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Old 06-06-2015, 14:20   #1
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Lack of Compression on a Kubota one cylinder

Hi All:

My generator has been out of commision for a few years now. It is a horizontal one cylinder engine. The exhaust valve rusted out because water got in the fuel and sat there for a few months. I had a seat recut. I have rebuilt the head and when I do a leak down test I have 90% hold. Seems like it should be ok. When I plug in the compression tester I only get 200psi. Should be 500 psi. I am doing the compression test with an old head gasket.

What should I be looking for?

1) I have a new head gasket but don't want to put it on until I am sure that everything is good. The original gasket meets the thickness speck of 1mm.

2)I don't hear any air coming out of the radiator.

3) Both the exhaust and the intake side of the engine do not leak much air as evidenced by the 90% retention on the leak down test.

I'm stumped. Would appreciate any ideas people might have on what to check next.
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Old 06-06-2015, 15:20   #2
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Re: Lack of Compression on a Kubota one cylinder

G'day Charlie,

Interesting situation! If you are convinced that the valves are not leaking, then there are only a few possibilities that come to mind:

1. The old head gasket isn't sealing well... have you tried gooping it up with Permatex or other sealant?

2. The engine isn't cranking at full speed, leading to low compression readings.

3. The rings are not doing well after sitting so long. Could try the o ld trick of putting some heavy oil into the bores to temporarily improve the seal (possibly hard on a horizontal engine!)

Have you tried starting it with the old gasket? Perhaps if it ran a few minutes things would bed in and the compression would improve.

Finally, you could bite the bullet and put the new gasket in, invoking whatever gods of things mechanical you subscribe to and hope for the best!

Good luck, mate!

Jim

PS Used the hose removing tool just yesterday... successfully. thought kind thoughts about you!
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Old 06-06-2015, 17:19   #3
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Re: Lack of Compression on a Kubota one cylinder

Had a quite similar situation with a Phasor 3.5 generator last year. Took it in trade as partial payment for a job and it sat for about 5 years before I looked at it.

Condition seemed almost identical to the one you describe, the exhaust valve was shot, plus the liner looked a little corroded.

I was able to get it running by honing the cylinder, and grinding the valves but it didn't run very well at all, lots of blowby Wound up replacing the exhaust valve, liner, rod bearing (since I was in there anyway) and rings. If I remember correctly, together the parts were less than 150.00 from the local Kubota dealer.

If the liner isn't too bad you might be able to get away with just honing the cylinder and new rings.

Jim's idea of siliconing the old gasket to test the compression is good, I actually initially ran mine with the old gasket siliconed to seal it so I could test that the generator end had output.
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Old 06-06-2015, 20:22   #4
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Re: Lack of Compression on a Kubota one cylinder

500 psi is being way over optimistic unless it has a comp ratio approaching 35.1 look for closer to 320-350psi given you have 90% pressure retention possible piston top erosion and valve regression your engines problem.
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Old 06-06-2015, 20:37   #5
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Re: Lack of Compression on a Kubota one cylinder

Did you check the compression release lever? It could be stuck on, limiting how much the valve closes. Its does not hold it much, just enough to lower the compression for hand starting.
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Old 07-06-2015, 09:36   #6
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Re: Lack of Compression on a Kubota one cylinder

Hi charlie,

You helped me a year or so back with my next gen 3.5, so maybe i can return the favour!

I had a similar starting problem, turned out one of the valves clearances had closed and hence no comp!

Might be worth checking? Should be 7/1000 of an inch for each.

Incidently, my genset has decided to not start now too!! Another day or so in the bilges for me i think!

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Old 07-06-2015, 10:49   #7
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Re: Lack of Compression on a Kubota one cylinder

Quote:
Originally Posted by jimbunyard View Post
Had a quite similar situation with a Phasor 3.5 generator last year. Took it in trade as partial payment for a job and it sat for about 5 years before I looked at it.

Condition seemed almost identical to the one you describe, the exhaust valve was shot, plus the liner looked a little corroded.

