gregjo1948
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What causes engine surging and how is it fixed?
engine surging
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bertsmobile1
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Governed engines surge because the ratio of fuel added to the air is not sufficient to maintain the speed constantly.
So as the engine slows, the governor allows the throttle butterfly to open wider.
Carburettors are designed to run rich momentarily as the air flow increases to provide the extra power so when the governor allows the throttle to open the rush of extra air pulls excess fuel in even when the passage ways are partially blocked so the engine accelerates from the extra fuel then when trying to stabalize, there is not enough fuel so the engine slows down causing to governor to allow the throttle plate to open up creating the in rush of air drawing the extra fuel to enter the engine then when that is burned off it slows again, thus the cycle continues at infinitum.
Not enough fuel is caused either by a blockage in the fuel passageways or an air leak after the main jet in the carb which can be a worn throttle pivot hole, cracked , broken or warped manifold , leaking gaskets or loose carb.
Most times it is a blockage as Rivets has mentioned but before you rip into the carb get a trigger bottle of WD 40 or similar, start the engine and soak the manifold & engine side of the carb with the WD 40, avoiding any going directly down the carb throat.
While you do this move the governor arm with your finger back & forth to get maximum manifold vacuum.
If there is a leak then you get clouds of white stinky smoke out the exhaust pipe
Takes about 5 minutes to do and is a lot easier if the blower housing is removed , which has to come off most times to remove the carb for cleaning in any case .
So as the engine slows, the governor allows the throttle butterfly to open wider.
Carburettors are designed to run rich momentarily as the air flow increases to provide the extra power so when the governor allows the throttle to open the rush of extra air pulls excess fuel in even when the passage ways are partially blocked so the engine accelerates from the extra fuel then when trying to stabalize, there is not enough fuel so the engine slows down causing to governor to allow the throttle plate to open up creating the in rush of air drawing the extra fuel to enter the engine then when that is burned off it slows again, thus the cycle continues at infinitum.
Not enough fuel is caused either by a blockage in the fuel passageways or an air leak after the main jet in the carb which can be a worn throttle pivot hole, cracked , broken or warped manifold , leaking gaskets or loose carb.
Most times it is a blockage as Rivets has mentioned but before you rip into the carb get a trigger bottle of WD 40 or similar, start the engine and soak the manifold & engine side of the carb with the WD 40, avoiding any going directly down the carb throat.
While you do this move the governor arm with your finger back & forth to get maximum manifold vacuum.
If there is a leak then you get clouds of white stinky smoke out the exhaust pipe
Takes about 5 minutes to do and is a lot easier if the blower housing is removed , which has to come off most times to remove the carb for cleaning in any case .
gregjo1948
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Thanks guys for your explanations. If the idle is set too low, could that also cause a gallop/surging?
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If you are referring to idle speed then yes it can. As throttle plate (vane, butterfly) can block the idle ports and engine tries to die and governor spring opens up the plate as if the engine was not running thus causing the surging.
Also as mention it usually cause by a dirty carburetor, leaky intake manifold gasket, or improper set static governor adjustment. It is also to note several the Briggs are borderline too lean jet wise to start with. They will surge until fully warmed up and even then they have some surging off load. Plus those v-twin Briggs with governed idle speeds must be tuned correctly per the service manual using a tachometer.
One note on the Nikki carburetors they have main jets that have tiny o-rings that hold the jets in place. These tend shrink and harden which leading to excess fuel making pass the jets which can also cause surging as the jet moves around leaking and not leaking excess fuel.
It not as simple as just adjusting screw always as you must find the root cause and it correct it first.
I even had one aftermarket clone Niiki carburetor last year that had an idle jet that was too small and had to resize it.
Also as mention it usually cause by a dirty carburetor, leaky intake manifold gasket, or improper set static governor adjustment. It is also to note several the Briggs are borderline too lean jet wise to start with. They will surge until fully warmed up and even then they have some surging off load. Plus those v-twin Briggs with governed idle speeds must be tuned correctly per the service manual using a tachometer.
One note on the Nikki carburetors they have main jets that have tiny o-rings that hold the jets in place. These tend shrink and harden which leading to excess fuel making pass the jets which can also cause surging as the jet moves around leaking and not leaking excess fuel.
It not as simple as just adjusting screw always as you must find the root cause and it correct it first.
I even had one aftermarket clone Niiki carburetor last year that had an idle jet that was too small and had to resize it.
It can but we are talking about trying to idle an engine at 1200 that normally idles at 1700 So I doubt that is the issue. On rare occasion you can get a governor surge, but I have only seen it 2-3 times in almost 25 years, and one of those I caused. If the issue is carb released the surges should stop if you engage the blades on the mower. the extra load allows fuel from the high speed jet. An engine even running at full throttle without a load still runs on the idle circuit of the carb, and the main high speed jet is not in play until there is a load applied.
frenchriverco
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As the previous replies have confirmed, this is normally associated with a dirty idle circuit in the carburetor. I wrote an article about this topic here: https://frenchriverco.com/how-to-fix-lawn-mower-surging