Older Briggs and stratton 8hp horizontal shaft runs fine but the exhaust heats up to red hot in about 40 seconds you have checked it is clean good condition why this?

There are a few possible reasons why the exhaust on your Briggs and Stratton 8hp horizontal shaft engine is heating up to red hot in about 40 seconds. Here are a few things to check:

1. Check the fuel mixture. If the fuel mixture is too lean, it can cause the engine to run hot and the exhaust to overheat. Try adjusting the carburetor to richen the fuel mixture and see if that makes a difference.

2. Check the air filter. A clogged air filter can restrict airflow to the engine, causing it to run hotter. Try cleaning or replacing the air filter and see if that makes a difference.

3. Check the muffler. A clogged or damaged muffler can cause the exhaust to overheat. Try removing the muffler and see if that makes a difference.

4. Check the cooling system. Make sure the cooling system is working properly and that the engine is getting enough coolant. Check the coolant level and make sure the radiator is clean and free of debris.

If you have checked all of these things and the exhaust is still overheating, it is possible that there is a more serious problem with the engine. You may need to take the engine to a mechanic for further diagnosis and repair.