I
have a Craftsman mower/tractor with a Briggs & Stratton 31H777 0297 E1 18 hp.
It has always run great until it didn’t. lol Here is what it does. I took the
mowing deck off and only use it to tow the little yard trailer. It starts and runs at low idle just fine,
sounds good and runs smooth until I bring up the throttle. Somewhere between 1/4
& 1/3 throttle it starts to run
rough and the higher the throttle the rougher it runs and the exhaust turns
black. When I pull back on the throttle it evens out and will run that way for
as long as I want, however if I put a load on it and try to pull the trailer it
gets bad again.

Here
is what I have done:

Cleaned
the carburetor.

Rebuilt
the carburetor.

Replaced
the fuel line & filter, and made sure the tank was clean of debris.

Checked
to be sure that fuel was flowing freely. Pours out of fuel line next to
carburator at a very fast rate.

Changed
oil & filter.

Bought
a new carburetor.

Checked
to be sure the anti-fire solenoid was working.

Tried
running it without the anti-fire solenoid and the hole plugged.

Adjusted
the valves.

Removed
and checked the flywheel key, reinstalled.

Checked
and reset the gap on the ignition module.

Pulled
the head cleaned the combustion chamber and checked to be sure there wasn’t any
carbon under the valves, installed new head gasket and readjusted valves.

New
spark plug.

New
fuel cap and tried running it without the cap.

Checked to be sure the choke is fully open.

Please
let me know if I missed something besides taking it to the shop or flat out
replacing it.