Small Engines (Lawn Mowers, etc.): Snow blower stalls out after 30 minutes of use., yard machine snow blower, clearance problem

Question
I have a 8HP Yard Machine snow blower that is 7 years old but used only once a year at most.  For the last three years the snow blower stalls after 30 minutes of use.  If you wait about 20 minutes it will run for about 10 mintues before stalling again.  After each subsequent wait it runs shorter and shorter until it won' start unless you wait hours.  There is no back fire it just stalls out.  The weird thing is this only happens when it is being used to move snow, heavier the load shorter the run time.  During the off season I fill it up with gas and let it run completely out of gas - over 2 hours without stalling.

I have taken it to a repair shop last year and they filled it up with gas and let it run for hour plus without an issue.  It seems to only stalls if being used.  I have checked the carb bowl and it is clean.  It has a good spark plug and I have both loosened and removed the gas cap during use to make sure there is enough air flow and it still stalls.  I have ran it with "gum out" and still no change.  Runs fine when idling but stalls when being used.  The only other thing is when it has ran in the idle state to check for the stall it is always warmer out than when being used for snow.  I did this during the fall.

The unit is like new, very little used over the years.  I drain the gas each season and run it out to make sure nothing left.  Fresh oil every year.

Any suggestions?

Thanks.

Answer
Hi Steve,
It sounds like a valve tappet clearance problem. What brand engine is it? Model numbers? If that is the problem, you'll have to remove the valves and grind the appropriate clearance (depending on brand, model, etc.). If this is more involved than you'd like to get into a local shop should have no problem with it. If you want to try it, i'll try to walk you through it.
Thanks,
Keith