Does this popping/backfiring only happen at idle/fast idle speed? Nothing during acceleration or deceleration? Does it happen if you run the engine until warm, then let it cool for an hour or so, then run it again?
Ignition timing might be involved, but that shouldn't change with engine temperature. There is a possible connection, so check that, and make sure the idle speed is correct after setting the timing. Carburetor ice will cause stalling at idle and low speed, but I can't see that causing the exhaust noise you have. If the heat riser valve is stuck open, it is possible to have carburetor ice causing stalling during warm-up, especially after an over-night cold soak, so that possibility is worth checking into. Does this condition happen predictably, regardless of air temperature and humidity? If so, carburetor ice is less likely, but still possible. The only way to determine with certainty if your carburetor is icing is to run the engine with the air cleaner off until the engine stalls. Then look into the carburetor throat to see if you see frost around the throttle valve. If you do, you have to get the heat riser working to help prevent its formation. You have to be quick with that inspection, because the ice will melt pretty quickly. But water droplets on the throttle valve are strong evidence that ice was there.
As for loosening the heat riser valve, soaking the shaft with penetrating oil of your choice, and tapping on it fore and aft will eventually loosen it. But remember, you aren't trying to drive the thing through the manifold- only loosen it. It takes patience and perseverance. When it starts loosening, try turning by hand to help get it free. When free, it should move freely by hand and the weight should return to the raised position- when cold that is.
Harvester