I was able to get it running by honing the cylinder, and grinding the valves but it didn't run very well at all, lots of blowby Wound up replacing the exhaust valve, liner, rod bearing (since I was in there anyway) and rings. If I remember correctly, together the parts were less than 150.00 from the local Kubota dealer.

If the liner isn't too bad you might be able to get away with just honing the cylinder and new rings.

Jim's idea of siliconing the old gasket to test the compression is good, I actually initially ran mine with the old gasket siliconed to seal it so I could test that the generator end had output.
The part that is throwing me off is that the leak down test is holding such high numbers. I have no problem spending money to repair the engine but I don't want to throw new parts at it. I am not an expert at engine rebuilding. My understanding is that if the leakdown test holds then the rings are good. Is that correct?

Quote:
Originally Posted by shakey doug View Post
500 psi is being way over optimistic unless it has a comp ratio approaching 35.1 look for closer to 320-350psi given you have 90% pressure retention possible piston top erosion and valve regression your engines problem.
Seems high to me as well but that number was taken from the manual. Actually it was 501 psi with a minimum of 377psi just double checked it.

Quote:
Originally Posted by sailnow2011 View Post
Did you check the compression release lever? It could be stuck on, limiting how much the valve closes. Its does not hold it much, just enough to lower the compression for hand starting.
Actually had to run my test without the valve cover on because my adapters wouldn't fit with the valve cover on.
Quote:
Originally Posted by Sulaire View Post
Hi charlie,

You helped me a year or so back with my next gen 3.5, so maybe i can return the favour!

I had a similar starting problem, turned out one of the valves clearances had closed and hence no comp!

Might be worth checking? Should be 7/1000 of an inch for each.

Incidently, my genset has decided to not start now too!! Another day or so in the bilges for me i think!

Charlie .


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Thanks Sulaire but I adjusted the valves to the middle of the spec.

Quote:
Originally Posted by Jim Cate View Post
G'day Charlie,

Interesting situation! If you are convinced that the valves are not leaking, then there are only a few possibilities that come to mind:

1. The old head gasket isn't sealing well... have you tried gooping it up with Permatex or other sealant?

2. The engine isn't cranking at full speed, leading to low compression readings.

3. The rings are not doing well after sitting so long. Could try the o ld trick of putting some heavy oil into the bores to temporarily improve the seal (possibly hard on a horizontal engine!)

Have you tried starting it with the old gasket? Perhaps if it ran a few minutes things would bed in and the compression would improve.

Finally, you could bite the bullet and put the new gasket in, invoking whatever gods of things mechanical you subscribe to and hope for the best!

Good luck, mate!

Jim

PS Used the hose removing tool just yesterday... successfully. thought kind thoughts about you!
Jim glad the tool was of use. I have one on the boat and one at home use them alot. I'm going to try removing the head and coating the gasket.

Another thought that I have just had is that perhaps the tester is not accurate. It is only getting 200 psi so I am going to try my gas engine tester and see how that works. Just seems strange to me that you can have a good leakdown test and low compression.

Thinking out loud here -- if the rings are sealing tight ( I think I might have replaced them) and the cylinder is at TDC on the compression stroke then would the lead down test not test the holding power of the head gasket.

Ok I'm off to goop up the old gasket and see if that raises my compression.
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Old 07-06-2015, 13:15   #8
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Re: Lack of Compression on a Kubota one cylinder

Did you do a top and bottom leak down test?
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Old 07-06-2015, 13:55   #9
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Re: Lack of Compression on a Kubota one cylinder

Don't know what a bottom leak down test is I take it you do BDC does it matter which stroke?
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Old 07-06-2015, 14:13   #10
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Re: Lack of Compression on a Kubota one cylinder

Quote:
Originally Posted by Charlie View Post
The part that is throwing me off is that the leak down test is holding such high numbers. I have no problem spending money to repair the engine but I don't want to throw new parts at it. I am not an expert at engine rebuilding. My understanding is that if the leakdown test holds then the rings are good. Is that correct?

