B&S 21 HP Running Rough

Have a Craftsman mower with a B&S 21hp single cylinder with numbers 331877-2371-G5-110408ZD. Lately it has started surging with the governor seeming to control it. It will not idle except with the choke half closed and even at full throttle needs some choke. All its passages seem clear, but I haven't removed the Welch plug yet. Is there something behind there I need to worry about? When running at full speed it is laboring and I can hear some misfires, all this while I'm holding the governor to prevent the surging. I rebuilt the Nikki carb last summer and have disassembled it several times this spring looking for the problem, but I'm admitting defeat and asking for help. Changed the rubber gasket from carb elbow to block, but no evidence of any leak or damage. Have also flushed out the fuel tank, running it on non-ethanol gas, and changed the fuel filter. I have adjusted the valve lash. I'm going in the morning to get a spark plug since the old one is nearing a year old. A compression test looked a little strange, seemed to not climb every compression stroke, then climbing another 15 to 20# on the next stroke. It averaged around 160 to 170. I tried a leak-down test with my cheap Harbor Freight tester that is only designed for 15# pressure, but I'm not sure I trust my results so I'm skipping that. I'm frustrated and out of ideas and hunting some more ideas. Oh, haven't adjusted the governor yet as I didn't want to create another problem if I can't get it to run with no choke.

Thanks,
Dave

Well, I'm totally stumped. Today I drilled out the Welch plug, blew all the passages out with carb cleaner then air, and reassembled it. I also put in a new plug.

It will run fairly well at full speed if I hold the governor steady and close the choke almost or all the way. Haven't replaced the fuel line, but if I remove it from the carb inlet and open the shutoff, it runs a steady stream.

Only other idea I have is could the bowl o-ring be damaged? I don't see any damage. I wasn't able to find that part last summer at my local shop and ordered a complete kit.

Thanks,
Dave

I would remove the valve cover and check the action of the valves, is everything in place, both moving about the same. I can send you a Service Manual for your engine IF you like. Address below, put in proper format and remind me engine model number and what you want.

Walt Conner
wconner5 at frontier dot com

Walt,

Thanks for the ideas and the manual. I didn't notice anything with the valves when I cranked the engine except oil on the toes of my shoes. I changed the o-rings back to my older set and adjusted the governor. Now only have two issues.

1. The engine is hunting at all throttle levels.

2. Running at about half-choke, the engine runs smoothly at full throttle. No hunting. It will hunt at lower throttle, even with half-choke.

Giving up for today, and tomorrow has other commitments. I'll read the manual when I have time.

Thanks,
Dave

Sounds like there is some blockage of the main jet.

Well, it's running great now! When I bought a carb kit from eBay last year, it had some problems. What I noticed was (1) the fuel inlet o-ring was very tight making it hard to shove the assembly into the hole and (2) the float level was low. So the only part I'm now using from the new kit is the float valve. I did have to use the trick someone else used of putting a small amount of electrical tape around the main jet to hold it into place. One new issue is I believe I've somehow lost the kill wire function because it runs for another second after turning off the key, apparently until the fuel solenoid closes. I'll solve that another day as I have mowing to do today.

Thanks for everyone's help and next time I'll pay more for a genuine B&S kit if I can find it.

Dave