I'm losing my marbles on this one. I can concoct and build all kinds of crazy stuff but I can't keep a stinkin lawn mower running.

I've got a 5 or 6 year old Husqvarna push mower with a B&S engine (metal carb, not plastic). It has always been a great runner that fires on the first pull 90% of the time. Before this issue I can only remember it taking a third pull on a couple occasions. I've got a somewhat small yard and the mower doesn't get used all that often.

I screwed up and fed it some bad gas. The mower would start and die. i drained the tank and cleaned it out. The screen in there was iffy at best so I punched it out and installed a see through in-line filter. The carb had a little bit of scunge in the bowl so I cleaned it thoroughly, including an inspection of the needle and seat as well as holding a properly sized drill bit with my fingers to clean the jet. I then changed the oil, air filter, and plug. It got a tank of fresh gas, too.

When it all went back together it fired up and ran great for about two minutes. Then, it struggled to stay alive. At full throttle it would slow down to a weak idle before picking back up and revving to about 3/4 speed and repeating the process.

It acts like it's running out of fuel but I can see that it's getting plenty. The heat choke seems to be opening properly.

Spark seems to be a non-issue since there is no missing or smoke from incomplete burn.

What am I overlooking?

You are not alone! I bought a new cheapie Troy Bilt walk behind for occasional touch-up mowing around close quarters stuff - I have a Deere X-300 that I bought back in 2013 for the main mowing. The Troy Bilt won't start in the spring no matter what I do - and I run ethanol-free fuel. Last year I took it to a small engine guy & left it with him. He got it running no problem - he said there was junk in the carb. This spring - no start. It has a Briggs one-pull start engine. It's a 20-pull no-start. Here it is 2020 & some things are worse-engineered than things from 60 years ago.

Buy some Seafoam and some premium fuel.

What do you expect Seafoam to do in this application? The system appears to be clean.

Float level or still trash in the jet?

i have a old craftsman 9hp snow blower with a similar problem, has 9hp kohler motor,.
when i got it there was a big surging problem with it. took off the bowl and cleaned everything up. ran better but still had a surge problem,. bought a new carb for $12. didn't surge but barely ran, acts like it not getting fuel. dropped it off by my brother in law who works on small engines on the side it to look at it. been there for 4 months he hasn't had time to look at it yet.

I'd temporarily bypass the inline filter you added to see if maybe it's too restrictive. I've seen this issue once before. If that doesn't work I'd opt for a new $12 Chinese ebay carburetor. They work amazingly well and seem to last longer that the originals.

What I usually find is the jet is clogged with a white looking crap,
I believe it's corrosion caused from ethanol in the gas.

Sometimes the vent in the gas cap plugs up and it starves for fuel. Take the cap off and see if it runs longer than two minutes.

Check your idle speed and the governor linkage. I have a similar husk that did the same thing.

I found that somehow the linkage got caught, possibly by a branch and tweaked. It was binding and wouldn't regulate smoothly.

What happens if you manually cover the intake to restrict the air flow into the carb?

Vacuum/intake leaks can have goofy symptoms.

Did you replace the fuel line? I had that issue this past spring line was plugged.

I've taken to not filling the tank full, just estimating how much gas I need then run it till it's dry, even if I have to tie the safety handle down to do that.

I just tuned up my brother in laws generator. Ordered a new carb from eBay for $40 and put a new plug in it. It would do the same thing as yours until I removed the inline filter. It didn't come with a filter originally but the carb came with one. With that gone it runs great.

Hmmmmm appears. This crap gas that we now get with alcohol causes corrosion and plugs orfaces. Regular use of Seafoam prevents this. Like others have said also check the fuel cap. My tractor was cutting off while cutting the grass. I replaced the fuel filter and it still did it. Left the cap off and I ran great sitting there. Put the cap on and backed it off a turn or two and it ran well cutting last week. See what happens this week.

I'm NOT a small engine guy but I'm turning into one just to keep the fleet running. The most common problems I'm finding are carburetor problems caused by debris from tank corrosion or just the affects of today's gasoline. I religiously run a mix of Seafoam, red HEET, and premium gas and even after running a carb dry and draining the tank, I still have carburetor problems. BTW, just because it "looks" fine doesn't mean it is.

After checking all the easy stuff already posted, replace the carb.

Been there, fought the same battles, won with the new carb.

I’m not crazy old (64) and own two acres in the country since 1985 and have never had to replace the carb on any small engine. That includes mowers, tractors, motorhome generators (3), weedwackers, chain saws, power washers, boat engine, go carts, quads, motor cycles, three race track generators, snow blower, rototiller, etc. take care of the fuel and it will take care of you. Today’s fuel is JUNK. It loves eating rubber and plastic fuel lines too.

Time for EFI and a turbo.

Kevin

FYI my last push mower, bought several years ago from Sears has a B&S 'never change oil' engine and the cap says it'll take E10-E15, but not E85. So far no issues with the fuel system.

i NEVER have fuel problems with a lawnmower !
the neighbor kid does a GREAT job on my lawn !