Small Engines (Lawn Mowers, etc.): 8 hp Briggs with glowing muffler, hp briggs, homelite generator

Question
Hi,

I have an 8 hp Briggs&Strattion
 Model      Type      Code
190417    3093-01   89012510 on a Homelite generator.

This is a new aquisition and it it driving me crazy. When I purchased the unit I was told it hadn't run in years. A little gas and a fresh battery and off it went. Unit sounds good turns over fine and starts quite easily. Once it runs for approx 10-15mins the muffler is glowing red at the bottom. I tried adjusting the carb with no results. I then swapped the carb with one I had on another motor and started the unit the next morning after sitting all evening, same results muffler started glowing after 10-15 minutes. The motor sounds fine it just doesn't seem right. I pulled the head everything looks fine there, the valves appear to be opening and closing all the way. I plan on hand lapping them just incase but I do not feel it will correct my problem. Would you be able to point me in a direction? Any help you could provide would be greatly appreciated.
Thank you
Kind Regards
Bart

Answer
Running way too lean or not cooling properly.  Have you removed all the sheet metal to make sure rodents have not built homes inside the cooling fins on the flywheel and engine block?

Check the valve clearance.  Here is the spec link:

http://www4.briggsandstratton.com/miscpdfs/RNT/Engine%20Specifications%20Chart_m...

Are you using SAE-30 oil?
Let me know.
Eric