Dave wrote on 06/19/13 at 04:53:55:When you turn off your ignition the engine is still turning and is till pulling air through the carb.....where it picks up fuel the same as if it were idling. Since the ignition is off....the fuel/air mixture just gets pumped through the engine and into the hot exhaust system. When the temperature of the fuel air mixture is elevated by the hot exhaust system.....it can become hot enough to spontaneously combust in the exhaut pipe and "bang". Leaner mixtures seem to make a louder pop than a rich mixture. I believe you should adjust the mixture screw for the best engine operation.......not to eliminate backfires or afterfires.
I believe you should adjust the mixture screw for the best engine operation.......not to eliminate backfires or afterfires./agree
I choose not to adjust the carb to eliminate backfires... but I have a sequence that I use to virtually eliminate the issue.. or at least reduce it to a mere phoot...
When getting ready to shut down:
Stop the bike,
reach down and pull out the "choke" ( it isn't really)
Half generally works but full out if needed
when you first do this the RPMs will drop
while the rpms are down, use the kill switch to shut down
Remember to push the choke back in
Remember to return the kill switch
Always park in gear ( generally 1st)
The bike requires a high idle to maintain proper lubrication to the head, but when shutting down the high idle allows as described above (intake of gas rich air mixture ) which is then shoved out a very hot exhaust. Other possible contributing factors:
Fresh air leaking into exhaust manifold at the head, allows for leaner and O2 rich condition in the manifold. Check the header bolts. I have read here that sharp edges inside the muffler absorb the heat and can create the "hot ember" needed to ignite the gases in the muffler. I suspect that over time this would be more common as components break down.
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