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My Gravely ProTurn 148 with Kawasaki FX691V runs perfectly for 30 minutes then begins to slightly surge a little. As run time increases surging increases, and it begins to act as though I were slowly increasing the choke handle. Like it wants air. For instance, when I disengage the blades then re-engage them, it strains the engine, almost killing it, sometimes backfiring. Just like choking it while running.

It's got 350 hours or so on it. I use it on only one 3 acre yard. I have new fuel filter, plugs, air filters. I checked for clear air and fuel lines. And I've run SeaFoam fuel additive through it for a couple of tank-fulls.

Do I need new gas caps? Hopefully not a dirty carburetor because I'm not sure I completely know what I'm doing with this one.

Any suggestions?

Surging is a lean running condition caused by too much air, if you can choke it will it's running, it will probably smooth out and run good. i would start by loosening the gas cap when it's surging. if it goes away, it's not venting properly.
It could also be an intake gasket failing when hot and letting air in past the carburetor causing it to run lean.
get some WD40 in a trigger spray bottle, with the engine running and surging, saturate the area around where the carb bolts to the engine and where the intake manifold bolts to the cylinder head. If the engine, runs better or blows white smoke, or has any change in RPM, you've found your vacuum leak causing a lean running condition.

Scrubcadet10 said:

Surging is a lean running condition caused by too much air, if you can choke it will it's running, it will probably smooth out and run good. i would start by loosening the gas cap when it's surging. if it goes away, it's not venting properly.
It could also be an intake gasket failing when hot and letting air in past the carburetor causing it to run lean.
get some WD40 in a trigger spray bottle, with the engine running and surging, saturate the area around where the carb bolts to the engine and where the intake manifold bolts to the cylinder head. If the engine, runs better or blows white smoke, or has any change in RPM, you've found your vacuum leak causing a lean running condition.

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OK great! I'll give it a go and report back later. Thanks for that thoroughly thorough suggestion!!

Scrubcadet10 said:

Surging is a lean running condition caused by too much air, if you can choke it will it's running, it will probably smooth out and run good. i would start by loosening the gas cap when it's surging. if it goes away, it's not venting properly.
It could also be an intake gasket failing when hot and letting air in past the carburetor causing it to run lean.
get some WD40 in a trigger spray bottle, with the engine running and surging, saturate the area around where the carb bolts to the engine and where the intake manifold bolts to the cylinder head. If the engine, runs better or blows white smoke, or has any change in RPM, you've found your vacuum leak causing a lean running condition.

Click to expand...

When surging, when I re-engage the blades, it's backfiring. Isn't that due to running too rich? No?

could be misfiring due to not enough fuel. Which can sound like a backfire.
What causes surging is, (the pro's may need to correct some info in the following)
Engine does not have enough fuel (or too much air)
engine slows due to not enough fuel, the governor senses this as a load bogging down the engine, governor opens throttle (Wider than what you can open with the throttle control) so it gives the engine more fuel, which causes the sudden raise in RPM's, the cycle then repeats.

Scrubcadet10 said:

could be misfiring due to not enough fuel. Which can sound like a backfire.
What causes surging is, (the pro's may need to correct some info in the following)
Engine does not have enough fuel (or too much air)
engine slows due to not enough fuel, the governor senses this as a load bogging down the engine, governor opens throttle (Wider than what you can open with the throttle control) so it gives the engine more fuel, which causes the sudden raise in RPM's, the cycle then repeats.

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I see. Thanks for that explanation!

You can also get floating debris in the petrol tank that gets sucked into the outlet as the mower is running like hairs in your shower drain .
Then when you turn the mower off they float away .
So you start mowing and all is well, then surging starts and if you turn off for a while it goes away till you start mowing again.

bertsmobile1 said:

You can also get floating debris in the petrol tank that gets sucked into the outlet as the mower is running like hairs in your shower drain .
Then when you turn the mower off they float away .
So you start mowing and all is well, then surging starts and if you turn off for a while it goes away till you start mowing again.

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I thought about that. How can I fix it? Remove the tanks and clean them? Easier way?

drain the tank & let it dry right out
Shove a feather duster into the tank & swirl it around to free up stuck on debris
Put an air line on the fuel outlet and have a long blow
Those miracle dust magnet cloths work well too.
To confirm it is the tank hook up an alternative fuel supply , pinch the tank off your push mower , motorcycle , boat etc .
Problem goes away confirms fuel supply blockage.
If yours is a duel tank mower the debris can also get caught in the switch over valve .
Some cheap fuel lines can also go soft & pinch off when hot .

bertsmobile1 said:

drain the tank & let it dry right out
Shove a feather duster into the tank & swirl it around to free up stuck on debris
Put an air line on the fuel outlet and have a long blow
Those miracle dust magnet cloths work well too.
To confirm it is the tank hook up an alternative fuel supply , pinch the tank off your push mower , motorcycle , boat etc .
Problem goes away confirms fuel supply blockage.
If yours is a duel tank mower the debris can also get caught in the switch over valve .
Some cheap fuel lines can also go soft & pinch off when hot .

Click to expand...

Very helpful. Thanks!!

Pull the carb and inspect the main jet.

Have you ever, pulled the engine cover, and cleaned the cooling fins and block?



Down on power quoted below.

Really got to narrow the cause as it can be fuel supply, carburation, or even a failing ignition system.

The twin barrel Nikki due develop loose main jets which can cause over rich conditions. But with bogging engaging the deck it sounds like you are dropping a cylinder.

StarTech said:

Really got to narrow the cause as it can be fuel supply, carburation, or even a failing ignition system.

The twin barrel Nikki due develop loose main jets which can cause over rich conditions. But with bogging engaging the deck it sounds like you are dropping a cylinder.

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So if I might be "dropping a cylinder", what can I do to check for that and/or fix it?

I have all but eliminated a fuel supply problem. Tomorrow I'll be testing to see if a cylinder is dropping. I have an ignition coil assembly ordered and on the way because all indications (not listed here) lead to that. I will update this when I learn something – for the sake of future Googlers.

The problem has been fixed. I replaced the ignition coil assembly that tested bad, and I spliced a severed kill wire that led to both coil packs.

When I lifted the engine cover to get to the coil packs, I saw that the kill wire was broken. Weird, right! No rodent marks or signs of heat-related damage. Since the engine has been hard to turn off ever since I bought the mower (new, ten years ago), I assume the factory guy didn't secure the kill wire properly, allowing it to move around and be chopped by the spinning flywheel the first time I used the mower. Who knows.

The engine kills as it should now.

I hope that the electrical tape I used to splice the kill wire isn't going to melt. I didn't know if I should use a plastic wire nut or electrical tape. Any thoughts?

Thanks for all of your help!!!!

I use butt connectors myself or better crimp a Ferrel on the wires and cover with heat shrink tubing to make a neater job.

Thanks, if I ever go back in there – for instance, if my electrical tape splice fails – then I'll take your advice, for sure.