Start with a valve adjustment. It's possible you've got too little, or no clearance at all on an intake valve, and as the engine warms up, the valve stem grows in length enough to let compression and/or combustion pressure pop back into the intake. A sticky valve will do the same thing. If the clearance is OK and it still pops, pour about 4 ounces of Marvel Mystery Oil down the carb throat at a fast idle until you get a heavy smoke out of the exhaust. Shut the engine off and let it sit overnight. Next morning, start it and goose the throttle hard five or six times. That should dislodge any carbon buildup on the valve stems.
The last-resort fix would be to pull the head and do a valve job.
Jerry
