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Old 08-22-2011, 05:21 PM   #1
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120 lehman fuel economy

I have a 34* MT with a 120 lehman turning a 21''x16'' prop*tru a 2-1 gear** I have put over 1400hrs in the last 4 years often running 12hr*days* I have cruised at 1650 rpm to 1800 rpm, my question is...* What is the best rpm for this motor for economy?* It seems the temp rises just a bit at 1800* maybe I am running to easy at 1650 rpm* Is there reallt ant difference?* How often are you guys adjusting the valves on these motors** This has been a great motor never missing a beat
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Old 08-22-2011, 05:34 PM   #2
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RE: 120 lehman fuel economy

I've attached an engine performance curve chart - hard to read, I'm afraid, but it does give a few hints:

Engine efficiency is the SFC chart on the bottom - 1600 to 1800 is the sweet spot and there's virtually no difference between the two.* Your fuel burn per mile is probably higher at 1800, but that's just from trying to make the boat go faster - the engine is giving the same amount of power per unit of fuel burned.

Note that the "continuous operating range" on the bottom goes from about 1250 to 2500 RPM, so I doubt you're running it too hard or too easy.
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Old 08-22-2011, 08:10 PM   #3
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RE: 120 lehman fuel economy

From everything I've ever been told about the FL120 from people who made careers working on the base engine in England to American Diesel to our diesel shop to others, the engine is "happiest" in the 1500-1800 rpm range. We cruise at about 1650 which gives us a combined fuel burn of about 5.5 gph. I've not measured that with any accuracy, it's just what the burn seems to be based on the day tank fuel gauge.

The FL120 operators manual calls for the valve clearances to be checked/adjusted every 400 hours. Bob and Brian Smith at American Diesel say to adjust them every 2,000 hours. Take your pick......
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Old 08-22-2011, 08:14 PM   #4
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120 lehman fuel economy

Chris
thanks that is great the motor sounds like its runnung easier at 1650 then higher motor runs great but even in netural will not turn over 2105rpm had a the injector pump replaced same rpm but runs smooth and clean

after hundreds of hours I have loged fuel burn between 2.2 to 2.8** gph** but mostly 2.4gph does this sound rite?** Starts at the turn of the key and smokes a little untill warm


-- Edited by motion30 on Monday 22nd of August 2011 08:20:11 PM
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Old 08-22-2011, 11:08 PM   #5
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RE: 120 lehman fuel economy

(Disclaimer:* I'm no Lehman expert...)

But 2-3 GPH sounds very reasonable.*

Not making full RPM in neutral sounds odd, however.* How certiain are you of your tachometer calibration?* The injection pump likely governs maximum RPM, so the fact that it's indicating the same before and after a pump replacement seems like there may be an issue with the instrument.

I use an inexpensive handheld photo tach to set idle speed and check the tachs:

http://www.amazon.com/gp/product/B000EUT9ZS

*
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Old 08-23-2011, 05:38 AM   #6
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RE: 120 lehman fuel economy

Quote:
More interesting it makes the same 2150rpm underway as in neutral and no I am not sure about the tachs but both upper and lower tach agree* not really a problem as I never run it that hard

*

*

*

*

*

(Disclaimer:* I'm no Lehman expert...)

But 2-3 GPH sounds very reasonable.*

Not making full RPM in neutral sounds odd, however.* How certiain are you of your tachometer calibration?* The injection pump likely governs maximum RPM, so the fact that it's indicating the same before and after a pump replacement seems like there may be an issue with the instrument.

I use an inexpensive handheld photo tach to set idle speed and check the tachs:

http://www.amazon.com/gp/product/B000EUT9ZS

*
*
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Old 08-23-2011, 05:47 AM   #7
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RE: 120 lehman fuel economy

2150 underway , 1800 cruise sounds just fine to me .

300RPM under max is usually all that is required.
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Old 08-23-2011, 09:09 AM   #8
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RE: 120 lehman fuel economy

Fred,

Do you mean that 300rpm down from max is good for continuous operation? I agree for older engines but 200 down could be fine for newer engines and perhaps older engines rated for industrial use. Is that what your'e say'in Fred?
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Old 08-23-2011, 09:37 AM   #9
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RE: 120 lehman fuel economy

I usually run mine between 1600 and 1700. I find that if I run it around 1900 the fuel consomption rises considerably (all things being relative here) for very little gain in speed.
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Old 08-23-2011, 09:41 AM   #10
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RE: 120 lehman fuel economy

The slower you turn the engine, the better the economy on the 120. If you really want to stretch your fuel run 1500-1550 rpm and get 1.5 gph burn. At 1750 you can expect 2.0 - 2.1 gph. My boat loses 1 knot of speed if I drop the rpm from 1750 to 1500. (7 kph down to 6 kph) While many tend to scoff at FloScan meters, it is a useful device if you want to see exactly how much fuel you are burning at any given time.

If you are in a hurry, run at hull speed, burn 2 gallons an hour and wave to all the Bayliners burning a gallon or more per mile.

Watch your water temp and/or oil temp either by the guage on the dash or with an infrared no contact thermometer and see that everything stays up to temp.
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