Gravely 260Z - Rpm's bog down when moving under load

[Davenj4f]

Hey everyone. I'm new to zero turns but very mechanical in experience (ex Navy fighter jet mechanic).

Got a used 2005 260z (Kohler CV740 Command Pro 27) for cheap that starts and idles (and throttles up and down) perfect. Sounds good sitting still. Even starts mowing like a bat out of hell for the first few seconds. But then after 20 feet of moving at speed (and even worse when blades are engaged) the motor quickly starts winding down to about 20-30% rpms (or so it feels) and barely mows. Like someone stuck a banana in the tailpipe. If I disengage blades and pull back the controls to a stop, the rpms pick back up and she idles/sounds normal again. But then the behavior returns when I push the sticks forward.

I put new plugs in it and after mowing for five minutes I pulled both plugs out and they were both dry and thick black (rich).

1. This behavior remains unchanged after installing a brand new carburetor, new fuel pump, fuel filter, spark plugs, air filter, and fresh gas.
2. Compression tests on cyl1=170psi and cyl2=175psi
3. There is no smoke in the oil dipstick (crankcase) after running and the oil level is correct.
4. Behavior does not correct when unscrewing the gas cap (*even though "lean" doesn't appear to be the issue)
5. Governor linkage was set per specs when I installed the new carb.
6. Battery is good and I hooked up my multimeter while running and it maintains 13.x+ volts throughout the whole behavior. Also, there is no spikes when engaging PTO clutch. That system appears to be operating normal??
7. I had the deck off last week and all three spindles seem to spin fine.

I'm not sure what else to think. The basic behavior is that the motor slows down under load. So either the normal load is affecting something wrong with the motor, or an abnormally heavy load is too great for the healthy motor.

Any ideas on what to check or fix to get this right?

Check your spindles for wear. If they have any play, the mower will bog down under a load. I just went thru that on my mower. Also, a VERY dirty air filter will produce the same effect.

 

[TobyU]

Try pulling your plug wires with rubber handle pliers while its running. If you pull one and it stalls then that side is your problem. Most likely no spark on one side. Surprisingly those kohlers run fairly well on one cylinder until its time to work.

Don't you mean if you pull one and it stalls... Then the other side is the problem??
It's running on the one you pulled when pulling it makes it die so the other one was doing nothing.

 

[jviews12]

Best diagnostic thread in a while. Thanks to all people who added thoughts.

 

[Turbodriven]

Don't you mean if you pull one and it stalls... Then the other side is the problem??
It's running on the one you pulled when pulling it makes it die so the other one was doing nothing.

The idea is that if somehow your mower is running on only one cylinder, you wouldn't necessarily notice it outright as it will still run and sound fairly ok with no load. But to check for this, you pull one spark plug cable (say the #1) and if by doing so the mower shuts down, then you know the #2 cyl wasn't working (as you can run on one but not on zero cylinders).

 

[Turbodriven]

As a follow up (hopefully to close the thread) I've narrowed my motor problems down to either rings, valves or a head gasket issue (despite having good compression). Everything else has been looked at and tested good, so I pulled the heads. My valves are sealing and within specs in all ways. Cylinders look good with visible cross hatching still after 1200 hrs (what!!?). My #2 head gasket seemed to be ok too. But the (OEM) #1 head gasket had some carbon tracking on the exhaust side fins which I guess is a typical failure. However, the head gasket itself didn't look that bad to be honest. But I checked flatness, both well within specs, then put updated gaskets in and buttoned it up per manual specs.

As an interesting note... in this process I also discovered that my 5 rib hydro drive belt was missing 2 of the 5 ribs, just disintegrated. I'm sure under load it was struggling to hook up. This may have been at least part of my problem. New belt was delivered last night so I'll put it on and see how things go with the new head gaskets and new belt installed. Fingers crossed!

 

[TobyU]

The idea is that if somehow your mower is running on only one cylinder, you wouldn't necessarily notice it outright as it will still run and sound fairly ok with no load. But to check for this, you pull one spark plug cable (say the #1) and if by doing so the mower shuts down, then you know the #2 cyl wasn't working (as you can run on one but not on zero cylinders).

The idea is that if somehow your mower is running on only one cylinder, you wouldn't necessarily notice it outright as it will still run and sound fairly ok with no load. But to check for this, you pull one spark plug cable (say the #1) and if by doing so the mower shuts down, then you know the #2 cyl wasn't working (as you can run on one but not on zero cylind

Precisely what I said. I believe you misspoke in your first statement when you said if you pull a spark plug wire off and it stalls then that one is your problem.
As you just clarified, if you pull one off and it stalls, it was the other one that wasn't giving you power so that one would be your problem.

