Noobish Tomos Question

Ive read many posts that talk about tomos jets. Like #56 or something like that. I dont know what jet there all talking about nor do I know were to buy one. So can anyone explain what the hell they are? Thanks!

Re: Noobish Tomos Question

A jet is a small brass screw in the bottom of the carb with a hole in the center to control fuel flow. The size of a jet is marked on the body, a stock main jet would be something like #53 and if you added parts requiring more fuel you would be required to upgrade to a larger jet (like a #56) and so on so you dont run lean. <> has a selection of dellorto jets.


Live to ride, Ride to work

Re: Noobish Tomos Question

I have a biturbo pipe. What jet should I get?

Re: Noobish Tomos Question

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.

Re: Noobish Tomos Question

Don Pflueger /

anything from #54-#56 depending on your altitude and plug chop results. there is no set number jet that you need to use. its a trial and error sort of situation. you have to install a new jet, then install a new spark plug, run the bike full throttle for at least one mile, then remove the spark plug and see what color it is. you do this with each different size jet and a new plug until you get a nice even tan color on the plugs ceramic core. white or grey= too lean. black and sooty= too rich. improper jetting will cause a siezure.

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