Briggs Intek, replaced everything, still surging.. Ideas?

[Tiger Small Engine]

I was recently gifted a Craftsman YS4500 with a 20HP Briggs single cylinder. It had been sitting in a neighbor’s garage for at least 5 years, probably longer, because “it was blowing smoke out of the muffler.” It turned out to be a blown head gasket. Looking at the consumer comments of those who bought head gaskets for Intek engines, I learned that these big Briggs Inteks go through head gaskets like the Gabor sisters went through husbands. In my research I also read that a bad head gasket could cause surging. I notice that you have replaced everything else. So, maybe do a leak down test and eliminate the head gasket as the cause. Or, just replace it. Intek head gaskets are really cheap and easy to replace and if your engine is typical, you’ll need to replace it in a year or two anyway. Good luck.

Briggs 17.5 to 21 HP single cylinder engines are more prone to blow head gaskets, then comes the twin cylinder Briggs. However, it is not a common occurrence as stated above. If you replaced OEM carburetor with aftermarket carburetor and engine is hunting and surging, obviously it is running lean. You have an air leak, a carburetor that needs special attention, or learn how to live with it.

 

[loopback1957]

What RPM is this happening at? Are you running her at low revs? If you are under 3400 revs, not recommended due to lack of air cooling and lubrication.

Yes, did that when I replaced ignition coil, also checked the flywheel key to ensure it was not torqued or broken

 

[loopback1957]

What RPM is this happening at? Are you running her at low revs? If you are under 3400 revs, not recommended due to lack of air cooling and lubrication.

Yes, did that when I replaced ignition coil, also checked the flywheel key to ensure it was not torqued or broken

All those OEM Briggs parts? If not, every one will require testing. Could have a Chinese carb. Counterfeit plug. Over torqued gasket sealing area. Set valves wrong, IN and should of been EX.

Tank needs to be inspected for full flow into a glass jar or fuel can. Any elbows will catch trash and clog the flow from the tank.

No load it hunts. Under load with blades on, it smooths out. Would of thought the other way around.

Where is it backfiring from exactly? Carb or muffler? Could be a slipped flywheel key.

What RPM is this happening at? Are you running her at low revs? If you are under 3400 revs, not recommended due to lack of air cooling and lubrication.

 

[Auto Doc's]

Check the fuel line back at the tank (should be on left side top of tank and under the driver platform) where it comes out and bends down and goes the front. Old fuel lines on this series would collapse internally. Do not use cheap Amazon or Ebay purchased fuel line. Go to the parts store and find some Gates brand hose if you can.

Bulk fuel line off the internet is genuine garbage that will break down with Ethanol fuels. I had to throw a bunch of that stuff away.