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

[Turbodriven]

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?

 

[bertsmobile1]

Check both plugs are firing
The ignition is via flywheel magnetos so the timing is fixed
The coils use a hall effect trigger which is embedded into the coil
They work on the negative side of the induced voltage
Each has a common kill wire that grounds the coils
When each coil fires they send a ripple down the kill wire and to prevent this there is a set of diodes either in the coil as part of the trigger chip or in the wire itself the latter is usual Kohler practice
When these diodes fail the timing goes off so you are only running on 1 cylinder
Usual test is to remove the kill wire and see if the problem persists .
Before doing this check that the solenoid in the carb is working or there is no way to stop the engine short of pulling the plug leads off
NOW forget every thing you learned in the navy
Mowers are made down to a low ticket price not up to military reliability standards
When idling or driving on relatively flat ground the engine runs on the idle circuit alone because there is not enough load on the engine to cause the carb butterfly to open enough to activate the main jet
this is why the manual calls 3600 rpm "High Idle"

Also new does not mean good
At least 1/2 of what is sold on Amazon & Ebay is either fake , wrong or defective parts bought as scrap then sold as "fits a ..."
When going down to face book you are looking at 75 %

Kohler have reasonably good service manuals available from the Kohler web page as a free download
If you can follow navy instructions , the Kohler manual should be a doddle

Finally thank you for your years of service

 

[Turbodriven]

Thanks bertsmobile1. I'm reading through the Kohler CV740 book as we speak. And ironically I'm at the ignition coil section. Tomorrow I'm going to look further into that chapter and do some testing like you suggested. It sounds like low (or no) spark is a very good possibility. But then again, so did all the other things I've checked prior to this, so who knows. In the Kohler manual it has a troubleshooting guide for "low engine" power. Every one of their suggestions passes.

Note: I've been out of the Navy for more than 3 decades. But in the last 1 decade I have rebuilt 30+ jet ski motors. And if there is one thing I know from that, it's that a motor can run great on the bench, and then crappy under load if something isn't right. This seems to be no different.

​

 

[bertsmobile1]

Thanks bertsmobile1. I'm reading through the Kohler CV740 book as we speak. And ironically I'm at the ignition coil section. Tomorrow I'm going to look further into that chapter and do some testing like you suggested. It sounds like low (or no) spark is a very good possibility. But then again, so did all the other things I've checked prior to this, so who knows. In the Kohler manual it has a troubleshooting guide for "low engine" power. Every one of their suggestions passes.

Note: I've been out of the Navy for more than 3 decades. But in the last 1 decade I have rebuilt 30+ jet ski motors. And if there is one thing I know from that, it's that a motor can run great on the bench, and then crappy under load if something isn't right. This seems to be no different.

​

And I forgot to mention the coils are polarity sensitive so if fitted upside down they will not work
There was an interesting batch of B & S magnetos that appeared on Evilpay down here that were wired backwards or put into the mould upside down
So the instructions cast into the coil ( engine side ) was wrong & they had to be flipped .
I must have has at least 30 of these turn up with owners bemused why their magnetos made no spark .
Thus SOP has always been ry it right way up & if that does not work then upside down .

 

[Turbodriven]

Pulled the dust cover off this morning. What a pain. For a dust cover this thing is bolted stronger than the motor itself. Sheesh.

In any case, are we feeling like this is an issue? I haven't done any multimeter testing yet to the coils but that pickup (?) looks awful crusty. Can I just sand that down to clean it up? Or is it going to need replacing? Coils aren't looking too clean either, but I'll do some Ohm testing to see what they look like.

As a side note, if the pickup needs replacing, what fastener is that holding it on?? Or would I just need a whole new flywheel?

 

[bertsmobile1]

Do not touch them
They are not a set of Barbie doll toys so looking grotty makes no difference to how they work
And no you can not diagnose a faulty magneto coil with you multimeter
Even if you could it would make no difference as they are not repairable

 

[Turbodriven]

Do not touch them
They are not a set of Barbie doll toys so looking grotty makes no difference to how they work
And no you can not diagnose a faulty magneto coil with you multimeter
Even if you could it would make no difference as they are not repairable

So wait, then what is this whole chapter for in the Kohler manual then? The whole point of testing is to verify that it's internal workings aren't shorted. If it fails testing then it needs replacing. I know you can't repair them.

 

[Turbodriven]

Well as I continue down this path I've discoverd that my ignition module gaps were way too small. Kohler books specs .28-.33mm but mine were almost zero. I could barely put my smallest feeler (like aluminum foil thickness) in the gap. In order to set the proper gap I had to clean up both the flywheel pickup and the coil. Lightly with sandpaper. All ohm tests on both coils were perfectly in spec. I'm now at .30mm gaps with a lot less corrosion so we'll see how this affects things.

Gaps too small as I understand it will advance timing though. I know too far retarded timing will cause power loss, not sure about the opposite. We'll see though. Either way I suspect this will improve how the engine runs no matter what. Tomorrow I'll finish buttoning it up and take her for a spin to see. I'll report back.

 

[bertsmobile1]

Those particular coils can be tested because they have an external timing module
And there was a series of Kawasaki engines that used external modules although Kawasaki like to call it an Igniter Module
Without serial numbers we do not actually know if your engine has the SAM module, the MDI module or internal chips
The Kohler manual covers all 3 systems because all 3 were used over a period of time that overlapped
Usually Courage's came with the std internal chip while Commands came with any of the 3 systems

However the point remains that they do not need to be cleaned other than to make sure there is no corrosion between the laminations and the mounting pillars so the coil gets a good ground
Magnetism goes through wood , paper plastic & rust \
If it was a problem then the coil wold come painted or plated

 

[bertsmobile1]

Well as I continue down this path I've discoverd that my ignition module gaps were way too small. Kohler books specs .28-.33mm but mine were almost zero. I could barely put my smallest feeler (like aluminum foil thickness) in the gap. In order to set the proper gap I had to clean up both the flywheel pickup and the coil. Lightly with sandpaper. All ohm tests on both coils were perfectly in spec. I'm now at .30mm gaps with a lot less corrosion so we'll see how this affects things.

Gaps too small as I understand it will advance timing though. I know too far retarded timing will cause power loss, not sure about the opposite. We'll see though. Either way I suspect this will improve how the engine runs no matter what. Tomorrow I'll finish buttoning it up and take her for a spin to see. I'll report back.

Goes like this
Closer = bigger spark slightly advanced
further away = smaller spark slightly retarded
In fact you can "tune" the timing by varying the air gap
However the take away is unless the laminations actually make contact with the coil , if it is good it will generate a spark.