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Kohler 7.5R Marine Unit, Surging/Hunting Under Load Only

stevegnh

Registered
Hi All,

I've made some pretty good progress doing some updates to the Kohler 7.5R genset in my boat. Spec is 106275-A. I bought the boat last year and the genset has been flaky from the start. I paid someone to fix it, and it worked for about a week, then they didn't call me back. And I found a few leak so I knew I needed to get into it. I'll say what I've done, and what I'm seeing. From there, I'd appreciate any advice.

First, I have completely cleaned the carb and used a new carb kit. So all new gaskets, needles (main and idle), carb bowl needle, checked the float height etc. And the leak is fixed. Next, I deleted the mechanical fuel pump and installed an ignition protected electric fuel pump kit from Kohler. I then replaced all the plugs, changed the oil (end of last season), the impeller in the water pump, and changed the thermostat. The carb screws were set to spec as outlined in the service manual (which I have).

Upon first starting it after doing all this work, it fired right up, surged/hunted a bit for about 2 minutes, then settled down and ran great. Nice and smooth and ran for over an hour with no issues. The load was pretty light at this point with just some lights on, so I went to adjust the main fuel screw on the carb per the service manual, but I never got the response I expected. It did slow down when turning it in, but it just kept speeding up when turning it out, it never slowed again from being too rich. I found that odd. So I couldn't find the mid point between lean and rich per the service manual. I set it back to 2.5 turns out. It was producing 122vac at this point.

I then turned on my A/C unit, and that's when things went sideways. The machine started hunting/surging (not sure which it really is, meaning how each is defined - and it did this last year in addition to hardly running at all). As it was doing this, voltage was dropping though, causing the Air Conditioner to reset. Voltage was dropping to about 88vac. Adjustment of the fuel screw settled it down and it stabilized, but when the load was removed, the engine ran way too high, around 150vac. With the A/C running, if I manually held the linkage between the carb and governor it would be stable. I again set the fuel screw back to 2.5 turns out.

The carb itself is spotless inside. Not much to it really, and it has all new guts. Nothing is leaking. I've been wondering if maybe in the past someone messed with the governor and it's not sensitive enough? Or just due to its age (1989) it's worn and needs adjusting to compensate. It seems to me the genset isn't managing the load properly and opening the throttle when a load is placed on the unit.

I'm a bit nervous to mess with the the governor without fully understanding it, so that's what I'm doing right now, trying to learn what I can. I read a bunch of threads, but wasn't sure they were similar symptoms to what I saw.

Any thoughts, advice, pointers, are very much appreciated. Thanks!
 

RETCPO

Registered
As no one else has given advice. It sounds to me like you still have a carburetor that has passages clogged and this is the bigger problem you are having.
What carburetor do you have? This information will help with better help being provided.

Respectfully
Keith
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
Another possibility is an air leak between the carburetor and engine, possibly the intake manifold gaskets. One of my Onans had this trouble, it ran perfectly with no or low load, but surged badly when the load was over 3,000 watts. The problem was an air leak in the two piece intake manifold.

I think I would avoid testing this generator using your air conditioner. Purchase a few 1,500 watt heaters and use them for the load because they will not be damaged by the surging and voltage fluctuations.
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
And then there is the old adage that says that 90% of carburetor troubles are ignition related... :brows:

I would try a known good condenser in the magneto. Cheap and easy to do, nothing to lose.
 

stevegnh

Registered
I hope you are right, as that is the easiest to remedy! I'd be surprised though, as I've cleaned a lot of carbs, and this thing was clean, unless I missed something. I could always do it again. Although not the easiest carb to get off and on (screw access), at least it's not a major thing to try. The carb per the service manual is number 50 053 03 variation 61.

As no one else has given advice. It sounds to me like you still have a carburetor that has passages clogged and this is the bigger problem you are having.
What carburetor do you have? This information will help with better help being provided.

