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

[Turbodriven]

No change. Back to the drawing board. Both cylinders are getting good spark (checked).

I did notice something different though. Note that before I did any work, right after I bought this mower I pulled the plugs and the right side was golden brown and the left side was dry soot black. I put new plugs in it and ran around the yard a little bit and as stated earlier both plugs were dry soot black. Not sure how that anomaly happened because I've just done two more clean plug tests and both times the left cylinder comes in dry soot black and the right cylinder comes in white. So, considering this is a single carb engine, there is something definitely wrong with the one of the cylinders for sure. Also considering both cylinders had equal and good compression and both are getting good spark (tested) my gut is telling me it's something with a head? Cams, valves, rockers... dunno? But I need to do a leak down test and/or pull the covers off the heads to check things for sure.

Still looking for answers if anyone has experience here. Getting frustrated...

 

[bertsmobile1]

I still think your engine is out of time
If you have a ignition strobe light
Hook it up to one cylinder and take a video of where the magnet is with respect to one of the coil legs
Stop the engine and mark the magnet with a white paint pen
Won't be in exactly the right spot but that will be good enough for this test
Run the engine again with the strobe to confirm the position of the lines .
Then swap over to the other cylinder and see if the lines are in the same position relative to that sides magneto legs .

The other thing that has been overlooked is a leaking inlet manifold est test for that is to run the engine again with the blower cover off and saturate each manifold with liquid WD 40 or similar from a trigger sprayer .
While doing this work the governor by hand from WFO to slow idle for maximum manifold suction
Any leak will show up with a lot of white smoke from the exhaust .

Another thing to check is the actual valve positions
The cam lobes & valves are identical so the protrusion inlet to exhaust and left to right should be identical
While not common with Kohlers , a valve guide can slip in the head if the head gets hot enough to overcome the heat treatment and become soft .

 

[Turbodriven]

I still think your engine is out of time

Ok, I've dug into your suggestions and have a question at the end.
1. My Kohler has 'Smart-Spark' and I've run the timing advance test per the manual.

• 1. Make a line near edge of flywheel screen with a marking pen, chalk, or narrow tape.
• 2. Connect an automotive timing light to cylinder that had good spark.
• 3. Run engine at idle and use timing light beam to locate line on screen. Draw a line on blower housing next to line
• on screen. Accelerate to full throttle and watch for movement of line on screen relative to line on blower housing. If
• both cylinders had good spark, repeat test on other cylinder.

Results were in spec. Line moved away from the mark similarly on both cylinders when throttle increased. And cylinder1 and cylinder2 marks were 90deg apart. So that checks.

2. I've exposed the inlet manifolds and all the nuts are tight at each cylinder head and there doesn't appear to be any gasket issues but I have yet to spray carb cleaner or whatnot to check for vacuum leaks. Based on visual inspection I don't believe this to be any issue but I'll run the test anyways.

3. I pulled both valve covers and rotated the engine a few times. Everything seems tight and functions as it should. I measured the rocker/valve travel on both sides to compare and both are the same with no loose rockers or anything. I can't see anything wrong in there tbh.

Now the question. Back to timing... Isn't the flywheel magnet suppose to be at the ignition coil when its corresponding cylinder is at TDC (or slightly past TDC)? My flywheel magnet is 4" past each when the appropriate cylinder is at TDC. (see cyl1 and cyl2 TDC pics). I could be wrong on this. I just thought the trigger was tied to an immediate firing of that cylinder. And that should happen a few milisecond AFTER the engine is at TDC. So it should all line up at that event. No?

 

[Cajun power]

couple suggestions from easy to difficult:

1. check the choke valve...should have a spring that opens that valve when choke is not applied...sometimes the spring is too weak, and sometimes the choke valve is gummy stuck...and so as air is drawn into the carb, it will have a tendency to get sucked into the close position (reducing air flow).....causing the engine to go rich! sluggish performance.
2. check all your safety switches...seat and drive controls...intermittent problems with power can be caused by gummy, loose safety switches or where wiring connections to them are shorting to ground intermittently...inspect all wiring to safety switches and make sure no damage to insulation and connections are solid and no crude. do the shake test while the engine is running.. a little tricky to accomplish but you have to be creative.
3. does your gas have some water in it? disconnect fuel line, drain into a glass container and look for separation.
4. check oil levels...sometimes overfilled causes oil to get into carb from breather and even cause crank pressure problems...check you oil levels...under load an overfilled oil will cause smoke...that's typical.. but always check oil levels.
5. ENGINE OFF - look under the mower and inspect all blades, spindles and pto...make sure unloaded all spindles are not seized, including tensioner pulley..if there is something wrapped up around a wheel or both, can cause excessive loads.
6. take all spark plugs OUT...rotate engine by hand..should rotate easily and little resistance. if it's hard to turn with spark plugs out, you need to address that and find out what is causing it. (valves, crank, piston, rings, etc)
7. hand test the governor while engine is running..it should return automatically after release.
8. head cylinder gasket failure...engine running, shroud off, spray a little carb cleaner near each head cylinder...any change in rpm indicates loss of power due to gasket failure...classically a very small blow by and loss of power can happen only under load and in a hot cycle.