 

[TobyU]

As a follow up (hopefully to close the thread) I've narrowed my motor problems down to either rings, valves or a head gasket issue (despite having good compression). Everything else has been looked at and tested good, so I pulled the heads. My valves are sealing and within specs in all ways. Cylinders look good with visible cross hatching still after 1200 hrs (what!!?). My #2 head gasket seemed to be ok too. But the (OEM) #1 head gasket had some carbon tracking on the exhaust side fins which I guess is a typical failure. However, the head gasket itself didn't look that bad to be honest. But I checked flatness, both well within specs, then put updated gaskets in and buttoned it up per manual specs.

As an interesting note... in this process I also discovered that my 5 rib hydro drive belt was missing 2 of the 5 ribs, just disintegrated. I'm sure under load it was struggling to hook up. This may have been at least part of my problem. New belt was delivered last night so I'll put it on and see how things go with the new head gaskets and new belt installed. Fingers crossed!

Well, your belt has nothing to do with your engine in any way shape or form. Now the machine could very well slow down going uphill if your bell to slipping etc but your engine RPM and sound wouldn't change in any way.

I'm sorry to say I feel your diagnostic procedure has been all over the place and throwing a lot of time and parts into the engine that I would have never done.
It does no good to troubleshoot and diagnose certain aspects of things unless you first are stepping back and making educated guess on what the potential problem could be in the first place.
In other words if a lawn mower had some sort of noise in it only when you turn on the blades then it would be senseless to first start out worrying about the engine causing the noise and I understand this is not quite that simple because it is certainly an engine thing AND not to say that a noise problem with the blades on couldn't be coming from the engine or engine related because I have seen that happen too but it's highly unlikely.

 

[Turbodriven]

I'm sorry to say I feel your diagnostic procedure has been all over the place and throwing a lot of time and parts into the engine that I would have never done.
It does no good to troubleshoot and diagnose certain aspects of things unless you first are stepping back and making educated guess on what the potential problem could be in the first place.
In other words if a lawn mower had some sort of noise in it only when you turn on the blades then it would be senseless to first start out worrying about the engine causing the noise and I understand this is not quite that simple because it is certainly an engine thing AND not to say that a noise problem with the blades on couldn't be coming from the engine or engine related because I have seen that happen too but it's highly unlikely.

I respectfully disagree. To begin with, not including the parts the mower actually needed (like new filters, belt, etc) I've only spent about $46 on this $600 mower. And most of that was on a new carb. Mostly because I just needed the gaskets it came with after tearing the ones on the mower. I figured the delta between just new gaskets and a whole new carb with gaskets was worth the few extra dollars.

As far as diagnosing, I think I've been very methodical. Starting with the fuel system, then moving to electrical, and then the engine. Checking each system per the manual from front to back. Maybe my list was a bit jumbled. But I'm not a writer. And I rather enjoy tinkering on engines. So the time is a non-issue.

For what it's worth, my problem still isn't solved and I'm open to any ideas. I hooked a tinytach up to it while mowing my lawn an hour ago and when under normal load, the rpms dip to about 1700-2000. Occasionally less, and sometimes it holds at 2500 for a short while (depends on load). But at rest idle is 1200rpm and full throttle is 3700-3900rpm.

You're right about listening to the problem though. But that's what is so frustrating here. The problem is telling me there is a fuel/air mixture problem between Cyl1 and Cyl2 but nothing seems to be wrong anything that could cause that. Good compression both sides. Good valves both sides. New head gaskets both sides. Good coils both sides (even swapped them after testing them with no change) and I'm still getting rich plugs on cyl1 only (and possibly lean on cyl2). Plugs in pic are today after a full mow. Plugs were clean white when starting.

 

[TobyU]

I respectfully disagree. To begin with, not including the parts the mower actually needed (like new filters, belt, etc) I've only spent about $46 on this $600 mower. And most of that was on a new carb. Mostly because I just needed the gaskets it came with after tearing the ones on the mower. I figured the delta between just new gaskets and a whole new carb with gaskets was worth the few extra dollars.

As far as diagnosing, I think I've been very methodical. Starting with the fuel system, then moving to electrical, and then the engine. Checking each system per the manual from front to back. Maybe my list was a bit jumbled. But I'm not a writer. And I rather enjoy tinkering on engines. So the time is a non-issue.

For what it's worth, my problem still isn't solved and I'm open to any ideas. I hooked a tinytach up to it while mowing my lawn an hour ago and when under normal load, the rpms dip to about 1700-2000. Occasionally less, and sometimes it holds at 2500 for a short while (depends on load). But at rest idle is 1200rpm and full throttle is 3700-3900rpm.