Respectfully
Keith
I hope you are right, as that is the easiest to remedy! I'd be surprised though, as I've cleaned a lot of carbs, and this thing was clean, unless I missed something. I could always do it again. Although not the easiest carb to get off and on (screw access), at least it's not a major thing to try. The carb per the service manual is number 50 053 03 variation 61.

Another possibility is an air leak between the carburetor and engine, possibly the intake manifold gaskets. One of my Onans had this trouble, it ran perfectly with no or low load, but surged badly when the load was over 3,000 watts. The problem was an air leak in the two piece intake manifold.

I think I would avoid testing this generator using your air conditioner. Purchase a few 1,500 watt heaters and use them for the load because they will not be damaged by the surging and voltage fluctuations.
Good tip, thanks. The gaskets between the carb and intake elbow is new, as is the one between the intake elbow and manifold. I believe I'm talking about the same thing you are. I'm sort of learning the more involved stuff as I go, but very capable...

And yes, I won't be using my A/C anymore.

And then there is the old adage that says that 90% of carburetor troubles are ignition related... :brows:

I would try a known good condenser in the magneto. Cheap and easy to do, nothing to lose.
So this is something I haven't messed around with before. Is it involved to put it in? I'm guessing no based on your comment. I'll look and see what the part is, if I can find a reference. I don't recall seeing that in my travels through the parts diagram so far.

So we have, 1. Carb still dirty, 2. Leaking gaskets, 3. condener
 

Vanman

Subscriber
Last Subscription Date
07/10/2019
Condenser, and I should have added to check and clean the points. These are inside the magneto, which is on the opposite side of the engine from the carb. Easy to do, assuming it is reasonably accessible in your installation.

Also good to take a look at the plugs and wires.

After you know all of that is in good shape, then we can get back to carburetor and governor adjustments.

One trick for finding vacuum leaks is to spray a bit of carb cleaner at the suspect areas while the engine is running, and listen for changes in how it runs.
 

G Sams

Registered
I agree with RETCPO. Since adjusting the needles doesn't change anything, there is a passage that's plugged.
 

stevegnh

Registered
Condenser, and I should have added to check and clean the points. These are inside the magneto, which is on the opposite side of the engine from the carb. Easy to do, assuming it is reasonably accessible in your installation.

Also good to take a look at the plugs and wires.

After you know all of that is in good shape, then we can get back to carburetor and governor adjustments.

One trick for finding vacuum leaks is to spray a bit of carb cleaner at the suspect areas while the engine is running, and listen for changes in how it runs.
Thanks. I'll read up on a bit on the condenser and points. Those are items I'm not familiar with servicing. And the back side of the unit is fairly accessible from above via the genset hatch.

Plugs are brand new, and wires look fairly new, but I have not checked them for continuity issues (breaks), which I could do as well.

I agree with RETCPO. Since adjusting the needles doesn't change anything, there is a passage that's plugged.
I wouldn't say the main needle didn't adjust. It did slow down when dialing it in (CW), and it sped up when opening it up (CCW), but when going CCW it never seemed to slow down again like the service manual said it was.

This is such a basic carb, I'd be surprised if it still had an issue, but maybe. Always a chance. I cleaned up and ran it through an ultrasonic cleaner.
 

G Sams

Registered
Sometimes the only way to clean out a passage is with compressed air. I use 150# pressure to make sure it's clear of all debris.
 

LWB250

Registered
Has anyone tinkered with the governor? Let's hope not. Also, the length of the throttle linkage between the governor lever and carb is very critical. Again, hopefully no one has tinkered with that either....

Once you get it running and stable, using either a hand held tachometer or a frequency meter, set the running no-load speed at 1890 rpm or 63 Hz. As you apply load the speed should drop or "droop" to around 1800 rpm or 60 Hz under full (boat) load. You won't be able to judge this with the AC unit, as the load from it is transitional, being large at startup and dropping considerably as the load on the compressor drops off.

I have attached a picture for governor adjustment from the manual. Adjust speed FIRST, then adjust sensitivity if necessary. Small adjustments go a LONG way, so don't get carried away.

