Briggs 407577 surges high speed or under load,cleaned carb,changed carb. comp 155

you adjusted your governor incorrectly.

ill bet this is an older briggs engine ? as they seem to need a little tlc on there governor speed control spring for it's feedback to the throttle , this makes the engine surge a bit (hunting) till a load is placed on it then it runs smooth , now if it doesn't smooth out under load id suspect a carb fuel flow problem

put more tension on the governor spring.

comp air wont do it.. soak in carb cleaner to make sure all orifices are clear.. use fishing line to clear them out THEN comp air

really sounds like a carb problem if it only starts acting up under load

Has it been sitting for a long time without running?

Sorry, this section is for Measurement Tool Questions only.

Sounds like the low speed idle circuit is clogged or the inlet needle tip is clogged in the carb. About the only way to clean these is disassembly.

Careful with the high octane.. That can burn a hole in the piston..

It might be fludding.. At the bottom of the carb should be a bowl.. One bolt at the bottom.. Take it off and clean it.. Spray some carb cleaner up in there where the float is..

If that dont do it --

Maybe the float is bad & taking in fluid..

If it has sat for long piriods of time - the carb might need to be cleaned & rebuilt..

When gass sits in there for a long time it shalacks and wont work correctly..

Pop off the carb & take to your local small eng shop. Shouldnt cost to much..

Hi, Check that you have the correct spark plug fitted or fit a new one as it sounds like your plug is getting hot then shorting out.
Sometimes the magnito can do this as well. but I would start with the plug.