I recently purchased a 2010 vanguard Briggs and Stratton 36 hp motor. I’ve cleaned the carbs, as well as done a compression test. (180psi and 160psi). I’ve installed new spark plugs, as well set my valves to .005. The prop is pretty new.

Working on my boat/motor out of water. No problem cranking. Idles close to 1000 rpm’s. When I give it gas, the RPM’s gradually increase as expected. However, Once it reaches above 2000rpms, and you continue to increase throttle, the RPMs start to jump around from 800 to 1700 and everything in between. The motor starts to shake.

I haven’t down a leak down test. The Carburetor is a 2 barrel - and it’s stock I believe, One spark plugs is really dry while the other is wet. As far as the little black Knob on top of the carburetor, one is broken. If I understand correctly, doesn’t that control the air/fuel mixture? I’m assuming by the way it’s broken, it can’t be fixed.

Any suggestions would be Greatly appreciated!


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honestly, with a broken set screw, I would just buy a stock carb and replace the one you have.
The spark plug that is really dry, may not be getting fuel at all, and you are running on 1 cylinder.
OR, the plug that really wet may not be getting fire and you aren't burning the gas, in which you are only running on 1 cylinder.

if you change out the carb and still have this problem, you aren't getting fire to 1 of your cylinders.

Arc flashing on the spark plug boots...
Fire it up in the dark and look for spark jumping from plug boot to the metal air dam. Sometimes you can't see it if it's grounded out. Make sure the boot isn't touching the metal shroud. If so, you can wrap in good electrical tape but best to replace the boots with NGKs LB05EP