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Hey guys i’m new here, i run a small engine repair business and in at a loss with this problem. Customer dropped off a rider with a 15HP Briggs and Stratton that wouldn’t run right (they had tried replacing the carb and bent the choke bar so it was sticking). I fixed the choke lever and got it fired up but it was surging BAD so i tried cleaning the carb with no success, swapped out for a known good carb and still same issue, even tried one off an 18.5 and still surging. I adjusted the governor and that didn’t help, adjusted valve lash and nothing, new fuel filter and fuel line as well as ran it with the gas cap off to make sure there wasn’t any fuel restrictions and once again same issue, replaced with a brand new carb twice as well as new carb gasket and still surging. So at this point my last resort was something broken on the mechanical side of the governor so i pulled the engine off and tore the housing apart and everything looked good, i had a known good governor gear assembly laying around so i slapped that in there and it’s still surging. I’m at a loss now and short of having the customer come pick it up or scraping it im not sure what else to try. What else could i possibly be missing?? Thanks in advance!!

Can be as simple as the base idle speed is set too low...

Hopefully you will have a spare tank kicking around the workshop.
Rig it up to gravity feed directly to the carb.
Problem goes away = fuel supply problem.
Bad fuel lines, plugged tank outlet etc.

It surges at idle and when revved up, I’ll try using al alternative tank but i blew through the tank outlet and tried new fuel lines and filter, and when i disconnect the fuel line from the carb it’s got a nice steady flow

If no difference is detected with a known good carburetor, I'd suspect governor issue or intake air leak.

what's the model number?

When the engine is surging observe the the throttle linkage. Is the throttle butterfly opening and closing with the surging?
If you hold the throttle in the idle position does engine run smooth at idle or run rough or die? If you hold the throttle butterfly at about 1/3 open does engine run smooth or rough or die. You need to eliminate the govenor system. holding the throttle in one position and seeing how the engine runs will help eliminate the govenor.

Hammermechanicman said:

When the engine is surging observe the the throttle linkage. Is the throttle butterfly opening and closing with the surging?
If you hold the throttle in the idle position does engine run smooth at idle or run rough or die? If you hold the throttle butterfly at about 1/3 open does engine run smooth or rough or die. You need to eliminate the govenor system. holding the throttle in one position and seeing how the engine runs will help eliminate the govenor.

Click to expand...

Holding the throttle open it runs smoother which points to governor issue, but governor is adjusted properly and mechanically sound inside the engine

If you hold it in the idle position does it run smoothly and when it surges is the throttle butterfly opening and closing with the surging?

The logic behind holding the governor is to eliminate it surging so you can hear what the engine is actually doing.
So you start the engine then slowly increase the throttle opening.
At some point in time the engine will start to faulter.
As you do engine repairs how it starts to faulter should point you in the right direction.
Surging is nearly always a lean running condition.
It could be something as odd as the stud hole for the manifold penetrating the inlet tract.
While the engine is faultering saturate the manifold region with WD 40 from a trigger pack using your other hand
Do little bits at a time so you can find where the air leak is , assuming there is an air leak.

Have you tried the alternative tank yet ?

bertsmobile1 said:

The logic behind holding the governor is to eliminate it surging so you can hear what the engine is actually doing.
So you start the engine then slowly increase the throttle opening.
At some point in time the engine will start to faulter.
As you do engine repairs how it starts to faulter should point you in the right direction.
Surging is nearly always a lean running condition.
It could be something as odd as the stud hole for the manifold penetrating the inlet tract.
While the engine is faultering saturate the manifold region with WD 40 from a trigger pack using your other hand
Do little bits at a time so you can find where the air leak is , assuming there is an air leak.

Have you tried the alternative tank yet ?

Click to expand...

Alternative tank did nothing, i tried WD-40 all around the intake and couldn’t find any leaks. Also tried brake cleaner to see if the engine would race and nothing.

How did you go with manually opening the throttle slowly till it faultered ?
You are really only left with things like a leak around the inlet guide or a cracked head