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Discussion Starter · #1 ·
I just bought a JD F680 w/54" deck and will be picking it up in a few days. Owner reports mower has a new short block, PTO clutch and carb installed by dealer w/ 2 hrs on them and it runs/drives fine but revs pull down when the clutch is engaged and you start to mow. The clutch and carb were installed separately by the dealer as attempts to cure the issue and eventually the previous owner and the dealer grew pretty aggravated with each other and threw up their hands and gave up. JD owner became a Dixie Chopper owner and I became a JD owner.

I am thinking governor but can't imagine that the dealer didn't try adjusting it 1st thing but the old owner doesn't think so. I would also guess carb but that has hopefully been addressed. Since it was a short block, it am wondering if the valves were reinstalled a little tight and maybe it's down on compression? I don't know yet if this engine has the smart spark feature but am wondering if this could be an issue since I read about a similar problem in an old forum?

I'd like to hit the ground running as soon as I get the mower home and have a list of all possibilities to check. I really don't expect this to be a major issue since the heads were supposedly checked out and OK, the engine has a new factory short block and carb and the work was done by a (questionable???) dealer but do need some help on possible ideas that would cause this symptom...any ideas out there?
 

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Get it to replicate the problem (bogging down under load) then test the cylinders by pulling the spark plugs (one by one). My guess is that you may have a bad coil. I've had this happen several times with our Kohler CV20s - may be as simple as a coil that cuts out under load. Post back.
 

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I wouldn't assume anything. Check the adjustment. The engine has hydraulic lifters, but, I would never reuse the heads if the engine had over 1000 hrs on it. It's 1/2 ass and doesn't save the end user squat. That doesn't mean that your problem. Brand new Kolher engines have about 150psi compression. I would definitely make sure there is no abnormal parasitic load on it.
 

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As mentioned,

Check both coils by removing each plug wire one at a time at idle (use rubber handled pliers) and see if the engine stalls completely, If so the coil/plug on the opposing side is not functioning.

Check governor adjustment.

Check high speed RPM's, Can be set at 3750 no load.

Check ALL deck spindles and any idler pulleys for binding.

That should be enough to get you started....
 

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Discussion Starter · #6 ·
Appreciate the replies. Going tomorrow to pick it up...about a 450 mile round trip. I'm starting to feel a little nervous on this deal, as long as I don't have to spend much, I'll have bought this machine right ( $1925). Hopefully, diagnosing the problem will be a quick and easy deal. The owner admitted he isn't very mechanical and didn't try anything himself to remedy the problem and he felt like the dealer didn't do much either, so I am praying the easy stuff has been overlooked and that's where the problem lies.

I sure don't need to be chasing ghosts on this thing, I've got my tractor spit right now and am heading out to the shop as I write this to finish putting a new clutch in it. I've got a Grasshopper that's got a small electrical bug that I'm sorting out and my old Chevy is waiting in line for a new clutch install..... that's on this weeks agenda. I work a lot of double shifts to boot so I don't need any more on my plate right now.

I'll post back what I find as soon as I get her home and take a look at a few things, hopefully I'll be saying that the beast is cutting grass.........
 

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Discussion Starter · #7 ·
Well, got the mower home last night and didn't have time to do anything with it until this evening. Checked the coils by pulling the plug wires one at a time and that checked out OK. took the air cleaner off to make sure the choke butterfly was opened all the way--that was fine. Doesn't appear to be any unusual drag caused by deck pulleys or spindles. Fuel filter is good and pump is fine-good gas supply to carb. I noticed that the a small reduction in revs even occurs when you give it full forward drive, not to the extent that it has when you engage the PTO but it is sometimes noticeable. It also is worth mentioning that these problems don't occur every time you engage the PTO. The previous owner mentioned that sometimes you could get on the mower and it would seem to work OK but most of the time not as well as it should and I noticed this to be so.

I did move the governor spring one hole toward the carb and also adjusted the throttle a little for higher revs, seemed like the top end was a little lacking. This seemed to take care of a lot of the issue. I am going to barrow a Tach from a buddy and see what she is turning before I run it any this way. I may get lucky and find this was all it needed. Still open to any ideas. I'll keep ya posted on what I find when I put a tach on it.
 

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Discussion Starter · #8 ·
OK, now I've got a new question concerning the correct RPM setting for this engine. The Kohler online service manual states when running a vacuum test to do so at 3750 RPM but under section 11 "reassembly" Intended Maximum RPM with 2 categories:

High Idle

3888
3780
3672
3564
3456
3348
3240
3132
3024

WOT
3600
3500
3400
3300
3200
3100
3000
2900
2800

With each number corresponding to a particular governor spring and position.
The John Deere F680 manual states this engines operating range is 1350-3350RPMs , with a fast idle speed of 3350......what gives? What would be the reason for Kohler and JD stating the wide differences on RPM's on this motor? Is it safe for me to run this engine at 3600-3700 rpms?
Thanks in advance
Darryl
 

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One question, was a proper break-in done on the engine that your aware of?

Also an addition from the service manual.

For a new engine, change oil after the first 20 hours of
operation. Thereafter, change oil after every 100 hours
of operation.
For an overhauled engine, use 10W-30 weight service
class SG, SH, SJ or higher oil for the first 5 hours of
operation. Change the oil after this initial
run-in period. Refill with service class SG, SH, SJ or
higher oil as specified in the “Viscosity Grades” table
on page 6.1.
 

