How can i know if my bike is lean ?

What the best way to know if my bike is correctly jetted. Is looking spark plug a good way on those 4 stroke? When I'm idle and i open 100% throttle fast the engine want to stall. Its the only bug i have on the whole power band, when i open the gas to 100% really fast. My bike is doing some backfire when i release throttle too.

If it's popping I'd say it's lean.

The popping on decel indicates it's too lean. Perfect jetting will pop just a little on hard decel.

The flameout when you whack the throttle is related to the accelerator pump timing, the leak jet and the pilot circuit. Once you get the fuel screw and pilot jet set right and it still bogs, do some searching here for some AP timing threads.

Basically the AP squirts fuel into the motor when you twist the throttle quickly. The motor will stall if the squirt is too much / not enough or too long / too short.

Best way to tell if is jetted right is by feel and the way it runs, don't rely on the old SP color method unless it is sooty black, then it will be running like crap anyway.

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