Mad Dog said:In many, if not most, cases, an exhaust leak will produce this. What happens is when you let off the throttle, the decelerating engine speed causes a vacuum which sucks air into the exhaust pipe leak. This 'fresh' incoming air, mixes with the 'unburnt' fuel exiting the combustion chamber, gets heated by the already hot exhaust pipe and ignites it. Kaboom!
With the engine running, take a bottle of soap/water solution, spray it on the areas that may cause an exhaust leak: where the pipes attach to the head, where the pipes attach to the muffllers, etc. and look for 'bubbling'. If you see any, that's where your leak is. If it's where the pipes attach to the head, change the exhaust gaskets. If it's where the mufflers slip into the pipes, remove them, put a thin coat of hi-temperature silicone around the end of the muffler, slip it back on, tighten it up, let it sit for 5-10 minutes and fire her up.
If after doing all this, it still doesn't go away, you at least have eliminated this as a potential problem.
Next, do the same with your carbeurator/manifold setup. If this doesn't help, your carb could be too lean or too rich, which in any case, needs to be looked at by a competent mechanic to adjust.
The absolute worst case situation could be any of the following:
a burnt valve(s), bad ignition, ignition timing off, the list can go on forever, so try the easy stuff first.
Good luck, Wes.