John J @ 6:14PM... The problem is going to be either sparks or juice, you need to decide!

Cheapest way to start..... do what Jonn said for spark (a water mist from a spray bottle on the spark plug wires is a good idea too.) For juice, start with a can of carbeurator cleaner/can of injector cleaner as the case may be.

Sometimes, the cleaners just won't be enough to solve the problem, but they might point you in the right direction.