I working on a 2001 Ranger 500 built 7/26/2001. It idles fine but has a severe backfire through the carb when accelerating. It's so bad it unseated the carb at the rubber intake manifold. My first suspicion was an intake valve not seating, I checked my valve clearances-.006, inspected the cams, checked compression-150psi, (a little high I thought) and checked cam timing, which is nearly impossible to by yourself in the vehicle, but appears to be correct. Then I thought maybe it's a lean missfire. Before rebuilding the carb I applied the choke during the misfire-no change, I sprayed starter fluid in the carb-no change, Now I'm thinking ignition system, so I replaced the spark plug-no change. Could it be the module or pickup. If so what's the best way to isolate it? I considered monitoring the TDC mark on the flywheel with a timing light while it's misfiring, but very difficult to see.

 

Since my original post I've performed a cylinder leakage test. Only 2-3-% leakage, so I'm ruling out any kind intake valve not seating issue, including valve/camshaft timing. I also looked a TDC on the flywheel compared to piston travel. As near as I can tell with the naked eye, TDC on the flywheel indicates piston at top of stroke, (keyway in flywheel ok-hasn't slipped). So I'm leaning more towards an ignition problem, your thoughts.