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Is it safe for me to run this engine at 3600-3700 rpms?
Do you think for a minute I would have typed 3750 above if it would cause issues ?

Most commercial Z's come from the factory set at 3700-3750 no load....
 

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you can safely turn up the rpms to 3750 . Kohler command engines are internally balanced to withstand close to 7000 rpms.You will never get an overspeed broken rod in these engines. The plastic gov gear may come apart but not the rod. ... With top speed at 3350, you are near thetop of the torque curve,as soon as you apply any load the engine will start to labor. JD de-tunes engines ,(maybe so the rest of the mower wont wear out so fast...I dont know..).....The only problem that may arise is the engine may surge from too soft of a gov spring , easy enough to replace.
 

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Discussion Starter · #12 ·
Do you think for a minute I would have typed 3750 above if it would cause issues ?

Most commercial Z's come from the factory set at 3700-3750 no load....
Restrorob, I did not mean to question your knowledge here, especially when you're helping me, so please don't take offense by this question. I really appreaciate your help and input! It is obviouse to me after reading the Kohler site that it is OK to run the engine at 3750 rpm...at least under certain conditions (no load) maybe WOT? my main concern..and question is, since Deere states in the manual that the operating rpm is 3350rpm, is the JD spec engine significantly different than the other CH 20's and won't tolerate the revs and if not, am I going to damage something like a hyd. pump or something else by spinning it faster than Deere intended? Piston Slapper kind of touched on all this. I was also pondering a different spring while mulling over all this last night at work. The mower certainly runs a lot better at the higher revs, whatever they are. I am going to put the tach on it this evening and see just what she's turning and as long as I'm under 3800, I think I will probably leave it like it is because it seemed like it was mowing pretty darn good for the short time I ran it this way. I'll check back and thaks again everyone.

Thanks again men. You guys are a Godsend. I'm over on Tractorbynet forums a lot and I love how everyone is so willing to help and I want to thank you for making Lawnsite the same type of forum!!!
 

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is the JD spec engine significantly different than the other CH 20's and won't tolerate the revs and if not, am I going to damage something like a hyd. pump or something else by spinning it faster than Deere intended?
No difference in engine build, There are only a couple companies that build hydro pumps for all the OEM's so Deere uses one of these. So if a pump works fine on one unit at 3750 it will work fine on another brand unit at 3750.

You should be fine at that RPM.....

please don't take offense by this question
None taken..... Thumbs Up
 

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Discussion Starter · #15 ·
ARRGH! I'm about to pull my hair out! My buddy builds cart engines and he loaned me his Digitron DT31 tach a few days ago and I thought I set it up correctly after reading the directions but could never get a reading unless I pushed the max. recall button. The idle reading sounded right at about 1300rmp but full throttle was only reading about 2500-2700rpm...engine sure sounded like it was spinning a lot more than that and it's actually mowing pretty well. I told my buddy about the tach and he explained how to set parameters etc again and so tonight I set it up again and get the same results from the tach--that is to say, I could not get an instant reading like it is supposed to-- I have to press max recall buttonto get an RPM reading and I also got about the same rpm readings again tonight at idle and WOT.

The mower is actually running fairly well although I think it could still stand to see some more revs. Guess I'm going to have to get a tach that I know for sure is giving an accurate reading. Any suggestions on an inexpensive but accurate tach? Just need something that I can use to dial this engine in with and doubt I will need it for anything else so I really hate to spend a ton. I wonder if any of the auto parts stores rent anything that would work for this application?
 

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Two words, Tiny Tach; http://www.tinytach.com/tinytach/gasoline.php

It's also a hour meter so you can keep track of maintenance, This is the same one we use on all our rental equipment. If you mount it on the machine do so out of direct sun light.
 

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One thing I have run into with inductive tachs, when used on some Kohler engines. I have had the "Smart Spark" module affect readings, if the tach is used on the side that the box is (usually cylinder # 1). Try taking a reading from the other side, if it is equipped with that feature. Or get a "VIBRA-TACH". Picks up rpm's via vibrations. Extremely accurate when used correctly.
 

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3750 on the kohler is fine. there is a problem if deere isnt reving it to 3700 no load speed. Once you turn deck on, drive unit, use engine, a 10-20% drop off is normal under load. If it falls on its face, that is a problem.
Remember if it is a 20 horse, it takes 3-5 per hydro to drive+ deck. It should have plenty of power but it wont be unstoppable.
 

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Discussion Starter · #19 ·
Restorob- I had looked at Tinytachs before my buddy offered the use of his tach. After you posted that, I think I will be ordering one to supplement the stock hour meter. I noticed that the stock JD unit is electronic (12V) and stays on when the key is on. This mower is to replace a Grasshopper for my 16 yr. old sons 2 yr. old mowing business and he has been known to leave the key on accidentally from time to time. I really want to keep accurate hours on this mower and the Tinytach would be ideal...thanks for posting this.

Ricky86-I did read that having the pick up too close to the spark plug boot or various electronics could cause false readings so I was conscious of keeping the cord away from the engine but I will try it on the other side. thanks for pointing this out.

Thanks Dennis. I have to agree. Others have said it's their opinion that Deere sometimes detunes in the interest of longevity(?) but I think after everything I've read and been told the Kohler has no problem with the higher revs and the way this thing's built, I doubt anything else would be hurt by 200 or so extra revs either. This thing is a tank in every aspect, build quality and weight and with the hydro drives, deck losses and only 20 HP, it could benefit from revving at the full rated HP.
 
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