.
I'm a little confused. All the compression testers I have have a check valve in the part that screws into the head, if one doesn't relieve the pressure the gauge can be unscrewed from the head and it will retain the reading. I've run tests with a homemade gauge without check valves and have never seen an engine that will hold it's compression for more than a few seconds in a static position. My understanding is that the pressure reading is a cumulative, dynamic number that represents the pressure that a particular cylinder is capable of producing while running.

I think that if your engine won't produce somewhere around 400 psi, there are problems with either ring sealing, head gasket sealing or valve sealing.

Did you do a fluid leak test after working on the valves?

I don't like the parts-changer approach either, much better to figure out what's wrong first.
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Old 07-06-2015, 17:35   #11
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Re: Lack of Compression on a Kubota one cylinder

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Originally Posted by jimbunyard View Post
I'm a little confused. All the compression testers I have have a check valve in the part that screws into the head, if one doesn't relieve the pressure the gauge can be unscrewed from the head and it will retain the reading. I've run tests with a homemade gauge without check valves and have never seen an engine that will hold it's compression for more than a few seconds in a static position. My understanding is that the pressure reading is a cumulative, dynamic number that represents the pressure that a particular cylinder is capable of producing while running.

I think that if your engine won't produce somewhere around 400 psi, there are problems with either ring sealing, head gasket sealing or valve sealing.

Did you do a fluid leak test after working on the valves?

I don't like the parts-changer approach either, much better to figure out what's wrong first.
I did a fluid leak test on the head. It didn't drip at all after 45 minutes.

Interesting point about the compression tester. Mine was not holding -- as if there was no check valve. I wonder if there is a problem with the tester.

I didn't remove the head and silicone the gasket yet. I think that is my next attempt. Just being lazy this afternoon.
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Old 07-06-2015, 18:20   #12
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Re: Lack of Compression on a Kubota one cylinder

Which model Kubota do you have? Have you checked the bump clearance? Have you checked the valve regression? What pressure are you using when per doing the leak down test?
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Old 07-06-2015, 18:40   #13
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Re: Lack of Compression on a Kubota one cylinder

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Don't know what a bottom leak down test is I take it you do BDC does it matter which stroke?
Yes you do a TDC and BDC leakdown test. You would want them both to be on the compression stroke when both valves are closed of course. BDC at the start of the compression stroke and TDC at the end of it.

I did a search and couldn't find any reference to testing at BDC. One reference says that the results would be erratic if the engine is not at TDC but I think that might be as a result of piston movement. I can't see why the results would be any less valid at the bottom though, can you? Maybe I'm thinking of aircraft testing top and bottom. Ah, memory...
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Old 07-06-2015, 21:38   #14
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Re: Lack of Compression on a Kubota one cylinder

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Originally Posted by shakey doug View Post
Which model Kubota do you have? Have you checked the bump clearance? Have you checked the valve regression? What pressure are you using when per doing the leak down test?
it is the EA 300 NB I ran the leak down test at 50 psi

Valve regression and bump clearance are over my head. I don't know what they mean.

@ Deep Freeze. I'm not an expert on this type of thing and have never heard of a BDC leakdown test.

So the latest is that I get smoke coming out of the exhaust when I run it but the smoke is not very warm and it is not oily. I think that my next effort should be to take the head off and put some gasket sealer on the head gasket and see if I get a better compression rating. I ordered a bolt that will fit in the injector hole and I plan on manufacturing an adapter for my compression tester.
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Old 07-06-2015, 21:55   #15
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Re: Lack of Compression on a Kubota one cylinder

I had a similar problem on my YSM 8 which is horizontal too. I would try startn it with a squirt of engine oil in the air intake when you are cranking it over, this should raise the compression high enough for it to fire. ( I have been using this method of starting for months) Run it for half an hour till things settle down as your rings may have got a bit sticky. I recommend Loctite 510 master gasket for re-using your head gasket as I have used this multiple times with no problems with a damaged gasket. My liner is stuffed which is why I need the oil injection to start. If your exhaust valve rusted I would be a little suspicious about the state of the liner too.
Good luck with it
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