I would check the cap and wires for cross firing. Was it damp out, high humidity? Check the cap for carbon trails from one terminal to the next. If there was enough humidity in the cap, the arc will burn the carbon out of the water and form a track. It will look just like a pencil mark.
On the timing chain, why would it jump a tooth under a load? The camshaft doesn't know the engine is working hard. High rpm it could jump, but would it just jump one tooth? Any more than one, would the engine run at all?
A valve could of hung up and caused it to back fire? Do you hear anything from the carburator or out the exhaust pipe? Can you pull one plug wire at a time with engine running and make it better or worse?
I have allways heard of timing chains jumping a tooth but never seen one do it. I have seen nylon teeth break off but never a chain break or jump. I have seen some very loose ones that should of jumped. Its just one of them stories I here about but never see . Joe
