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Old 01-16-2020, 06:36 AM   #1
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Fuel difference

I have twin 275 hp Ford Lehman Diesel engines . When I fill up the tanks they are usually about 5 gallons apart . Now what is going on is there is about a 28 to 22 gal difference in the tanks upon fill up .. we had to have the motor mounts replaced and props balanced.. ever since then there has been this difference . Also I am now running at 1900-2000 rpms where before I was going 1800 rpms . It is the port tank that is requiring the extra fuel . I am not running the generator.. your thoughts ?
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Old 01-16-2020, 06:46 AM   #2
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Three questions
- is there a cross over line between the tanks?
- could it be that either fuel returns or fuel supply valves are set differently?
- are the tachs accurate?
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Old 01-16-2020, 06:52 AM   #3
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No cross over .. don’t know about returns . Tachs I think are ok , cause when I put both at 1800 the boat stays straight, does not go to one side or the other
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Old 01-16-2020, 06:54 AM   #4
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Greetings,
Mr. K. Could you clarify further please?

-There used to be only a 5 gal difference and now there is a 22 gal difference? Both tanks taking more fuel?

-Were motor mounts replaced on both engines or only the port engine?
-Is it only the port engine that is now running @ higher RPM?
Thanks.
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Old 01-16-2020, 06:57 AM   #5
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Both motor mounts replaced.just port tank taking more fuel . I keep both engines at same rpms
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Old 01-16-2020, 07:09 AM   #6
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Greetings,
Mr. K. With my very limited knowledge, the only thing that may have changed, is the prop pitch, assuming both engines have been aligned accurately.
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Old 01-16-2020, 07:18 AM   #7
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Both were aglined
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Old 01-16-2020, 08:05 AM   #8
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Greetings,
Mr. K. From your description and assuming no changes were made or damage caused to linkages/engine settings (timing etc.) AND everything is aligned as it should be, pretty well the ONLY thing it can be is the props.


Might it be that at some point in the past the props were re-pitched and the changes not stamped in the prop so when you had the props redone, the prop shop returned the stamped settings to "original"?



This IS a head scratcher...


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Old 01-16-2020, 09:00 AM   #9
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The props were done to the original specs as per the docs that came with the boat .
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Old 01-16-2020, 09:37 AM   #10
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Are gears the exact same ratio? It is common for gears on older boats to have a slightly different reduction ratio, so to balance load one prop will have slightly different pitch. If you tuned props to be exactly the same, now load will be not be balanced.

What gears do you have? What is the ratio on each?

Can also phototach both the engines and prop shafts and calc the ratio.
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Old 01-16-2020, 09:42 AM   #11
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F.L.s only return a small amount of fuel, but then again, they don't burn much either. Could be that they return at least part of the discrepancy . There has to be a crossover there somewhere and one tank must be sitting slightly lower.

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Old 01-16-2020, 09:57 AM   #12
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I have been told FL 120s return little fuel but the other models of Ford Lehmans can return a lot more.

Different injection pumps and injectors...so again make sure you double check ANY internet advice and even pro advice including American Diesel should withstand a "second opinion".
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Old 01-16-2020, 10:10 AM   #13
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Quote:
Originally Posted by KEVMAR View Post
No cross over .. donít know about returns .
Chasing down how and where the returns are plumbed seems a worthwhile endeavor.
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Old 01-16-2020, 11:17 AM   #14
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Do both engines reach the same WOT?
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Old 01-16-2020, 11:20 AM   #15
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Some additional info would be helpful. How big are your tanks and how far down are they when you fill them up? A 5% delta in fuel consumption between the engines would not be a concern. 20% would. When you say 22-28 gal difference, I assume you have filled up more than once - same fuel dock? Clogged or constricted vent on your tank could cause the fuel hose to shut off prematurely, though odd coincidence (as would be the return lines, but agree with Sunchaser its a worthy exercise).

What prompted the motor mount/prop replacement?
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Old 01-16-2020, 11:23 AM   #16
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Quote:
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I have been told FL 120s return little fuel but the other models of Ford Lehmans can return a lot more.
My understanding is Detroit's set the hi-watermark for returning fuel and is the defacto benchmark for comparison. Everything else is a far, far, distant second. Doesn't mean they return negligible fuel, just in comparison to the 2-stroke DDs, it's a drop in the bucket. That said, I've never seen any stats on exactly what the volumes actually are.
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Old 01-16-2020, 11:51 AM   #17
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If you donít have a crossover, are you drawing from both tanks simultaneously while underway? If so, and youíre drawing through a manifold, you may have a crimp in a fuel line somewhere that causes a slight restriction leading to the imbalance. (All this is total conjecture.)
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Old 01-16-2020, 11:52 AM   #18
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I don’t know what the problem is or what exactly you are asking but the difference may be because one transmission is running in reverse to get the counter-rotation and the ratios will be slightly different. Your wheels should have been adjusted for this difference.
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Old 01-16-2020, 12:01 PM   #19
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Quote:
Originally Posted by mvweebles View Post
My understanding is Detroit's set the hi-watermark for returning fuel and is the defacto benchmark for comparison. Everything else is a far, far, distant second. Doesn't mean they return negligible fuel, just in comparison to the 2-stroke DDs, it's a drop in the bucket. That said, I've never seen any stats on exactly what the volumes actually are.
OK...it IS helpful to know FACTS when eliminating troubleshooting issues.


If Ford 120's return almost zero, 225's return near consumption numbers (such as 2-3gph) and.....well ....whatever Detroits return....I would say finding out a more exact number might be more useful than worrying what Detroits bring to the party.


Over the course of a fill up/tank burn.... it only takes a small amount to account for 20/30 gallon difference.
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Old 01-16-2020, 12:42 PM   #20
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I have SP225s in my President 41. Typically one of my tanks takes a bit more fuel but not enough to concern me. My boat does have a crossover but it is closed. I suspect that your President does have a crossover but mine isnít very noticeable so you may have one and just not noticed it yet. I didnít notice mine for about a year.

How many gallons are you putting in to get the 20+ gallon difference? If it is 100 gallons then I would be concerned. If it is the entire tanks worth then 20 or so gallons may not be a problem.

Have you talked to Brian at American Diesel? I would get his opinion and also ask him how much does the 275 return as compared to a 120. Bob Smith said that a 120 would return about a pint to a quart in a days running. I donít know about the turbo charged engines as to how much they return. Maybe you are returning fuel from both engine to the same tank.

Keep us informed as to what you find out.
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