21HP B&S Engine

I have a Sears lawn tractor which is 6 yrs old. It came with a 21HP
B&S engine. A few days ago it started to run real rough at idle. If I
add about 50% chock, it run ok, but still at times can misfire.

At a 100% speed, it seems ok and can cut grass and move around ok. I
got this condition a few years ago and was able to clear it by
cleaning the air filter. So far I have clean the air filter, replaced
and gapped both plugs, replace oil, replaced the fuel filter. With all
of that no change.

The engine surges up and down at idle real bad. I removed all the gas
and tried new one, no change. In the Sears manual it talks about a
idle mix adj. I have none of that. No screws to adj other the the RPM.
I used carb cleaner and cleaned the carb. No change.

What could this be? I called Sears and they want $96 to come over and
provide diagnostics. Then parts plus about $100 or so per hr for
labor

Thanks,
Fred