Kohler CV20-65991 loss of power hissing from exhaust and blowing air/fuel out carb.

The Engine runs good at an idle. It begins hissing through the exhaust and blowing air/fuel mixture out the carburetor throat with mid speed thru full throttle. It will almost cut grass with the air cleaner housing off. It will not cut grass with air filter housing on. It loads up with fuel and smokes black from the exhaust with air filter on.

First I checked and clean the carburetor. I have two carburetors I have tried them both. No change. Change the spark plugs and Check the ignition coil‘s. No change.
Checked keyway under flywheel looks good. No change.

Removed the heads took them to a machinist. Had them cleaned and the valves lapped. He reported that everything looked good. New head gaskets. No change! Cold Compression 165psi both cylinders!

Removed The hydraulic lifters. Three out of four were not spinning during operation. Replaced the three bad lifters. Cleaned and rebuilt the good one. No luck same sshhtuff!??

I’m at the end of my skill level! I need serious help! Love to learn this stuff! Any suggestions would be great!
Is there a way to check the cam and timing without taking the engine apart?
Should I be able to depress the lifters by hand after running them?
What are the specs on valve movement or rocker arm movement?

try running for a few minutes without the muffler on.
sounds like a blocked muffler or a badly worn exhaust cam.
Don't run for too long without any exhaust pipes as you can burn the exhaust valves or the hot exhaust can melt plastic parts, blister paint or set fire to the mower

OK I finally had time to take the muffler off and run it. Seems like that’s the problem! Makes perfect sense now!
The muffler is the original muffler. It’s probably 12 or 13 years old. Going to replace it with the new one.
Many Many thanks to you bertsmobile1! Many thanks to the people behind this website as well.

Sit down when you get a quote for the muffler.
In most cases it is part of the mower and not part of the engine.
Go to a engine vendor and look at the photos / drawings of the engine .
If the muffler shown is the same as yours, don't bother buying lottery tickets you have just used all of your remaining luck for 2018.
Mice like to nest in there, then they get baked & block the outlet.

Where are you at in Louisiana ????? If you are close to me I can get you one for a decent half price......

Let me know Mon Ami ~!~!

Yep, I fell over when I saw the price $351.00. Searched repair/cleaning options for mufflers on YouTube. Saw some footage on putting in a fire and getting the muffler red hot. Mostly smaller mufflers nothing this big though. Decided to try heating it up on my crawfish pot burner. Basically it’s a propane torch on steroids capable of boiling a huge pot of water approximately 30 gallons. Can cook 50 pounds of crawfish, sausage and vegetables all at once. I had nothing to loss and grass growth slowing down as winter approaches.
Put it on there for about 30 min. Moving it around every few minutes to get different sections glowing red. It was to big to get the whole thing glowing red all at once. Also blew compressed air through it while cherry hot.
After it cooled down I put the muffler back on and to my surprise it’s running great! When I first cranked it up I got a lot of red dust or smoke. Probably rust. I’ve run it for about 3 hours now cutting high grass. It’s running great! Plenty of power! Bought myself a bottle of whiskey since I save $350. Thanks again everyone. Go Saints!