my xr 70 1981 won't run . it has spark and it is getting fuel. i cleand the carb and no difference,air filter is clean. I tryed doing a compression test on it but my tester dosn't fit the spark plug hole so i just stuck my thumb over the the hole and cranked it over with my hand. I can almost hold back all the compression with my thumb , just alittle sqeek of air passes by my thumb. is this may be the problem? (not enough compression?