It's difficult to say what the exact problem may be. If the engine tends to accelerate as it warms up, then something's making it go lean. It's possible that vibration is agitating air into the fuel that's in the tank, which then leans the mixture. It's also possible that there's some kind of air leak that opens-up after the engine's been run a bit. Rubber o-rings may be bad due to their age. The engine is quite old, and the rubber seals in the carburetor may not be doing their job. You may also want to replace the front and rear cover gaskets. You can easily make some out of baker's parchment paper. We'd not have any gaskets for the engine in the parts stock...it's been out of production for too long.