Briggs and Stratton 16HP Cast Iron Surging

[gtermini]

The old man has been out splitting wood with a splitter I bought him a while back. It has a 16hp B&S cast iron single cylinder. The exhaust valve was stuck open when I bought it, making for a bargain price. Pulled the head and hit the valve with a hammer. I stuck a new china's finest carb on it and it fired and ran like a top for the first 5 or so hours.

Now it is having a surging issue. It will start right up at any temperature, and if it isn't at full operating temp, runs smoothly and evenly. As soon as it get fully hot, the rpm's will drop like it's out of gas, then it will catch itself without dying, and rev to the governor. Repeat ad infinitum.

I'm pretty sure I have the carb needle set close enough to right, and the fact that it only happens when it's hot leads us to believe it is some kind of ignition problem, rather than the bowl float. Tried running without a fuel tank cap to rule out a plugged vent/suction fuel issue. Same as before. I think this is a points/condenser/coil system. Anybody have experience with an issue like this?

I think it is a 326437 model



Greyson

[lostmind]

Try adding a little kerosene or Marvel mystery oil to the gas. the new gas changes the float level with higher temps.
Not sure of the chemistry , but my Briggs 2 cylinder does it.

[bonneyman]

Sounds plausible. My B&S 3.5 hp lawn mower had starting issues when hot. Insulating washers between the tank and cylinder head - plus some Supco 88 (basically ultra-purified kerosene) in the fuel tank - seems to have helped.

[gtermini]

I'll have him try that. There was a couple tablespoons of 2 stroke oil in the fuel, and the fuel was fresh after I steam cleaned the tank out. I know he was bogarting the clear gas for chainsaws because it's 50 miles round trip to the closest station that has it.

Greyson

[slip knot]

Surgeings usually a fuel delivery issue. does the carb bowl have a drain? if so open it and see if fuels streams out. Try filling a pint jar. This will tell you your getting enough fuel to the carb.  then look at it, that will tell you if its got water in it. I'm betting on the water.