Dan
 

Attachments

jack0

Registered
Age
60
Who knows how many fingers have worked on this set?
The gov. sensitivity adjustment could very well be off.
You held the linkage with the a/c running and everything was stable. One concern is at start up. The engine shouldn't hunt around for 1-2 min. They should settle down fairly quick.
You went over the carb so that should be good. 2.5 turns out is a good starting point. Should run okay.

Maybe you could post a close up picture of the gov. and we'll compare. Something might be obvious to a different set of eyes.
 

Kevin K

Subscriber
Last Subscription Date
07/12/2019
I wouldn't say the main needle didn't adjust. It did slow down when dialing it in (CW), and it sped up when opening it up (CCW), but when going CCW it never seemed to slow down again like the service manual said it was.
That is symptomatic the inability of the carburetor to supply a overly rich mixture. Try putting a load on the generator, say a 1500 watt heater. This will make sure the motor is running on the high speed jet. Then try turning the needle CCW. The motor should eventually slow down due to a rich mixture. If it doesn't, you have a carburetor problem, fuel supply problem, or an air leak somewhere.
 

nblack

Registered
Another trick with a heater as a load, is to start adding choke to see if the engine smooths out. If so, DEFINITE carb/ air fuel issue.
1. carb passages.
2.Earlier part of thread about spraying carb cleaner to joints and observe engine performance. (I use WD-40, but probably would not suggest the use of it in a boat, due to flammability )
3. Also, engine should NOT hunt at start up.
4. Check the timing with a timing light. #1 should light up just as SPK aligns in window. (16' BTDC)
5. be very careful with compressed air in a carb. i.e. get the freaking float OUT of the equation BEFORE introducing ANY air into the carb to "blast", "check",or"clean" passages.... just sayin'.
6.From your posts, I am leaning toward the main jet/ passageway being at least partially blocked, since you are not mentioning any condition of "four stroking" (running too rich).
7. I also notice no mention of the rpm dropping, which makes me also wonder about the governor. Please set it up as described.
8. I would strongly recommend starting with the carb first, and THEN worrying about the governor.
good luck, and please keep us posted. :wave:
 

dkamp

eMail NOT Working
All the above is good... but one question:

Did you load test it at all with it OUT of the boat?

If you did, and it passed muster, then check two things:

Raw water hose from seacock... if the interior of the hose is collapsing in and restricting coolant, it may be just enough to cause the engine to warm up...

Exhaust and muffler... if the exhaust path is becoming constricted by something (excess cooling water backpressure, etc), then the engine will SEEM like it's running lean AND... won't respond to the mixture screw position.

Do you have a water-lift type muffler? if so, make sure you don't have a mouse nest floating in it...
 

stevegnh

Registered
Sometimes the only way to clean out a passage is with compressed air. I use 150# pressure to make sure it's clear of all debris.
Sorry for the delay in my responses, I've been swamped at work. Haven't had time to think about the genset.

I don't use 150, but I'm around 100 actually. I would be shocked if it were dirty, but I certainly can clean it again. It is kind of a pain to get off though. :(

---------- Post added at 08:16:35 PM ---------- Previous post was at 08:14:28 PM ----------

Has anyone tinkered with the governor? Let's hope not. Also, the length of the throttle linkage between the governor lever and carb is very critical. Again, hopefully no one has tinkered with that either....

Once you get it running and stable, using either a hand held tachometer or a frequency meter, set the running no-load speed at 1890 rpm or 63 Hz. As you apply load the speed should drop or "droop" to around 1800 rpm or 60 Hz under full (boat) load. You won't be able to judge this with the AC unit, as the load from it is transitional, being large at startup and dropping considerably as the load on the compressor drops off.

I have attached a picture for governor adjustment from the manual. Adjust speed FIRST, then adjust sensitivity if necessary. Small adjustments go a LONG way, so don't get carried away.

