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Old 07-29-2019, 10:45 AM   #1
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Lehman SP135's not reaching full rated rpm

I am looking at a 48 defever with twin Lehman SP135's. They are only reaching 2150 rpm at full throttle, when top rpm should be 2600 rpm. Could this be over propped or something else?
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Old 07-29-2019, 11:06 AM   #2
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2600 is unloaded rpm in neutral. Once you put it i gear max rpm will be somewhat less. 2150 sounds low so I suspect it might be a bit over propped.
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Old 07-29-2019, 06:14 PM   #3
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Originally Posted by Dennyg View Post
I am looking at a 48 defever with twin Lehman SP135's. They are only reaching 2150 rpm at full throttle, when top rpm should be 2600 rpm. Could this be over propped or something else?
2150 is definitely a bit low. The 135 owners manual specs the max RPM in neutral should be 2850 and in gear 2600. If its twins and they're both doing the same thing, could be overpropped or very dirty bottom. Its possible that the tach's are not reading correctly if alternators were changed and I'm sure there may be another few odd items, but both engines doing the same thing limits the causes.

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Old 07-29-2019, 06:27 PM   #4
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Yes, it should reach 2600 in gear. I would check the bottom first thing because of the KISS principle. Do the easy thing first, check the bottom and running gear, then check and see if it is overpropped. Then look for engine problems. If you get to the engine problems, Brian at American Diesel is the guru.
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Old 07-29-2019, 06:54 PM   #5
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My 1983 Lehman manual states gross horsepower is 135 bhp at 2600. Industry standard defines gross horsepower as unloaded measurement. Note that brake horsepower at that time was probably measured without engine accessories such as alternator and water pump and does not address transmission loss. I would check with Brian before trying to redline the engine.

I would check the max speed stop in the injection pump speed control lever before anything else. PO may have set it wrong and put the screw all the way out to act as a governor of sorts? I would then check the tach accuracy with a laser tach.
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Old 07-29-2019, 08:46 PM   #6
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The low-hanging fruit, to me, is to check with a laser tach first in case—as kchase suggests—the analog tachs are misreading due to alternator change-outs.
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Old 07-30-2019, 08:25 AM   #7
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Agree with the numbers neutral at 2850 and gear at 2600. Mine had the same issue, I called american diesel and brian confirmed these numbers.

If you don't reach 2850 in neutral, it's because somebody dialed your limit screw back to prevent it from reaching that rpm. Unnecessary, but doesn't hurt the engine. If you do reach 2850 in neutral and don't reach 2600 in gear, it means you're over-propped. According to brian, bad for the engine - like leaving your car struggle in high gear as you're going up a hill.
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Old 07-30-2019, 02:10 PM   #8
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Thanks everyone for all the great advice. Talked to Brian at American Diesel. He suggested that it sounds over propped. Unfortunately this boat has had these large 4 bladed props on for 4 or 5 years. He also said that there would be excess soot in the oil analysis, provided that the oil was checked after 50+ hours of operation. This would show that the engines were not operating correctly. unfortunately the seller had just recently changed the oil. For this and a variety of other reasons I have decided to say no to this boat. The search continues...….
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Old 07-31-2019, 06:10 AM   #9
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"Unfortunately this boat has had these large 4 bladed props on for 4 or 5 years. He also said that there would be excess soot in the oil analysis, provided that the oil was checked after 50+ hours of operation. This would show that the engines were not operating correctly."

The PO ma have been knowledgible and installed cruising props to lower fuel burn extend engine life and make the boat quieter.

Ask how he cruises , if the engine can advance the throttle at normal cruise and you see at least 300 RPM and no black smoke , you are not "overproped.

Matching the engine cruise requirements to the prop makes the boat delightful.
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