the boots are good. my son and I just replaced the head gasket He set the timing at the center mark. Why are there three marks there and, which mark should he have set it at?
The other reply to this question is incorrect, please disregard it.
There are 3 lines on the flywheel. Two of them also have a radial line in between that connects them. This is the firing range. When the engine is running, if checking with a timing light, the mark on the case should be somewhere between these two marks.
The line by itself, which is the last line when spinning the engine in it's normal direction of rotation, is the mark for TDC, or top dead center. This is the mark that should be used when installing the cam chain and sprocket on the cam.
Regarding the backfiring.
Either an exhaust leak or a lean air/fuel ratio are the most likely cause, or a combination of both.
Like mentioned earlier, the "lean decel pop" can be tuned out, but sometimes at a tradeoff to throttle response and idle quality. I like to run them on the safe side of lean, if they decel pop, so be it.