I need help:

I have an older 18 HP B&S (422700-422799 model series)twin cylinder in a Garden tractor. The engine starts and runs fine at idle and at high speed while parked. It will run strong under load (3rd gear pulling mild hills in yard) until it warms up to operating temperature. Then it starts to surge and hunt for RPM, get an occasional backfire, and then stall. If I pulse the choke, it catches itself and keeps running. Otherwise, it will restart and idle fine but as soon as you put it under load - same thing.

This is what I have done so far:

- cleaned carb, inspected diaphrams and jets
- inspected coil and plug wires
- pulled heads and cleaned carbon. Inspected valve seats. Looked OK.
- inspected, cleaned, and re-gapped plugs.
- replaced fuel filter
- replaced fuel lines
- adjusted high speed mixture a 100 times per manual.

I rebuilt the entire carb last season.

I am at a loss. Anyone have suggestions?

The fuel is from October and I put the preservative in the tank when it went into storage. I am assuming since it runs great cold, that fuel is not the issue.

Thanks.

You may have a valve seat coming loose when the engine heats up, I would check the valve lash on all the valves and if one comes up with little or no clearance or way too much clearance then I would pull the head off and inspect the valve seat.