Briggs 18.5 surges at high rpm

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pastorbud

Member
I have a 18.5 hp Briggs that starts and idles good but cuts out at higher rpm’s. I have cleaned the carb, and replaced the fuel pump and fuel filter. I also took out that cheap plastic cut off valve, thinking it may be the problem, but that didn’t fix it. Two videos below.


 
If you add some choke, will it continue to run at higher throttle positions, and smooth back out?
If yes, it points to a fuel delivery issue!
(Does that engine have a fuel pump or is it gravity fed?)
My bad. . . I see the pump!
Thanks,
Doug Lee
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Sounds to me more like a miss fire of the ignition system. It can be either a bad spark plug or flywheel alignment problem. Some these 31 series had a poor flywheel to crankshaft keyway design that leading shearing of the key. I even one burst the flywheel.

Try changing the spark plug with a known good spark plug and the flywheel to crankshaft alignment.
 
Could be last years gas. Water in the gas.

Looks like the original fuel lines. Fuel hose deteriorates internally so you can't see it. Pull the hose off AT the carb inlet. Crank or run the engine verifying solid fuel flow. Should drain the tank. Look for water.

Most likely dirty carb. Lot of people use old open top spout fuel cans. Allows tons of grit, bugs, leaves, grass and water inside the can. Then at every fill up, can passes all that trash to the carb. Lots get clogged at the tank outlet nipple.

Could be a weak ignition coil. Taryl made a perfect video on it. Coil had spark at low revs. Mid and high revs, coil had no spark. Reason I preach about using open gap type spark testers. Starts at 4:05 in the video, stops at 23:30. Or watch the entire video and learn something.

 
You need to remove the top engine cover and the sheet metal pan that goes around the cylinder block. You have a large oil leak under the cylinder. Entire area is wet with oil and grit. Clean out the cooling fins all around the block and head area.

Still going with a dirty carb.
 
Did you clean and rod out the emulsion tube?

Your second video does NOT show a CLEAN carb at all. Just looking at the exterior. Unless you dumped half gallon of oil on top of the engine and freshly scalped your lawn. So stop passing fake news to us please. Looks like you've never been inside the engine to me.
 
Videos are hard to give a diagnosis opinion on but this sound like a coil breaking down at high RPM or a failed spark plug. If it a Torch plug, throw it away.
 
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