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Old 03-29-2016, 01:49 AM   #1
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Lehman SP135 Loses Revs

Towards the end of a 5 hour trip over the Easter break I noticed the revs on the engine drop some 200 rpm and then return to where the throttle was set. This happened in smooth water so I cannot lay the blame on water in the fuel.

Travelling at 6.5 kts at 1780 revs. Motor has 1360 hours on the meter.

Fuel filters checked and there was no water and no 'black death' to clog the filter.

Tanks were near full and distillate had been treated with anti-fungal additive.

This occurred 5 or 6 times on the return journey.

Any ideas what I should be looking at? Injecters. Fuel Pump or injector pump?
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Old 03-29-2016, 02:06 AM   #2
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My first guess would be air in the fuel line.

Check your connections.
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Old 03-29-2016, 04:12 PM   #3
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How long ago did you change any fuel filters?
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Old 03-29-2016, 04:14 PM   #4
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Although the Racor may look clean, the on-engine filter may be clogged.
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Old 03-29-2016, 04:58 PM   #5
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Quote:
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How long ago did you change any fuel filters?
The fuel filters were changed in January 2016 but I will check their condition.

Good suggestion.
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Old 03-29-2016, 05:27 PM   #6
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A

Could be a misaligned O ring on one of the filters. Which are actually square and not round O rings.
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Old 03-29-2016, 08:33 PM   #7
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Had exactly the same issue Saturday on my return trip but were only out for maybe 2 hours. We were running at 1800 and it dipped to 1500rpm for a few seconds then returned to 1800. Happened three times. Will check filters again but didn't think of air in the system. Engine is 120hp Lehman.

Well timed! Glad I read a lot...

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Old 03-29-2016, 10:01 PM   #8
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a real long shot would be something blocking the fuel tank pickup tube. Do you have a vacuum gauge?
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Old 03-29-2016, 10:28 PM   #9
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I had a cronic engine loosing rpm problem and it finally did quit. Some very nice people came along and towed us back to port.

Later I found that the issue was thread sealant falling away and drifting along in the fuel lines. I redesigned the fuel manifold (slighty) and replaced all the fuel lines and hose clamps. I used thread sealer (both teflon tape and goo) on all the pipe threads but kept the sealant away fron the ends of the pipe threads so the sealant could'nt get into the fuel. Only been one 4-5 day cruise since but rpm is steady now. It didn't fluctuate at all and I hope the problem is solved.
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Old 03-30-2016, 09:15 AM   #10
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Something is restricting the fuel, probably air, could be water, but you said yo checked carefully.

When was last change on engine mounted filters?
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Old 03-30-2016, 09:21 AM   #11
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I had a cronic engine loosing rpm problem and it finally did quit. Some very nice people came along and towed us back to port.

Later I found that the issue was thread sealant falling away and drifting along in the fuel lines. I redesigned the fuel manifold (slighty) and replaced all the fuel lines and hose clamps. I used thread sealer (both teflon tape and goo) on all the pipe threads but kept the sealant away fron the ends of the pipe threads so the sealant could'nt get into the fuel. Only been one 4-5 day cruise since but rpm is steady now. It didn't fluctuate at all and I hope the problem is solved.
You put T tape and thread sealant on the same fitting!?

Please say that was a misprint.
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Old 03-30-2016, 10:32 AM   #12
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Noooooo.. No tape ever, on fuel lines.
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Old 03-30-2016, 10:34 AM   #13
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To the OP:change all filters before worrying about exotic stuff
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Old 03-30-2016, 12:04 PM   #14
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"To the OP:change all filters before worrying about exotic stuff"

...and change the gaskets making sure the groove and mating surfaces are clean.
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Old 03-30-2016, 12:18 PM   #15
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Quote:
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"To the OP:change all filters before worrying about exotic stuff"

...and change the gaskets making sure the groove and mating surfaces are clean.
^^^^^^^^^^^THIS^^^^^^^^^^^

I could almost guarantee that if you change all filters this issue would go away. It is either a clogged filter or a gasket at the filter leaking air. If this does not solve the problem, THEN go looking elsewhere. But I am willing to bet this is all you will need.
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Old 03-30-2016, 12:21 PM   #16
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a real long shot would be something blocking the fuel tank pickup tube. Do you have a vacuum gauge?
Happened to me on my starboard tank.. Blew some compressed air through the lines and cleared it, never to return..
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Old 03-30-2016, 12:39 PM   #17
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Quote:
Originally Posted by Chubasco88 View Post
Towards the end of a 5 hour trip over the Easter break I noticed the revs on the engine drop some 200 rpm and then return to where the throttle was set. This happened in smooth water so I cannot lay the blame on water in the fuel.

Travelling at 6.5 kts at 1780 revs. Motor has 1360 hours on the meter.

Fuel filters checked and there was no water and no 'black death' to clog the filter.

Tanks were near full and distillate had been treated with anti-fungal additive.

This occurred 5 or 6 times on the return journey.

Any ideas what I should be looking at? Injecters. Fuel Pump or injector pump?
These symptoms are classic of a clogged filter and it is fairly common. You will not be able to "see" the filter is clogged. CHANGE the filter before you try anything else.

Ken
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Old 03-30-2016, 02:24 PM   #18
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Quote:
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You put T tape and thread sealant on the same fitting!?

Please say that was a misprint.
Bill,
Before you say something that tends to make one look like a fool you should read what is said. You obviously have a reading comprehension problem. Read the post and tell me where I said I had put both tape and goo on the same fitting.
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Old 03-30-2016, 02:58 PM   #19
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Bill,
Before you say something that tends to make one look like a fool you should read what is said. You obviously have a reading comprehension problem. Read the post and tell me where I said I had put both tape and goo on the same fitting.
"I used thread sealer (both teflon tape and goo) on all the pipe threads"

Here you go. Note the words "both" and "all".

I doubted you would really use "both" on "all". That's why I asked if it was a misprint.

It was just meant as a friendly jab Eric and to enlighten those who might think you should/could use both on all. So lighten up.
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Old 03-30-2016, 09:44 PM   #20
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Bill,
OK I'll put my guns down.
My wording was a bit foggy. I just assumed it would be obvious you would'nt use both on the same fitting. Sorry about all the shoot'in. One could assume the obvious though.
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