Running rich F engine on L21 mower

Standard Lawnboy L21 zpna mower with the F engine. Got it used, didn't look abused or tinkered with. One adjustment on the carb near the engine side, atmos. adj screw? Have the screw 1/2 turn out, adjusting it doesn't seem to do much. Engine makes loads of power, no excess smoke, starts and runs ok except for the 4 stroking/cackling from running rich. Have to really load it down to get it to smooth out. New float needle and seat, float level seems fine, carb is nice and clean, rodded out all the jets. Turn off the gas and run it out and it runs rich right to the end. Clean new air cleaner. Is this common? Any fix for this? Sure like to have it purr like it's supposed to. Walbro LMR carb, assume it's the original.

I might be able to help answer your questions. I have used and repaired a number of F powered Lawn Boys over the years. Based on my experience, I think the running characteristics you describe are quite normal for the F engine. Every one I have ever worked on or owned, seems to behave just like you have described. I am not enough of an engine expert to fully explain why this is. Maybe it’s a combination of the way the carb, ports, and overall design work together. I do know that my 1965 D engine Lawn Boy just purrs along without much burbling and my F powered R7070 runs less evenly. I just finished freshening up a 1981 4502 steel deck model—same thing, although I haven’t cut grass with it yet. A few years ago, I worked on a later one from the 1990’s and it was a bit smoother. This may have been due to the fact that it was a belt drive self propelled model, thus putting more load on the engine. Either way, I still like the F engine. It’s seems to be very durable, dependable and long lasting. In my opinion, it’s the best 2 cycle engine that Lawn Boy produced. I hope this helps a little.