Can't stop B&S engine surging

[Nibbo]

I'm trying to mend a Hayter Spirit 41 mower for a local disabled charity I volunteer for. They don't have the spare money to send to a mower repair shop and thought I could sort it .

I've fixed the height adjustment problem but the engine is surging. It's a Briggs and Stratton 10T602 0117 H1.
I assumed it would be the carb so stripped it down and cleaned it out including the petrol tank, replaced the fuel, new foam air filter, new gaskets. Still the same. Still convinced it is the carb I bought a new one. Still the same. New plug - still the same. Checked out the inlet manifold plastic pipe - no cracks. I'm a bit stuck for ideas now and would appreciate any input you guys could provide? Could it be a problem with an inlet valve?

 

[Scrubcadet10]

Get some wd40 or or parts cleaner in a trigger spray (not very smart to use it out of a aresol can with an engine running) and spray down the area where the carb meets the intake manifold, the intake manifold, and where the intake manifold bolts to the engine, if there is a change in engine speed or it blows some smoke, you found your leak., i know you said you inspected them but all it takes is a small crack, enough for air to get in, and that will cause the engine to run lean and surge, sometimes surging can also be an issue with the governor, not always a fuel issue.

 

[Nibbo]

Get some wd40 or or parts cleaner in a trigger spray (not very smart to use it out of a aresol can with an engine running) and spray down the area where the carb meets the intake manifold, the intake manifold, and where the intake manifold bolts to the engine, if there is a change in engine speed or it blows some smoke, you found your leak., i know you said you inspected them but all it takes is a small crack, enough for air to get in, and that will cause the engine to run lean and surge, sometimes surging can also be an issue with the governor, not always a fuel issue.

Thanks for suggestions Subcadet10. I did try squirting some some Easystart around joints but will try again .... just have to take all the casing off to get to the manifold. Unfortunately on this mower the carb to manifold joint is a push fit with a rubber O ring ..not ideal.

Cant see the governor mechanism as its inside the crank case so not sure what issues there can be with it. I'll see if I can find a diagram somewhere.

If I restrict the flow into the carb air intake it runs better but not sure what that means.

 

[bertsmobile1]

It means that the engine is not getting enough fuel to maintain a constant speed .
Try it again with the old carb/
Also try fresh fuel
Oddly enough that O ring does not leak all that much but it won't hurt to replace it .

 

[cpurvis]

Leaks between the throttle shaft and carb body can cause surging, too.

 

[StarTech]

I have seen the plastic crossover intake pipe to have holes rubbed into them on the underneath side. Kinda hard to check for as you the recoil to start the engine.

 

[Nibbo]

Thanks all for your input on this. I've finally managed to work on this again. Cleaned out and reassembled original carb. Removed plastic crossover pipe, inspected but all intact so re-seated. Reassembled and replaced fuel .. again. Restarted and bizarrely running OK. Not sure which change fixed it but am a happy bunny now. I think it might have been the crossover pipe as I had spent so much time fiddling with carb without joy.
Thanks again too all who had input.