I had a 1200 that did the same thing.
The exhaust valve is getting hot and sticking open.
First, retime the engine. The procedure is found here:
http://mgonitzke.net16.net/tools/static_timing.pdf
Then put a healthy dose of Marvel Mystery Oil in your gas and run it. Should cure the problem. After you run that tank of fuel through, run the MMO in the ratio recommended on the bottle every tank of fuel. It should prevent the problem from returning.