[Jim_Lou]

It could also be a plugged muffler or exhaust pipe, or a frozen heat riser valve.

The heat riser valve is between the exhaust manifold and the exhaust pipe on one side. If it sticks in the closed position, it forces almost all of the exhaust from that head to squeeze through a little passage under the intake to the other side of the engine. With the engine cold you should be able to rotate it freely. If it's stuck, work on it with penetrant and patience until it's completely free.

 

[Jim_Lou]

You may not have one. Look for a spacer about a half inch thick with a 5/16" shaft through it between the collector and the down pipe. Grab the shaft with pliers and see if it turns. If there's no spacer or the shaft turns the problem is something else. In that case, look for the link to RRich's Propane Trick in the welcome post.