One last jetting thread for me...

[WheelieMan4]

Well I got it pretty good, and its running great...The only problem I have is when I let off the gas, and the motor decels, it backfires? What is causing this? My buddy has a 450r that is doing the same thing right now...

I had my lid on and took that off, and that seemed to make the backfire worse. So, I'm not sure what to do...thanks for whoever helps!

[GPracer2500]

Confirm that your pilot circuit is correct.

Backfiring on deceleration with a closed thottle can be from being too lean OR to rich on your pilot circuit--but usually it's from being too lean. What happens is that a too lean or too rich mixture doesn't burn reliably in the combustion chamber. As the engine cycles there will be times that some portions of an incorrect air/fuel mixture won't be consumed (even when lean). This unburned mixture passes into the exhaust and will occasionally find conditions that allow it to light-off. There's your popping on decel.

What some people miss is that a correctly jetted pilot circuit will cause a mild lean pop (particularly at very high RPMs). This is because under closed throttle decel the pilot circuit mixture gets temporarily pushed to the lean side. The high vacuum of decel draws in more air through the carb than would normally flow. The fuel metering of the pilot circuit becomes overwhelmed and the mixture goes lean. The higher the RPM while decelerating, the greater the vacuum, and the leaner the pilot mixture becomes.

If your exhaust is completely silent under closed throttle decel than there is a good chance your pilot circuit is slightly rich. If you're getting sharp, rappid-fire pops that's an indication of a too lean pilot circuit. If you're getting a mild, mostly unobtrusive pop when decelerating from very high RPM then your propably close to where you want to be. BUT, don't try and jet your pilot circuit by sound alone. There are other much more precise methods.