Did you remove the idle mixture screw assembly and the brass jet behind it? There are three small holes in the casting that get clogged.

I removed the idle mixture screw, inspected it and cleaned it and put it back in. Not sure I understand what brass jet is behind it and how to access it. Also, I know there are 3 small holes in the idle mixing chamber that deliver the air/fuel mixture to the carburetor throat as the throttle opens. This is behind a welch plug so I didn't want to remove the welch plug unless I had to.

I have a basic question. It's my understanding (and I may be wrong) that the fuel/air mixture delivered from the idle mixing well only occurs when the engine is down close to idle speed. My engine at idle is fine. When the engine is at full throttle, the fuel/air mixture comes from the main jet and the air flowing through the main throat going through the venturi. Therefore, if my idle is fine, shouldn't I be able to assume the 3 holes mentioned are not the issue. Am I correct in those assumptions?

Sounds like it doesn't want to run warmed up. Check plug. If really clean there may be air leak past the carburetor. Sometimes the ignition coil fails when hot. Don't forget you could still have a bad diaghragm ...tiny leak will mess it up. How about bad spark plug ... fail when hot?

At WOT and no load it is still running on the idle circuit. The high speed does not kick in until the the engine pulls down under load and the governor spring forces the throttle wide open.
You didn't supply the engine numbers, I assumed you had the older version. They newer carbs don't have a serviceable main jet.

Thanks for the additional comments everyone. I replaced plug and ensured correct gap. I will check the ignition coil if when I put the carburetor back on, it still demonstrates the same issues. I also replaced the diaphragm in the carburetor.

Since I last posted, I took the caruburetor off the engine and thoroughly cleaned it again with compressed air and carb cleaner. When I took the gas tank off, the small fuel well at top of tank was full (the main tank was not full), so that demonstrates to me that the pump/diphragm system is getting fuel adequately to the small well.

This time, I took the welch plug off the idle mixing well and shot compressed air through the tiny orifices that deliver air/gas idle mixture to the throat. I blew compressed air through each hole and felt the air come out the throat so I know they are unclogged. I also put compressed air down the idle air blead that goes from the mixing chamber to the throat. I removed the idle mix needle and put carb cleaner down the orifice that brings fuel form the needle valve to the mixing chamber and from the needle valve down through the short pickup tube. I am convinced now that the entire idle air/fuel deliver system is clean. Now I'm waiting for the mail to get my new welch plug so I can put it back over the idle mixing well and reinstall the carubetor.

I will reassemble and start back up after I get my welch plug. If it demonstrates same behavior, I'll take a few minutes and scream and then proceed with the next suggestions which I believe is air leak past the carburetor and ignition coil fail when hot.

Thanks again for your suggestions.

I should have added, it's a B/S Model 135232 Type 1200E1. I believe 1997. It has a fixed jet at the bottom of the emulsion tube. For this part of the cleaning, all I did was put carb cleaner up through the fixed jet at the bottom of the emulsion tube up through the main nozzle in the carb throat. I was able to see the carb cleaner spewing out of the main nozzle as I can see the main nozzle as I look down through the choke. That makes me believe that the main jet is delivering air/fuel mix through the nozzle ok. I also sprayed compressed air down the main air bleed that delivers air to the fuel as it rises through the emulsion tube when travelling from the jet to the main nozzle. Wasn't sure what else to do to try to clean this part of it.

Update: I cleaned the carburetor again (including orifices in idle mixing well) and also replaced the idle needle and put it all back together. The engine now works well. I still get the muffled popping sound but that is something that is happening in the cylinder and/or muffler/exhaust. That doesn't seem to be impacting the performance of the engine.

Thanks for everyone's help. Paul

I have a question that I believe is related to this post. I have a 5 hp roto tiller with the same carb set up and I have a run time issue similar to what is described above. A bit of history. The tiller sat for a couple of years and when I got to using it I found that the 'bowl' well in the tank had corroded like swiss cheese and there was lots of rust in the tank. So, not wanting to spend a lot of money (I have the time) I cut out the top of the tank with a dremel metal cut off wheel, made a new well out of copper tubing, soldering the seams etc, cleaned the whole mess up pretty well, got the rust out of the tank, installed a new diaphragm in the carb. The engine starts right up and purrs for a minute or two and then once it gets a bit warm, it surges up and down, then it settles down and runs smoothly, then surges again, repeat, repeat. When it surging you can see the throttle being moved, I suppose by the governor attachment? Then after running for 20-25 minutes, with the on-off surging, it died and would not restart until it was cooled off and then repeat the process. Anyone got ideas of what the issue is? Plug is clean, the air filter on of off makes no difference. I think the carb is all cleaned out. I had it apart and cleaned out everything I cold see and spray carb cleaner and compressed air through every where. What's left? An intermittently bad when warm ignition?

Check the spark when cold for reference.

Compare when the problem occurs.

Good idea, never occurred to me. Once our storm of the century goes through tomorrow, I will give that a shot. I assume that the tiller will start with no issue when cold so that is a logical test. Thanks

One thought, if the spark is 'bad' when warm, does that necessarily mean the magneto/ignition is bad? I haven't researched far enough to know what ignition type this engine has. The machine is about 20 years or so old.

It sounds like the mixture jets are set right. Try setting both the idle mixture and the high speed mixture until it stops the cycling. Only turn them a tiny bit after warm up and listen for improved running performance.

Maybe a dumb observation on my part but I do not see any mixture screws on this carb. At least not like in the old days. Only thing that seems to resemble a mixture screw is a brass screw that when I took the carb off and cleaned it all up, was bottomed out tightly in its carb seat. When I was in the process of cleaning things up, I went to turn the screw in counting the turns so I could at least have a starting point but it was already all the way in, so that's how I reassembled it. The only adjustments are the idle speed screw which ought not to have anything to do with run speed? The frustrating part of all this is for the intermittent periods the engine runs well, it purrs nicely. What would happen if I disconnected the linkage to the governor and just attached the throttle cable directly to the carb? This has now become a quest!