I know we have some handy folks on the board, so I’m hoping someone can offer us a suggestion and prevent us from having to hire a professional.
We’ve got an older riding mower - an 18hp Murray. We just changed the fuel filter, the spark plug, and the oil. But the engine still surges, then sputters. That is, it runs fine, then acts like it’s starved for fuel, then runs fine again, over and over - the cycle is maybe 5-6 seconds. We thought the fuel filter would have taken care of that, but it hasn’t. Next guess is carb rebuild.
Any other ideas? Suggestions? Inspirations? Anyone wanna come cut our lawn?
Ya know, I’ve also had problems with governors on that type of engine (big ol’ bsic 4 strokes).
The gov’ may be a metal flap type thing that is mounted near the fly wheel. As the fly wheel produces more air flow, it pushes on the flap, connected to a rod that controls airflow into the carb. It can do some funky stuff, if it’s not adjusted right.
Comsidering the age of the mower, I’m thinking the carb rebuild is the best choice right now. Same with the two push mowers. It’ll be my chance to get my hands all dirty. woohoo
A gajillion years ago in 9th grade, we studied the internal combustion engine. We were graded on our ability to take apart and reassemble a lawnmower engine. I got an A. But that was in 1968 - I may need some refresher training.
Not that this helps, but the guy who lived across the street from us when we were growing up had a mower that surged like crazy, at a frequency of about half a Hertz. When he mowed the lawn, it went “RRRRrrrmmm…RRRRrrrmmm…RRRRrrrmmm…” the whole time. It sounded like a dragster revving up but never getting the green light. This went on for several years before he finally junked it.
“Surging” as it is called is a sign of lean operation. The mechanism described by enipla is the speed governor. Debris and other crud which binds the linkage can cause problems there.
However, it sounds like the carb passages aren’t clear. A small engine carb kit shouldn’t be more than $10 or $15. Clean out the carb interior, set your float drop per the spec sheet and after starting the engine with a basic idle setting, after warmup you should be able to adjust your curb and high idles for smooth operation.
To avoid this in the future, at the end of season, shut off the fuel supply at the tank or sediment bowl and run the engine until it starts to die, then add choke to suck as much fuel as possible out of the carb-that way there’s no residue left to gum up the works.
I’ll second (third? fourth?) cleaning the carburetor. There may also be some water in the gas. I’d drain the fuel tank and carburetor bowl and replace the old stuff with new.
