Briggs & Stratton 5kW Generator - Running Lean

[Urbish]

A friend of mine brought over her 5kW Briggs & Stratton Generator with a B&S engine on it so I could bring it back to life. This thing has me baffled. It had been left with fuel in it for years and had 1/4" of sludge built-up in the fuel bowl. I completely cleaned and rebuilt the carburetor. After reinstalling, it would run nicely but only on full choke. As you slowly move off full choke, it will start missing and shooting blue flames out the exhaust until about half choke where it completely dies. I've confirmed free flow at the gas line going into the carburetor.

I disassembled the carburetor again and double checked the main jet - all clean. The main jet is non-adjustable. After reassembling but before reinstalling, I used my air gun and blew air through the center of the bore with the butterfly held wide open (and fuel in the bowl). A nice mist of fuel/air blew out the other side. The only thing I hadn't removed was the idle mixture screw. It had a plastic limiting cap and when I pried it off the screw broke off. I extracted the portion of the screw stuck in the carburetor. I ordered a Chinese replacement carb and installed it (along with the new replacement .75" thick intake spacer that came with it) and new gaskets between all. The thing behaves exactly the same. I put the original carb back on with the idle mix screw from the Chinese one. No change. Adjustment of the idle mix screw seems to have no effect (even when fully closed) at both idle and full rpm. I sprayed carb cleaner on all areas of the intake while it was running and noticed no change. It's acting like it has a vacuum leak. The muffler appears to be flowing freely and the spark arrester screen is clean. The valves are properly adjusted.

I am at a loss. Any ideas?

[Slim140]

Does it have any little freeze plug looking things on the carb where they drilled some holes and plugged them? If so take some clear finger nail polish and seal them and see if it helps.

[Urbish]

There are bb's pressed in here and there. But I don't understand why it would exhibit the exact same symptoms even after replacing the carburetor with a new one. I sprayed down the outside of the carb as well when checking for vacuum leaks.

[Slim140]

Can you do a compression check on it? May be a sticking valve or a stuck ring if it sat that long.

[Urbish]

Yeah, probably should have done that first. Will check. It's an OHV engine. The valve clearances were good, so guessing not a stuck valve. Doesn't seem to have excessive blowby. It doesn't smoke and there was not a mad rush of air coming out of the PCV.

[Urbish]

Can you do a compression check on it? May be a sticking valve or a stuck ring if it sat that long.

You nailed it. 50psi. Good enough to run on choke but not otherwise.