Backfires when you let off the throttle - Motorcycles

check the valve adjustment a friend of mine has lost 2 sets of push rods they bent and fell into the motor

what make and model are we discussing here ?

you adjusted your governor incorrectly.

Dennis, Maybe try changing your plugs, unburnt fuel may be getting in exhaust because of weak plug.. Read the following links, very informative in telling causes of and some suggested fixes on backfiring. Check the attached links,instruction and guides, Good luck
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Exhaust backfire popping when letting off throttle Motorcycle Forum

What causes bikes engine to backfire Intake Jetting Fuel Injection

Preventing Backfire on Motorcycle Exhausts ' DoItYourself ...

www.doityourself.com/.../5-tips-to-preventing-backfire-on-motorcycle-e...

When the engine on your motorcycle has an emission system malfunction, such as anexhaust leak or a moment of running rich or running lean, backfire can ...

Exhaust backfire, popping when letting off throttle - Motorcycle Forum

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Apr 26, 2009 - 28 posts - ‎17 authors
Exhaust backfire, popping when letting off throttle General Motorcycle Discussion. ...You can damage the engine by running too lean or rich. Backfires on deceleration
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These vortec engines are prone to leak fuel from fuel pressure regulators inside the intake manifold ,an from injector pop it valves . How to replace the spider injector on Cheyv motor 4 3 or 5 7 liter check out this guys video

Carburetor needs adjusted. May need rebuilt or just cleaned. Could be one of the jets is plugged.

Backfire when throttle down can be;
1. dirty carbs that are running lean and need cleaning. Also the rubber grommets that hold the carbs can be leaking at the backside or even dry rotted.
2. no air filter
3. broken or open vacuum lines
4. an aftermarket exhaust that needs rejetting or an open exhaust system

the best thing to do is to clean or replace carb filter.. check spark plug for fouling clean or replace also if gas has been sitting in unit and stored outside I would drain gas because of water contamination..

I still think this is a timing problem, most likely with the crankshaft sensor. However, it could also be in the throttle positioning sensor, since you had raised the idle to get it to work momentarily. Also, the mass air flow sensor cannot be ruled out entirely. I would start with the crank sensor, then the throttle positioning sensor. Try these and let me know.