The timing is set by aligning the cam gear to the crank gear during assembly. There is no adjustment, and the only way for it to slip time is if physical damage has occurred inside the engine. The only way to verify timing is to pull the gear cover.
If your set is spitting gas out of the carburetor, you are more likely looking at too weak of a spark, fouled plugs, exhaust valve not opening, or a stuck float in the carb causing the engine to flood.
Did you check both cylinders for compression?
How did you check for spark - with an in-line tester or did you physically pull the plugs and verify the condition of the spark?
If you pulled the plugs, were they wet?
I would check all this out before pulling the gear cover - timing is the least likely cause of your set.
If you really want to pull the gear cover, click here for instructions.