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I was riding my troy bilt 42" lawn mower when it started surging, so I changed the gas filter and blew out the ports on the carb, reinstalled everything and it still did it.

So I changed the carburator with a new one, changed the head gasket because it was leaking and put all new seals on the carb, as well as a new filter.

Also the gas is new and the hoses are good, When I started it... IT still surges, but now it makes a loud "POP" in the engine compartment and dies, or just surges real bad at high and low idle

I'm about one idea from rolling this thing off a cliff if I cant find what it might be... The gas gap seal was checked and it seems alright

  • I am assuming, with all the other work you did, you cleaned/replaced the air intake filter? Your post does not mention it... – Jimmy Fix-it Jul 26 at 2:15
  • Not replaced, but I did take it out and the RPM's rose, then I put it back on and they dropped, but the surge remains... I just realized the idle butterfly was flapping, when I hold it wide open it runs just fine on the new carburetor, not sure how to adjust. – hello moto Jul 26 at 2:20
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If you were riding it when it started surging and the throttle is still work, I would guess that the choke cable either broke or the choke return spring fell off. The choke plate will be on the air-filter side of the throttle, so look and see if it is flapping around. If it is, then check the choke cable and return spring.

What is the make and model of the engine? Do you have any photos?

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  • 13AV60KG Kohler 535 Engine, Troy bilt mower, It looks like all the springs are there, although there are alot of empty holes near the arm mechanism... I tightened the idle screw all the way on the new carb, then backed it off 1 1/2 turns and it still surges 🤔 – hello moto Jul 26 at 8:36
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    Did you adjust the idle stop screw or the idle mixture? A lean mixture may backfire which could be the bang you mentioned. The mixture needs to be adjusted with the engine running and at idle. Unscrew the mixture until the engine runs rough and sputters (rich mixture) and then screw it in 1/2 turn at a time until it starts to lose speed and then back it out 1/2 to 1 turn which should be the best running stage. You can then adjust the idle stop screw (which holds the throttle plate open) for a normal idle. – Eric Jul 26 at 9:38
  • The carburator I bought only has one screw, I'm pretty sure it's the idle screw because it controls how open the butterfly is. – hello moto Jul 26 at 9:56
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    Thanks for the link with the photo. Yes, that is the idle set screw that you adjusted. Looks like it is indeed a fixed adjustment carburetor. In one of the comments you mentioned "the idle butterfly was flapping, when I hold it wide open it runs just fine on the new carburetor". Would that be the choke butterfly valve (the black lever / one with the holes in it)? – Eric Jul 26 at 10:48
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    OK, that is the throttle plate. You adjust the screw until the engine is running at a reasonable idle speed. Your throttle cable will connect into that. What is the "faint surging" you are hearing? Is the engine RPM changing much? – Eric Jul 27 at 9:36

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