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I have a Hustler 927806 with a Kohler CV740 engine.
I bought this mower used and have been restoring it and now that I have it running, there is a problem with it running rough under load.
It runs fine with full throttle and no load, but if I put some load on it such as blades engaged and going uphill, it starts running really rough.
My first thought was that it was starving for fuel. I had already replaced all the fuel lines, tank strainers, selector valve, and filter during renovation, so I then replaced the fuel pump.
The problem persists, so I had a new carburetor on the shelf and installed it.
The problem persists, so I went to the ignition coils. Someone had already replaced the DSAM system with the 25-707-03-S MDI kit, but I replaced the coils AGAIN.
The problem still persists, so I am now leaning toward a valve issue. Perhaps a valve is sticking, not seating, or a hydraulic lifter is bad.
That is the only thing I can think of that could be the issue, now that I have replaced about everything else.

Any thoughts???

If it runs good with no load it's doubtful you have a fuel or carburator problem, You need to look at the linkage to the governor first , maybe a spring is missing or the linkage is off , when a load is put on the machine the governor should react and maintain the rpms. Make sure you have good spark plugs etc.

I would be checking to see if you are down one cylinder.

Bertrrr said:

If it runs good with no load it's doubtful you have a fuel or carburetor problem, You need to look at the linkage to the governor first , maybe a spring is missing or the linkage is off , when a load is put on the machine the governor should react and maintain the rpms. Make sure you have good spark plugs etc.

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Governor linkage looks good, including spring. I really don't think it is the governor because it isn't simply bogging down, but starts running rough as if one cylinder is dropping out. If I stop moving, within a minute, the engine will smooth out again.
I just pulled the heads off today and #1 has a lot of black carbon buildup. Later today, I am going to remove the valves to verify that the seats look good or if they need lapping. While it is apart, I will replace the hydraulic lifters, head gaskets, intake gaskets, and exhaust gaskets. I read in the forum where someone said the lifters can cause this sort of thing and someone replace them and the problem cleared up.

rwhelipilot said:

Governor linkage looks good, including spring. I really don't think it is the governor because it isn't simply bogging down, but starts running rough as if one cylinder is dropping out. If I stop moving, within a minute, the engine will smooth out again.
I just pulled the heads off today and #1 has a lot of black carbon buildup. Later today, I am going to remove the valves to verify that the seats look good or if they need lapping. While it is apart, I will replace the hydraulic lifters, head gaskets, intake gaskets, and exhaust gaskets. I read in the forum where someone said the lifters can cause this sort of thing and someone replace them and the problem cleared up.

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Coils can also be a cause.

Carb float level could be set low. Restriction in fuel delivery to carb.

MParr said:

Coils can also be a cause.

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Already replace those. Same issue.

slomo said:

Carb float level could be set low. Restriction in fuel delivery to carb.

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I don't know the history of the mower as I just bought it and have been restoring it. Cooling fins are not too dirty, but while I have the heads off, will clean it thoroughly.

Sounds like you're pretty handy with this type of work, Let us know what it ends up being

Bertrrr said:

Sounds like you're pretty handy with this type of work, Let us know what it ends up being

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Yes, the next step is to order some parts. I hope this solves the issue. I'll keep you posted.
One thing I thought of after the fact...I should have did a compression test before pulling the heads off. If it ends up needing rings, I will be pulling the heads back off.

We need the full model and serial number to reference the correct parts catalogues.

This is what typically happens when a coil stops working under stress (load). The engine does not rock or shake, a coil just quits.

A quick test:

Run at high RPM, use a spray bottle and hit each exhaust head pipe with a quick stream of water. The cylinder that is working will burn off the water instantly. The one not working correctly will have a delay burning off the water.

Swap plugs to be sure it is not a plug that is failing and retest with the water. See if the issue switches sides.

Perform a compression test with the throttle in the wide-open position and compare the readings from side to side, they should be pretty close.

This may lead to having to pull the fuel tanks off and flush them out and do a thorough inspection. I've pulled some strange stuff out of fuel tanks like candy wrapper and small plastic bags. Old grass clippings and dirt are pretty common. Just looking down the fill opening is not an "inspection".

slomo said:

Carb float level could be set low. Restriction in fuel delivery to carb.

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After sitting for a year, I decided to work on this some more. I have really dumped a lot of money and my own labor into this mower and after trying everything else, I remembered what my local repair shop told me last year. They said that these cheap Chinese carburetors that I had installed are hit & miss. Some will work and some won't. I decided to rebuild the original OEM carburetor. I reinstalled it today and the mower now has plenty power and no spitting & sputtering under load. That Chinese carburetor was supposed to work on 25 & 27 HP. It may have been OK on 25 HP, but evidently it was starving the 27 HP when under load.

rwhelipilot said:

They said that these cheap Chinese carburetors that I had installed are hit & miss. Some will work and some won't.

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What I've found is don't buy the cheapest listed priced carb you can find off fleabay or scamazon. Say the lowest listed price is $19.95. Get one that cost $29.95 for example. Since I've started doing this, my good carbs have been way higher in luck for sure.

They are a crap shoot. I typically open then up and make sure they look legit. Some I've had to mod a bit to make them work. I pressure test the needle/seat at 7psi for 30mins. Make sure the float is level or slightly higher than level.

rwhelipilot said:

I decided to rebuild the original OEM carburetor. I reinstalled it today and the mower now has plenty power and no spitting & sputtering under load.

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This is typically your first and best move. Might take 3 or 5 cleanings to get the OEM carb to run right. I use a cheap 120 volt hot plate, garage sale pot, water and Pine Sol. Remove all plastic and rubber parts. Gently boil the carb for an hour. Blow out with compressed air. Get a hot plate and pot off facetrash market place.

After sitting for a year, I decided to work on this some more. I have really dumped a lot of money and my own labor into this mower and after trying everything else, I remembered what my local repair shop told me last year. They said that these cheap Chinese carburetors that I had installed are hit & miss. Some will work and some won't. I decided to rebuild the original OEM carburetor. I reinstalled it today and the mower now has plenty power and no spitting & sputtering under load. That Chinese carburetor was supposed to work on 25 & 27 HP. It may have been OK on 25 HP, but evidently it was starving the 27 HP when under load.

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Congrats and thanks for sharing the solution.