Cub Cadet/21" Cut Walk-Behind-Self Propelled Mower
Mdl 12A997A712

For some reason, I've never been fond of these mowers but this particular mower was given to me by a gentleman that let it sit for quite some time.
He had recently put it in the shop for service/repair if needed. The fuel system was flushed/carb overhauled/new plug/filter/primer etc.

When I received it, the mower ran and performed beautifully. I didn't use it often as I have numerous other equipment but I did run it fairly often/fresh gas and all that jazz to keep it in optimal condition.

The Issue:
Starts right up/1st or 2nd pull, runs steady for about 5 seconds and then the engine begins surging or loping (if that's the correct terminology).
Very noticeable rpm shift between the low and hi points of the surging.

If I manually increase the throttle to a higher rpm, it will run steady.
Obviously, the manual indicates that any Carb/Governor settings are factory adjusted and I'm not familiar with adjustments of these parts anyway.

Any recommendation as to what the issue cause may be and how I may remedy it myself?

Gulfman,

Your description is a classic lean mixture complaint. I think it is likely that, for whatever reason, some fuel varnish has formed in your carburetor.

This causes the engine's idle circuit to become restricted, and whenever the engine achieves target RPM's, the governor reduces the throttle into the idle range, the engine starves of fuel, the RPM's fall, the throttle opens back to main mixture, power is restored, rpm's rise, cycle repeats.

One way or another, the carb's jets need cleaning. The simplest way is to mix some Seafoam with the next tank of FRESH gas, at about 4oz per gallon (2x label's recommendation). Then run the mower for 5 or 10 minutes, and shut it down to sit for an hour. Restart, and things should be improved.

Failing that, it's carb dissassembly time. Special attention should be paid to any tiny drillings in the main jet, the brass bolt that retains the float bowl on the carb.

J

Thanks so much for the reply.

I'll definitely try the Seafoam in the near future, most likely within a week and then reply with any results.

I've had so many weird things happen to small engines lately that I don't even trust my own "fixes" anymore. I'm definitely an amateur with these contraptions.

Had one 17hp B/S engine that wouldn't do anything but idle, even when manually throttling up.
Fuel/carb/elec systems thoroughly checked/cleaned with no improvement. Just couldn't figure it out for the life of me.
Shoot, the darn push rod on the intake valve was bent and prevented the valve from opening correctly.
Lo & behold, the intake rod on this engine was Al, the exhaust steel - oh well, that's how the cookie crumbles.

Today was the SeaFoam attempt day.

Let it run for approx 15 mins, then sit for an hour or so, cranked back up on the 2nd pull and ran OK for approx 30-40 secs or so. I was hoping but just not to be.

Better than 5-10 sec previously prior to surging but obviously not a vast improvement.

Letting it sit overnight and attempting again tomorrow prior to going into the carb.

After sitting overnight and trying again, the mower actually surged/ran worse than it's ever run.

Going to drain it, then tackle the carb.

Frustration since you'd at least think (me anyway) some improvement would have been obtained, especially since this mowers fuel sys/carb was specifically cleaned, ran beautifully and then all of a sudden this started happening.
Hopefully, it's simply the carb.

Commenced cleaning of the carb. I was totally surprised that the main jet had ample evidence that a cleaning was in order and could definitely be the source of the surging issue.

After MY cleaning, the mower ran great w/no surging.

I surmise that the "professional" cleaning it supposedly had, may not have been as successful as it should have been.

Thanks.