Dan
Thank you for these tips. I have both a frequency capable DMM and a hand held tach. I can do both. I will verify the linkage length too, but I have no idea if it's been messed with in the past. I certainly have not, and a Kohler guy I bought parts from told me the same, don't mess with the governor linkage length. I do have the service manual too, but must have missed this value for me to verify.

---------- Post added at 08:17:44 PM ---------- Previous post was at 08:16:35 PM ----------

Who knows how many fingers have worked on this set?
The gov. sensitivity adjustment could very well be off.
You held the linkage with the a/c running and everything was stable. One concern is at start up. The engine shouldn't hunt around for 1-2 min. They should settle down fairly quick.
You went over the carb so that should be good. 2.5 turns out is a good starting point. Should run okay.

Maybe you could post a close up picture of the gov. and we'll compare. Something might be obvious to a different set of eyes.
I will get some pictures, and post them up. I'll also get a video of initial start up and I'll keep track of how long it takes to settle down.

---------- Post added at 08:18:46 PM ---------- Previous post was at 08:17:44 PM ----------

That is symptomatic the inability of the carburetor to supply a overly rich mixture. Try putting a load on the generator, say a 1500 watt heater. This will make sure the motor is running on the high speed jet. Then try turning the needle CCW. The motor should eventually slow down due to a rich mixture. If it doesn't, you have a carburetor problem, fuel supply problem, or an air leak somewhere.
I will do this and report the results. Good pointers, thank you.

---------- Post added at 08:20:44 PM ---------- Previous post was at 08:18:46 PM ----------

Another trick with a heater as a load, is to start adding choke to see if the engine smooths out. If so, DEFINITE carb/ air fuel issue.
1. carb passages.
2.Earlier part of thread about spraying carb cleaner to joints and observe engine performance. (I use WD-40, but probably would not suggest the use of it in a boat, due to flammability )
3. Also, engine should NOT hunt at start up.
4. Check the timing with a timing light. #1 should light up just as SPK aligns in window. (16' BTDC)
5. be very careful with compressed air in a carb. i.e. get the freaking float OUT of the equation BEFORE introducing ANY air into the carb to "blast", "check",or"clean" passages.... just sayin'.
6.From your posts, I am leaning toward the main jet/ passageway being at least partially blocked, since you are not mentioning any condition of "four stroking" (running too rich).
7. I also notice no mention of the rpm dropping, which makes me also wonder about the governor. Please set it up as described.
8. I would strongly recommend starting with the carb first, and THEN worrying about the governor.
good luck, and please keep us posted. :wave:
I've cleaned so many carbs, and this one was so easy, but I will check it again. I always blow a carb out with NOTHING installed. No float bowls, no needles, etc. And I verified the float bowl as well, that it was sitting at the right height.

---------- Post added at 08:23:06 PM ---------- Previous post was at 08:20:44 PM ----------

All the above is good... but one question:

Did you load test it at all with it OUT of the boat?

If you did, and it passed muster, then check two things:

Raw water hose from seacock... if the interior of the hose is collapsing in and restricting coolant, it may be just enough to cause the engine to warm up...

Exhaust and muffler... if the exhaust path is becoming constricted by something (excess cooling water backpressure, etc), then the engine will SEEM like it's running lean AND... won't respond to the mixture screw position.

Do you have a water-lift type muffler? if so, make sure you don't have a mouse nest floating in it...
The genset has not been out of the boat. That would be a big job. I took the carb off, brought it home, etc., and reinstalled in boat. Genset has new water pump impeller, new tstat and gasket, and new plugs (properly gapped). As for the exhaust questions you have, I'm not sure to be honest what I have. I mean it is is a wet exhaust where a canister fills with water to help muffle the sound and make it quieter. I will get pics of that setup too.

