I am refurbishing my old Craftsman (Tecumseh) engine model 143.394172 new about 1988. It has a 2 speed carb. Haven't used it for 3 years.

Anyway when last I was using it, if I put it on low speed it would run at low speed for a while then get progressively faster till it was running at approx 2/3 max rpm. Seemed like as the engine got hotter the rpm got higher. Somehow that throttle setting was advancing as the temp goes up.

This all occurred after the engine was maybe 20 years old. When new this didn't happen.

How can I fix it?