You're right about listening to the problem though. But that's what is so frustrating here. The problem is telling me there is a fuel/air mixture problem between Cyl1 and Cyl2 but nothing seems to be wrong anything that could cause that. Good compression both sides. Good valves both sides. New head gaskets both sides. Good coils both sides (even swapped them after testing them with no change) and I'm still getting rich plugs on cyl1 only (and possibly lean on cyl2). Plugs in pic are today after a full mow. Plugs were clean white when starting.

Well, the attitude is going to come out here but you can disagree all you want but all I'm saying is if that lawn mower was in front of me I would have been to the bottom of diagnosing it before I replaced a single part.
I don't care that something was old and needed to replaced anyways or was likely to fail etc etc. To me that's just a cop out..
I don't replace anything until it's broken or until it fails.
But I'm a cheapskate, and my goal is to get to the bottom of the problem and get the equipment running properly as quickly and as cheaply as possible.
That is the first concern..
Any additional concerns about longevity or how many times it's going to mow for is the secondary consideration which we can talk about later.

Regardless, since your problem still isn't fixed, your two new plugs you put in and after one mow showing that way definitely says something is still awry.
That number one plug is certainly too rich or possibly it's not firing all the time but that's not the only possibilities.
The other one could be a little lean but it could also be fine..
I wouldn't really worry about that one for now.

If I remember correctly this is a Briggs & Stratton twin but please correct me if I'm wrong because it's been a number of days and this thread is over three pages long.
I guess it doesn't make much different among the common brands but about the only ways I can think of off the top of my head that would be common for that plug to be that carbon footed up after only one mow of an hour or so would be extremely low air flow which is kind of hard to do on a twin since they share the same carburetor and intake and would be much more simple to happen on a single....then the second way would be if the plug just simply isn't firing every time it's supposed to, and the third would be a leaky head gasket or some other head or even valve problem.
It's not worth delving into the thought process right now as the which is more likely or what causes what and how.

The best procedure from here is to start with the easiest thing or anything you can 100% eliminate and then move on..
A lot of times the problem is people check something and rule it out but they haven't 100% ruled it out and often it ends up being part of the problem or the whole problem later.

So again, I would go back to cancelling out cylinders.
If I had to guess I would say when you take off that number one sooty plug wire, the engine still runs pretty decently on the other one.
Then, when you take off the number two plug wire which is much cleaner I believe you'll find there is a more drastic change and you're getting lower power from that number one cylinder.
Heck, my guess could be wrong and it could be reversed because maybe the number two cylinder plug is so clean because it's not getting any combustion at all but I doubt that.

So again, it all boils down to a performance test of how it runs on each cylinder or even if it runs on each cylinder.
So this would be more akin to a cylinder balance test they call it in the automotive field.
As I said, a compression tester is pretty much useless on these and I haven't screwed one into a machine in over 20 years and I do well over a thousand of these engines a year.
A link down test or simply but, blowing air through the cylinder and moving the Piston to a certain place so the valves move will tell you some, but I still feel it's a waste of time as there's more things you can do quicker and easier to get things diagnosed.

So do this cylinder canceling out test and report back with the findings..

I used to be an auto mechanic more so than a lawn mower mechanic so the first time I adjusted the valves on a Briggs & stratton, I didn't incorrectly and it wouldn't run right and when I checked them again I found I had one extremely too loose.

I find that people with other training like Auto mechanics, aviation, boats, or engineers, often have a hard time working on lawn mowers and often do things the very roundabout, hard, or time-consuming way.
I have streamlined the number of procedures that I actually do to mowers down to a science of quickness and repetition.
So basically, I am efficient as hell.
Often, the way I do it is a little bit harder and more convoluted as far as working blindly or in a tight area etc but it allows me to get the job done far quicker than your average YouTube person who takes everything apart and lays it on their work bench and everything like that.
I don't have time for that.

I have been come quite a master diagnostician but I will say it's very hard to diagnose things second hand and via text messaging etc and only a little bit easier when you're doing it via Skype or video..
There's nothing like getting a machine in front of a well experienced ear and set of eyes to quickly narrow down where the problem is coming from.

 

[Turbodriven]

Well, the attitude is going to come out here but you can disagree all you want but all I'm saying is if that lawn mower was in front of me I would have been to the bottom of diagnosing it before I replaced a single part.
I don't care that something was old and needed to replaced anyways or was likely to fail etc etc. To me that's just a cop out..
I don't replace anything until it's broken or until it fails.
But I'm a cheapskate, and my goal is to get to the bottom of the problem and get the equipment running properly as quickly and as cheaply as possible.
That is the first concern..
Any additional concerns about longevity or how many times it's going to mow for is the secondary consideration which we can talk about later.

Regardless, since your problem still isn't fixed, your two new plugs you put in and after one mow showing that way definitely says something is still awry.
That number one plug is certainly too rich or possibly it's not firing all the time but that's not the only possibilities.
The other one could be a little lean but it could also be fine..
I wouldn't really worry about that one for now.

