If I had to bet I would say it's the "injector pump" which regulates the amount of fuel mixture that is allowed to enter the cumbustion chamber thru the intake valve... A clogged injector would make it either not run or run lein...

So, that's my guess...

Of course a badly clogged air-filter, as someone allready made mention of, will do the exact same thing...

Good luck and keep us posted...

Bobert (motorhead and bluesman)