, the plug is getting too hot. An overheated plug actually looks kinda like it's been burnt ..

A larger main jet _may_ be called for.

However, if your carb or any part of the fuel system is dirty, or if your ignition timing is off, or if the spark plug's heat range is wrong, or if the comustion chamber is carboned up, or if you have an exhaust leak, or if the weather conditions are particularly unusual, etc, a plug chop cannot be used to check air:fuel ratio.. and it's pointless to do anything until the situation is corrected.

This single post is part of a larger thread. Start from the top or view this post in context.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account