I have an Xtreme Machine 90cc atv that won't start. Getting good spark, carb in clean, fresh gas and oil. Every now and then it sounds like it wants to pop but doesn't. How much compression should these have? Could it be a timing issue? Any help would be great. Thanks

 

Based on your symptoms, it sounds like valves. Bring it around to TDC and make sure they are set correctly. If your engine is the E22 style, it should at least start with anything above 80 psi.

 

I just had this same problem with my son's 90cc Extreme today. If your unit has an electric choke (located on the top of the carburetor), unplug the black connector. This worked for me.
You may have to let it crank over for a minute, but then it should run fine.
Hope this works for you