Small Engines (Lawn Mowers, etc.): briggs and stratton, underload, intake valve opens early and blows back through carburetor., briggs and stratton, valve clearances

Question
I work at a small engine shop, i had a customer come in with a mower that looked like it had been sitting for the last 5 years, i cleaned the carburetor to start with, replaced air filter, and oil. When i start the mower, its hard to start; if it doesn't flood first. When it starts it runs great, very smooth, no noises, no misfires. When i accelerate up to running speed it has to be done slowly, unlike what a customer would normally do. But once operating speed is reached, if you engage anything that would require a load on the engine i.e. blades, forward/reverse, the governor advances and it starts blowing back through the carburetor, unless i move throttle to idle position quickly, if left in operating speed while doing that it would die and not restart because/c then the carburetor and intake tube would be full of fuel., i have adjusted the governor lever to the best of my knowledge, which i may have done wrong, but that doesn't seem to help no matter where i put the adjustment. Its been a long time since I have been inside an engine and am unsure as to what makes the intake valve open too soon like it does.

Answer
First make sure the governor is adjusted properly.  Refer to this link, "static" setting.

http://www.briggsandstratton.com/support/frequently-asked-questions/Adjusting%20

What is the valve clearances?

http://www.briggsandstratton.com/support/frequently-asked-questions/Servicing%20

Make sure the intake valve is not sticking.  I have had engines that sat a while with gummed up intake valves that would not close properly as the valve spring was not strong enough to close the valve quick enough.

Finally, with the governor set properly and the valves at spec, I would check to see how it runs.  If it is still blowing lots of fuel into the carb I would check the flywheel key.

Let me know what you find.

Eric