I have a 1974 65 hp merc that has new fuel, good compression and wont start.?? I had it out at the lake put it in the water started up right away ran pretty good. battery was almost dead so after letting it run for a couple minutes i shut it off and charged the battery. after that it would not start at all???? cant figure it out at all.i have put fuel directly into the cylinder using spray bottle and still nothing.compression is 120 in all 3 cylinders and is getting great spark. really strange that with good spark, good compression and fresh fuel in cylinders that it wont fire?? it does back fire when cranking it over.!!?we took carbs off and they looked really good too. could the timming have slipped somehow?? any advise would be helpful. thanks.