What causes a 4 cycle engine to pulse when running?

Question

Started the roto-tiller this year with half a tank of old gas mixed with half a tank of fresh. After it was running and warmed up I took the choke off to the run position with the throttle in the high position and the motor pulses up and down continually. The machine was kept in the shop all winter and I did add some fuel treatment to it after it had been run a while. Could it have picked up something in the carburetor or is it something that will work it self out after it has been used for awhile. We have used it for about three hours so far and have started on a fresh tank of fuel with treatment in it also and it is still doing the same thing.

Answer 1

I would see if there is a leak at the carby manifold. It sounds like it is sucking extra air.~~

Answer 2

try a new spark plug and clean the air filter.