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?

My first guess would be air in the fuel line.

Check your connections.

How long ago did you change any fuel filters?

Although the Racor may look clean, the on-engine filter may be clogged.

Capt.Bill11 said:

How long ago did you change any fuel filters?

Click to expand...

The fuel filters were changed in January 2016 but I will check their condition.

Good suggestion.

A

Could be a misaligned O ring on one of the filters. Which are actually square and not round O rings.

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...

mike

a real long shot would be something blocking the fuel tank pickup tube. Do you have a vacuum gauge?

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.

Something is restricting the fuel, probably air, could be water, but you said yo checked carefully.

When was last change on engine mounted filters?

manyboats said:

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.

Click to expand...

You put T tape and thread sealant on the same fitting!?

Please say that was a misprint.

Noooooo.. No tape ever, on fuel lines.

To the OP:change all filters before worrying about exotic stuff

"To the OP:change all filters before worrying about exotic stuff"

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

bilge53 said:

"To the OP:change all filters before worrying about exotic stuff"

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

Click to expand...

^^^^^^^^^^^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.

meridian said:

a real long shot would be something blocking the fuel tank pickup tube. Do you have a vacuum gauge?

Click to expand...

Happened to me on my starboard tank.. Blew some compressed air through the lines and cleared it, never to return..

Chubasco88 said:

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?

Click to expand...

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

Capt.Bill11 said:

You put T tape and thread sealant on the same fitting!?

Please say that was a misprint.

Click to expand...

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.

manyboats said:

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.

Click to expand...

"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.

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.

manyboats said:

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.

Click to expand...

When I became a teacher, it wasn't long before I realized that even the most obvious, explicit instructions, would lead to a varied outcome. Pretty much the shape of a bell curve, 60% of the kids would understand reasonably enough. But 40%, would see things I though only existed in alternative universes or maybe Bizzarro World. And that 40% would be all over the place.

OK I told myself, they are kids.

When I became a principal, teachers acted exactly the same. Even having asked clarifying questions and having knowledge the kids don't have, made no difference.

SO, in Sum, nothing is obvious and when you are sure it is; it isn't.

manyboats said:

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.

Click to expand...

My 50 year old brass fuel manifold made of fittings and valves was loose, relied only on tape or sealants to work. What it does is take in 3 tanks and splits to 3 destinations.

I took the entire thing apart. Cleaned the brass. Then I soldered the whole thing threads and all. Now is one solid sealed manifold that cant let in any air.
https://photos.google.com/share/AF1...?key=V0RybFBIOTlFUWdNcXpOUVVac1U1UzRxMm81QWpR

Solved (temporarily?) my air and fuel leak problems by fabricating two fuel manifolds out of brass bars. Before that with three engines, a fuel polisher and four tanks I had no end of leaks.