Timing belt falling off strimmer motor - why?

[cl]

I have a Ryobi 4-stroke strimmer (it's the 'Expand-It' sort of one) and the timing belt has started falling off for some reason.

The rockers and head are bolted onto the cylinder and the belt is driven from the crankshaft so I can't really see any way that the belt can become misaligned.

Any ideas anyone?

Up until now the 4-stroke engine has been absolute magic compared with a

2-stroke one, easy starting (hot, cold or half-way), simple fuel, etc.

[harryagain]

Faulty timing belt (stretched) sounds likely to me.

[cl]

Could be I suppose.