Outdoor Power Equipment and Small Engines - engine running rough

My Scott's self-propelled mower is running very rough (by which I mean the rpms seem to surge every few seconds, and then settle back to a normal rate). The rpm surges every few seconds.

It seems to run smoother (a little bit) when the drive wheels are engaged, and also improves the faster I walk behind it. I was nearly jogging at one point, but it was not surging nearly as much as when I went at a normal pace or when the drive wheels were disengaged.

I was suspicious of the spark plug, but am also wondering if this might be carb related. I just had the carb cleaned / calibrated (not related to the current problem, but I wonder if it might now be a factor).

Any insights, and actual instructions on how to service it will be much appreciated.

You said you just got the carb cleaned. Is it adjusted properly? It sounds like the surging is caused by impropper high speed adjustment. I would adjust the highspeed screw untill it stops surging. Let us know how it goes. :)

Hello Dandylion!

It does sound like a carb problem. The engine is running too lean (not getting enough fuel). If you'll tell us the engine brand and HP, maybe even a engine model#, we can help you adjust/repair it. If it hasn't been long since you had it calibrated, you might get it re-calibrated for free at the same shop (since it's not right).

Furthermore, per cheese's suggestion and since I don't know if you had your unit as a whole in for service or just the carburetor alone, I suggest taking the machine back to the service center for their inspection. They will want to run the machine to diagnose. If it is a related problem to what was previously done, they should get right at it, thereby giving you little down-time. If it is an unrelated problem and they're anything like my shop, you will likely have to leave it for how ever long they are backlogged and have an added expense as well.
Answer these few questions and we'll be able to help you better. 1- What are your engine ID #'s? (They will be located on the shroud of the engine (not the frame)). 2- Was the carburetor serviced seperate from the machine? In other words, did you remove the carb. and take it to a service center? 3- If so, was this a reputable shop?

Here's some additional info, as per your posts:

Engine: Briggs & Stratton, 121682-0122-E1 / 98031157

Carburetor was serviced alone (I removed and brought to a local shop).

As for reputation of the shop, I have no prior experience with them (having moved to this area not too long ago), but I know they are registered authorized service dealers. They're listed on the Briggs & Stratton, Poulan, Toro (and other websites). I didn't find any info at the Better Business Bureau, so at least there are no unresolved complaints.

If the shop did the job right, they shouldn't have to do anything but turn a screw a little to set it. You can do this yourself if you want. The air-fuel adjustment screw is on the side of the carb, accessible from the front of the engine. Just back it out 1/4 turn or so.

Thanks to all for your assistance!

Terminator referenced the "highspeed screw ", and Cheese said turn back the "air-fuel adjustment screw". I don't see these, and don't see refernce to them in my manual (I'm looking at a general "fix-it-yourself" manual, don't have original owner's manual). If you can give more specific guidance, I'd appreciate it.

In the meantime, I stated the mower up and noticed that the surging of the engine corresponds exactly to the movement of the spring (between the governor linkage and carburetor). If I put my thumb on spring to keep the governor linkage from surging back and forth, the engine runs pretty smooth.

1) Does this change our opinion about adjusting the "high speed screw" or "air fuel adjustment screw"?
2) If not, wher can I get additional help (like pictures)?
3) If yes, what do you think now?

Thanks again for your help!!!

The carb you have may not be adjustable if you see no adjustment screws. I figured yours might be old enough to have them, but I don't have my parts diagram to look at right now, so I can't tell if you have an adjustable carb or not. I'll check and let you know what I find. I know that your engine does not have a high-speed adjustment, but some of them did have an air-fuel adjustment. The newest ones do not have either one. You may just need to remove the bowl screw and clean it out, and clean out the bowl.

Your engine is an overhead valve engine with a non-adjustable carburetor. It certainly sounds carburetor related in nature. I'll assume the carburetor was not serviced well enough. I would try replacing the main jet/bowl nut at the bottom of the carb bowl. If you have a primer-type carb, use part #692677, if a choke-type carb, use part #498978. Either one is less than three bucks. Try it and see what happens. These, by the way, are Briggs & Stratton part numbers.

You beat me to it with the carb info Puey! Thanks! :D

Try different gas, fresh gas. My neighbor had the same symptoms and it
turned out to be the gas.

Just a thought,

Otoe