If I remember correctly this is a Briggs & Stratton twin but please correct me if I'm wrong because it's been a number of days and this thread is over three pages long.
I guess it doesn't make much different among the common brands but about the only ways I can think of off the top of my head that would be common for that plug to be that carbon footed up after only one mow of an hour or so would be extremely low air flow which is kind of hard to do on a twin since they share the same carburetor and intake and would be much more simple to happen on a single....then the second way would be if the plug just simply isn't firing every time it's supposed to, and the third would be a leaky head gasket or some other head or even valve problem.
It's not worth delving into the thought process right now as the which is more likely or what causes what and how.

The best procedure from here is to start with the easiest thing or anything you can 100% eliminate and then move on..
A lot of times the problem is people check something and rule it out but they haven't 100% ruled it out and often it ends up being part of the problem or the whole problem later.

So again, I would go back to cancelling out cylinders.
If I had to guess I would say when you take off that number one sooty plug wire, the engine still runs pretty decently on the other one.
Then, when you take off the number two plug wire which is much cleaner I believe you'll find there is a more drastic change and you're getting lower power from that number one cylinder.
Heck, my guess could be wrong and it could be reversed because maybe the number two cylinder plug is so clean because it's not getting any combustion at all but I doubt that.

So again, it all boils down to a performance test of how it runs on each cylinder or even if it runs on each cylinder.
So this would be more akin to a cylinder balance test they call it in the automotive field.
As I said, a compression tester is pretty much useless on these and I haven't screwed one into a machine in over 20 years and I do well over a thousand of these engines a year.
A link down test or simply but, blowing air through the cylinder and moving the Piston to a certain place so the valves move will tell you some, but I still feel it's a waste of time as there's more things you can do quicker and easier to get things diagnosed.

So do this cylinder canceling out test and report back with the findings..

I used to be an auto mechanic more so than a lawn mower mechanic so the first time I adjusted the valves on a Briggs & stratton, I didn't incorrectly and it wouldn't run right and when I checked them again I found I had one extremely too loose.

I find that people with other training like Auto mechanics, aviation, boats, or engineers, often have a hard time working on lawn mowers and often do things the very roundabout, hard, or time-consuming way.
I have streamlined the number of procedures that I actually do to mowers down to a science of quickness and repetition.
So basically, I am efficient as hell.
Often, the way I do it is a little bit harder and more convoluted as far as working blindly or in a tight area etc but it allows me to get the job done far quicker than your average YouTube person who takes everything apart and lays it on their work bench and everything like that.
I don't have time for that.

I have been come quite a master diagnostician but I will say it's very hard to diagnose things second hand and via text messaging etc and only a little bit easier when you're doing it via Skype or video..
There's nothing like getting a machine in front of a well experienced ear and set of eyes to quickly narrow down where the problem is coming from.

Thanks for the response. A few weeks ago I moved this to the Kohler forum and streamlined the long list of things I've ruled out. Since they were all kind of all over the place here.
https://www.lawnmowerforum.com/threads/cv740-27hp-bogs-severely-under-average-load.75051/

It's becoming a long thread with an ex-Kohler mechanic at the heart of it with me. We've ruled out so many things. I'm at my absolute wits end with this thing and just decided to tear it apart at this point and rebuilt it from the ground up. If you dig into that thread you'll see why I chose that path. I've never been very good at giving up. LOL. Much to my detriment sometimes.

That said, I'm much like you. A cheapskate. And honestly I spent the money on filters/belts/etc before I realized I had a problem. But as far as the problem itself, I'm 100% onboard with don't spend money on a part unless you know it is the problem. But even getting into the engine to find a problem sometimes generates costs (gaskets, seals, etc). I pulled the carbs to clean them and ripped a few gaskets because they were glued on essentially. Gotta buy new gaskets now to put the carb back on and see if it's now fixed. That's money spent on parts that didn't fix the problem. Additionally, some parts just can't be tested accurately. Like my DSAM box. I have no one near me that has a DSAM testing tool. And even if I did they aren't always accurate. And a bad DSAM made 100% sense considering all the other things it wasn't. I jump started this mower a few times too which is reported to kill is Smart Spark box sometimes. After trying everything else, and with the symptoms matched the potential bad part, I spent money on the MDI conversion. It of course didn't fix it though.

In the end my goal is to have a motor/mower that will last me years. An investment. Outside of a hole in the case, there isn't much that can't be fixed on these things. And with such a small initial investment there is a lot of wiggle room before I'm anywhere near upside down on it. I guess that's my train of though in most of my projects. Will I be upside down if I spend this? If not, then let's get her done...