---------- Post added at 08:26:12 PM ---------- Previous post was at 08:23:06 PM ----------

One thing to mention too, is that I did add the Kohler electric fuel pump. That seems to work great. When I first try to start it, it will crank for about 5 seconds and I'll stop cranking (shut switch off), then I turn it on again and it starts right up. WAY better than last year and the mess of an incorrect electric pump (not properly wired) and the existence of the old mechanical fuel pump still being installed.
 

dkamp

eMail NOT Working
As for the exhaust questions you have, I'm not sure to be honest what I have. I mean it is is a wet exhaust where a canister fills with water to help muffle the sound and make it quieter. I will get pics of that setup too.
Okay, well, a NOT UNUSUAL problem, particularly for boats that get hauled out for winter storage... is rodents in the exhaust. Water-lift mufflers are excellent shelter for mice and rats, but they're also excellent traps, because they can get in easily, but oftentimes, not so easy out. i would NOT be surprised to find that there's a critter floating around in there, and when it gets to the exit, blocks it off enough to increase exhaust pressure.

Two important things to understand about marine submerged exhaust... is that when exhaust is submerged 14", that exhibits (in theory) 14" w.c. of static exhaust backpressure, which means an engine that would normally be running at one amount of manifold vacuum, is actually running at a DIFFERENTIAL vacuum substantially LESS.

Now, if you look at the exhaust discharge for your genset, you MAY have an exhaust port located 2" below the waterline, or 4" above... and that 2" may be negligible, or it may have an impact, the water lift exhaust ALWAYS has an impact.

Cooling water exits the engine on the BACK side of the manifold exhaust riser elbow, flows downhill into the muffler can. exhaust flows through the can, to the exhaust outlet, which is usually an inch or two off the bottom of the can. The muffler can eventually fills up with water high enough to cover the outlet, at which point, exhaust pressure pushes cooling water up the outlet, over another riser elbow, and downhill to the transom or gunwale outlet. The presence of water in the exhaust system both cools the exhaust (helps prevent Burning Boat Syndrome) and also substantially reduces noise in the harbor and cabin, and keeps carbon monoxide out of the cabin (we hope).

Anyway, in order to force water up and out of that muffler can, the static pressure differential is fairly easy to calculate... lift water an inch, is 1" of Water Column pressure.

It's like 1" of Mercury (1"HG)... but water is lighter than mercury, so...

13.59" of water column = 1" of mercury.

Most water-can mufflers will have about 24" of rise, in order to prevent following seas from getting pushed backwards into the boat.

When you put a vacuum gauge on the manifold of a submerged-exhaust engine, it will indicate vacuum pressure against atmosphere... you must not forget that exhaust pressure offsets that. When engineers marinize engines, they change the cam profile to 'protect' that engine's running character from changes in exhaust pressure from idle to on-plane, or to augment the exhaust lift pressure required of the water-can muffler.

If you've got something obstructing that exhaust, it'll have a dramatic effect on how the engine runs. Pop off the rubber couplings and put an inspection camera in there, and don't be surprised by anything... along with rats, i've found pop cans, plastic bags, ferrets, rope... oh... snakes... big, dead snakes...and a bird...:confused:
 

stevegnh

Registered
Wow, what an amazing post, thank you! So you are saying open up the wet exhaust can and take a look? I do have an inspection camera. The exhaust/thru hull is right at the water line, maybe an inch above it.

Also, while I didn't get pictures or anything else that was mentioned above looked at today, I did exercise the generator while I was doing other projects. Anyway, I had mentioned that it hunts at start up for a little while. It took 50 seconds from start for it to stop hunting and settle into a nice smooth status.
 

dkamp

eMail NOT Working
Yes, take a look inside, make sure there's nothing floating around in there. Make sure the incoming raw water is not obstructed, too.

Wet exhaust comes in on the side of the canister. Top is the outlet, there's a siphon tube on inside going from outlet down to about halfway to the bottom, that defines the cooling level when in operation.
 

stevegnh

Registered
Thanks. I will take a look asap.

What's the feedback on the fact that it takes about 50 seconds for the genset to stop it's hunting after initial startup?
 
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