Diagnosis!
1. ATV cranks over and will sputter and backfire into the air filter box.
2. Very low compression but very high suction at the spark plug.(Sorry don't have a compression tester).
3. Head was removed, cam manually cycled, valves move freely don't appear to be bent, valves appear to seat well.
4. Head was flipped upside down poured solvent on it to check for valve leakage, that tested OK "I think".




Picture #1 shows rotor timing mark "T" or a wide "H". Counter clock wise rotation the yellow arrow mark comes first which still has piston on an up stroke on compression cycle if the cam gear is reaching it's mark. Right?


Picture #2 shows rotor timing mark at what I believe to be "TDC". at this point the piston has reached TDC and starting it's down stroke. Right?


Picture #3 shows cam gear at again what I believe to be "TDC". If I remove the chain the cam will rotate about 20deg to either side of "TDC" before engaging the valves. so at this point I believe the valves are closed and should be on a compression cycle. Right?


HELP!

 

You are "over thinking" the problem. A mark on the flywheel just before TDC is going to be ignition timing, why they bother I don't know, as ignition timing is "fixed" by the flywheel anyway. Low compression is due to worn bore/rings, valves not seating or not opening far enough, timing out, or blown head gasket. All can, and should, be checked when you have the head and cylinder off.