Next update... I replaced all the gaskets from the air filter all the way into the block. Also cleaned the carb a third time while I had it all apart. Replaced the throttle return and governor springs too. I put on a new plug earlier in troubleshooting, and the only thing that I haven't done is replaced the air filter, but I've run the mower with the filter off completely and it still surges!!! I did find that if I manually play with the choke, I can get it to stop surging, so it sounds like it's just running lean at everything but WOT. I'm about out of options and I'm ready to just live with it, since it still works at WOT and that's where it is when I mow the lawn! The self propel thing is fine as long as I remember to push the lever all the way forward and don't just rely on the spring to completely bring it back! Still haven't found the time to take that lever apart and see what's wrong.

Say wot?? The big difference in operating an engine at WOT or less than WOT is determined by whether the engine really is running at WOT.....or if it is simply "running at governed RPM". If an engine is designed and built with a governor to keep the engine RPM below an arbitrary MAXIMUM RPM, it is perfectly OK to operate the engine at that RPM....continuously. In fact, a lot of engines that feature a governor, are operated at governed RPM throughout most of their life. You only have to consider all the big trucks on the highways to see the largest population of governed engines. On the other hand, trying to run an "ungoverned" engine at WOT will result in total destruction of the engine in a matter of seconds. The old arguments (countless) about low RPM being "easier" on the engine and "producing less wear"......are groudless. When an engine is run at low RPM, the lubrication of the engine is less effective, whether pump or splash (but we're splitting hairs here). As to cooling, the higher RPM circulates the cooling medium with greater force and velocity (and volume) at higher RPM, so cooling is more effective at higher RPM. Water pumps, and cooling fans, are both "centrifugal pumps" or movers. At very low RPM, either of these components could theroretically rotate WITHOUT moving any of the medium they are "immersed in" (whether air of liquid). The reason is because the medium exerts a certain resistance to the impeller to set the mass of the medium into motion. If you move the impeller (or "fan blade") slowly enough, the medium simply moves off the outer edge of the impeller and flows around the end of the impeller to the backside of the impeller without influencing any more of the medium mass than is situated inside the water pump, or fan housing. In order for a centrifugal pump or mover to actually affect "flow" into and out the housing, the impeller must be spun at a fairly high RPM. So, if you think you are "saving the engine" by shifting to a higher gear so the engine can turn slower, you could be doing more harm than good. Shifting to a higher gear actually increases the pressure applied to the crankshaft rod and main journals as well as the piston wrist pin. By shifting to a higher gear, you are denying the engine the benefit of "power multiplication" offered by the lower gears in the transmission. Because the throttle must be opened wider for the engine to operated in a taller gear, the combustion pressures in the cylinder increase dramtically in contrast to using a lower gear with less throttle. Sounds contradictory? Almost. The best RPM to operate an engine is the RPM that allows the engine to accomplish its job without bogging down or lugging down, If an engine is being operated at the proper RPM for the load, that engine will be able to accept an increase in load, without having to downshift to a lower gear. If the engine RPM can not be maintained without down shifting, you are putting too much pressure on the crankshaft AND, probably don't have the best lubrication as well. This is a very general description of why it is best to err on the side of caution, and in this case, "caution" lies in the higher RPM range (and lower gears).

The engine is the CV15S. I ran a compression test - 85 PSI. I think that's OK? I then pulled the carb and the throttle plate and screws were gone. I have to assume they were sucked into the engine, where else would they go? Does that mean the engine is trash? It did start, just WOT which I now beleive is because of the carb. Lot's of smoke and not sure what that was from. I think there may be gas in the oil. Any thoughts? Can you buy a replacement throttle plate and screws? Where? Thank you - Jim

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.