briggs and stratton engine on a countax?

hello all
just to let you know i repair, sell ,service,etc garden machinery for a living and i have got this problem that i cant get my head around at work.
so i thought i have helped a few people on this site as best as possable maybe theres someone out there who can give me a few suggestions to try.
what it is im working on a briggs engine(Model 28q777 Type 0150 ) and it keeps cutting the flywheel key in half as i am starting it knocking the timing out.
now i understand how it can do it on a walk behind mower but this is on a countax ride-on that has a electric clutch pulley on it. (could it be that?).
3 times it has cut the key now and i am banging my head on the wall.
i just want to get this back to the customer in a condition that wount let him down.
the only thing i can think of is putting so thread locker on the tapered shaft of the crank hoping it may take some of the jolt out of starting it because once started it runs very well.
any ideas?
adam