just some ideas to run through

 

[Turbodriven]

Thanks Cajun. All good suggestions. I'll run through the ones I haven't done yet. I've done a lot more testing but stopped posting in this thread for a few days so some of these things already "passed". At this point I'm 100% sure my ignition system is good based on further tests. Everything looks normal with the valve covers off and rotating the engine. Really the only thing it could be at this point is a head gasket or rings (or a binding drive system that externally taxes the motor). The part I can't get around is that I'm getting 160+psi compression numbers though. The mower has 1200 hours on it (I can only assume I'm on the current motor?) so no matter what an overhaul can't hurt at this point. I ordered new head gaskets which were super cheap. I may just pull the heads and see what I see. A hone and ring job couldn't hurt at 1200 hours too and that sort of thing doesn't scare me. Rings are fairly cheap too so it'd be a good investment no matter what at 1200hrs. I enjoy rebuilding engines and this is an easy one so we'll see. But maybe with the heads off I see a blown out oil galley and that'll explain things. *shrug.

 

[ohiodave53]

Learned a good lesson about after market Chinese parts after much aggravation with a Husky chainsaw.
Bought a replacement coil/"ignition module" on the cheap from the A online site. Looked like the old one so after installing the saw would not fire even on a shot of starting fluid. Checked for spark with inline tester and looked fine. So there I am pulling my hair out...good compression, good fuel and what I think is good spark. After much aggravation I got back online, ordered a genuine OEM coil (yeah, it cost at least twice what the china junk did). Installed it and put the saw back together, set the choke/throttle, about the 3rd pull away she goes and been using it since.

 

[Cajun power]

couple suggestions from easy to difficult:

1. check the choke valve...should have a spring that opens that valve when choke is not applied...sometimes the spring is too weak, and sometimes the choke valve is gummy stuck...and so as air is drawn into the carb, it will have a tendency to get sucked into the close position (reducing air flow).....causing the engine to go rich! sluggish performance.
2. check all your safety switches...seat and drive controls...intermittent problems with power can be caused by gummy, loose safety switches or where wiring connections to them are shorting to ground...inspect all wiring to safety switches and make sure no damage to insulation and connections are solid and no crude.
3. does your gas have some water in it? disconnect fuel line, drain into a glass container and look for separation.
4. check oil levels...sometimes overfilled causes oil to get into carb from breather and even cause crank pressure problems...check you oil levels...under load an overfilled oil will cause smoke...that's typical.. but always check oil levels.
5. ENGINE OFF - look under the mower and inspect all blades, spindles and pto...make sure unloaded all spindles are not seized, including tensioner pulley..if there is something wrapped up around a wheel or both, can cause excessive loads.
6. take all spark plugs OUT...rotate engine by hand..should rotate easily and little resistance. if it's hard to turn with spark plugs out, you need to address that and find out what is causing it. (valves, crank, piston, rings, etc)
7.

Thanks Cajun. All good suggestions. I'll run through the ones I haven't done yet. I've done a lot more testing but stopped posting in this thread for a few days so some of these things already "passed". At this point I'm 100% sure my ignition system is good based on further tests. Everything looks normal with the valve covers off and rotating the engine. Really the only thing it could be at this point is a head gasket or rings (or a binding drive system that externally taxes the motor). The part I can't get around is that I'm getting 160+psi compression numbers though. The mower has 1200 hours on it (I can only assume I'm on the current motor?) so no matter what an overhaul can't hurt at this point. I ordered new head gaskets which were super cheap. I may just pull the heads and see what I see. A hone and ring job couldn't hurt at 1200 hours too and that sort of thing doesn't scare me. Rings are fairly cheap too so it'd be a good investment no matter what at 1200hrs. I enjoy rebuilding engines and this is an easy one so we'll see. But maybe with the heads off I see a blown out oil galley and that'll explain things. *shrug.

before you make a big investment, I would look at any kind of crack in the crank, case, head cylinder. Sometimes you get these really small fine cracks that do not reveal until a high heat cycle and under load. A standard dye penetrate with blue light should reveal that. But will need to clean the engine up really well. Failed head cylinder gaskets will NOT fail a compression test. a leak down test "CAN" help identify a head cylinder gasket leak! (as well as many other things)

I recently did a head cylinder gaskets replace on a kawasaki fr691V (after chasing ghosts - symptoms was a surging engine at all RPM's). I opted for all copper gaskets (better squish and heat transfer), over the kawa oem gaskets.

I like rebuilding also. It's a side hustle to repair mowers and other small engine machines.

 

[MowerNick]

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.

 

[Turbodriven]

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.

Thanks. I'm miles past that though. It passed that test. Coils also passed all Kohler manual ohm tests. I rechecked timing per the manual and it passed. And I swapped coils left to right (verifying gaps too) with no change in the cylinder that was running rich. I'm near 100% sure my coils and the entire ignition system is working properly.

 

[moparjoe]

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?

Shorten the governor spring.