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Old 09-04-2018, 06:49 PM   #1
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Lehman 120, Low RPM under load, OK in neutral

My friend and I both own Grand Banks boats with the Lehman 120.

On his twin engine Alaskan one engine has an issue where it can only reach about 11-1200 RPM under load when traveling but stopped in neutral the engine can be revved up to 2000 RPM easily.

He thought he had maybe picked up something in the water and wrapped it around the prop so had a diver buddy check it out but both props spin freely and nothing was on the shaft.

We changed fuel filters and bled the system to make sure the fuel was not restricted and checked transmission oil levels and oil odor for any sort of issue.

The only other clue is that he says if he keeps running it at the low RPM the engine coolant temp starts to rise, that´s why he thought something was wrapped on the prop shaft.

The Exhaust color seems normal.

Any ideas on what to try next? Could it still be a fuel issue?

thanks, Andy
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Old 09-04-2018, 08:44 PM   #2
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Get some prop diameters and pitch numbers from guys running the same engine and gear.
With that if they are all not overpropped you should know what dia and pitch you will need to load your engine correctly. It’s that simple.
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Old 09-04-2018, 09:17 PM   #3
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Start by calibrating tachometers with digital tachometer from Amazon.
Max rpm unloaded is 2400 but if you can hit 2200 you're OK.
Next check linkages to make sure movement at throttle translates to equal movement at engine . Then disconnect linkage and check top rpm again.
If valve clearance hasn't been checked in awhile it's time to do it.
If exhaust is normal color it probably isn't worn rings, valves or bore.
That leaves us with injectors or injection pump. Pull the injectors and send them out for testing and/or cleaning. Injection pump servicing is not for beginners and I don't have any great ideas on that one. I'd probably put in a new fuel pump before tearing apart the injection pump. They're inexpensive and a leaky diaphragm could cause problem.

Rising coolant temp would have me pulling the end off the heat exchanger to check for obstructions.
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Old 09-04-2018, 11:02 PM   #4
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I thought max rpm (rated under load) was 2500 for the FL. Should rev in neutral about 2700.

But only using one engine it’s a bit like being tied to the dock/float.
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Old 09-05-2018, 06:38 AM   #5
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Something is seriously wrong. Most diesels will rev 2-300 rpm past their wot rated hp rpm, in this case 2,700-2,800 rpm, not 2,000. Also The maximum rpm you can get under way is half of what would be expected.


Severe binding of the prop shaft or fouling of the prop and hull can cause the latter, but I would expect lots of black smoke as well. Overheating might also occur as you try to produce max hp at lower rpms. But about the only thing that would limit wot rpm in neutral is a fuel restriction or a governor setting.



So you probably have 2 or maybe 3 problems occurring simultaneously: fuel restriction being the main one or a governor set way to low, combined with a compromised cooling system.


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Old 09-05-2018, 06:53 AM   #6
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You need calibrate the tachometer s first.
If not you have no idea what the engine is really doing.
You will only be guessing.
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Old 09-05-2018, 09:32 AM   #7
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Quote:
Originally Posted by Nomad Willy View Post
I thought max rpm (rated under load) was 2500 for the FL. Should rev in neutral about 2700.
You're correct. Max governed speed no load - 2650, under load 2500.

The transmission oil pump is always running so I presume there is some load. I'm also guessing a 1967 lehman 120 is not going to come close to original specs leading to my guess if you're hitting 2200 you're good to go. I would run through the common issues and shoot for 2400 but I wouldn't tear apart the injection pump to do it. How many are running their 120's over 1800?
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Old 09-05-2018, 11:46 AM   #8
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SoWhat,
You’re say’in old men can’t run fast nor should old engines?
I think if they’re in good condition they should run to specs.
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Old 09-05-2018, 01:07 PM   #9
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SoWhat,
You’re say’in old men can’t run fast nor should old engines?
I think if they’re in good condition they should run to specs.
Well if my knees are any example, an old engine could be worn out...
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Old 09-06-2018, 05:54 AM   #10
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"I think if they’re in good condition they should run to specs."

There are specs and there are specs.

An industrial rated engine decades old might take the loads ,
but the tractor engines are known for long life , no one uses them for high output.
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