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Old 05-29-2023, 01:41 PM   #1
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New GB32 owner here... normal RPM range?

I have finally brought my "new" 1969 GB32 home after a comically extended head gasket job (4 day job took 7 weeks, don't ask me, I don't want to talk about it and I'm still kinda mad).

Standard Ford-Lehman 120, single fuel filter (old model), Velvet Drive. I haven't even seen the prop yet so not sure of its specs.

On delivery I noticed that I couldn't push the revs up past 1800. Is this normal? At about 1775 or so advancing the throttle didn't do much, and we seemed to max out at 1800.

I would not normally motor at more than 6.5 knots anyway so this doesn't bother me too much, just curious. I seem to be getting about 5l/hr at 1600 RPM and moving pretty well considering the poor old girl has a lot of seafood salad underneath because previous owner hasn't hauled out for a while.

But since there was recent engine work I wanted to make sure they didn't make some kind of error reassembling everything.
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Old 05-29-2023, 03:25 PM   #2
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Congrats on your new boat. Not sure exactly what the max RPMs should be but around 24/2500. So something isnít right. Does the engine sound like it is straining at full throttle? It could be as easy as the control cable isnít adjusted correctly.
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Old 05-29-2023, 03:40 PM   #3
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Congrats on your new boat. Not sure exactly what the max RPMs should be but around 24/2500. So something isnít right. Does the engine sound like it is straining at full throttle? It could be as easy as the control cable isnít adjusted correctly.
Nope, no bogging down or straining (that I can tell). The bottom is quite foul (been a while since PO did last haulout) and the prop might be kind of mussely also. I will check on the control cable and throttle travel.

I am making 7 kts at about 1800 so is it possible my system is set up with more pitch and different gear ratio? it is a very old boat (1969).
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Old 05-29-2023, 04:46 PM   #4
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It's really impossible to make any assessment until the bottom is cleaned. That's definitely a contributor, if not the entire cause of the low RPMs. See where things are once the bottom is clean, and go from there.
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Old 05-29-2023, 05:33 PM   #5
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Verify the engine reaches high idle (max rated) rpm while in neutral. Then if your max rpm in gear is 1800, pushing it further should result in black smoke as it over fuels trying to gain rpm. If no black smoke it could be fuel restriction somewhere in the system. A clean bottom, clean air and fuel must be verified, a vacuum gauge on the primary filter would be helpful.

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Old 05-29-2023, 05:48 PM   #6
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Could be several things to cause failure to gain revs.

Is there any black or heavy smoke? Then that could be seriously fouled prop and /or bottom. You mentioned the "lettuce salad" on the bottom so maybe that has a lot to do with the low revs. Any prop. damage or to much pitch can contribute to lack of revs.
You will need to get this dealt with to eliminate the fouling as a cause.


If there is little or no heavy/dark smoke then maybe the engine is not getting enough fuel to power up any further. Dirty filters, clogged , can do this also.
If the engine cannot get enough fuel it cannot make power.

Once done with these two items then try again.

With a FULLY WARMED engine, a 1 hour run, you might try going to full throttle in NEUTRAL called a HIGH IDLE check for just long enough to find out if indeed the engines will get to their full revs.
I think that is about 2,400 rpm. Only long enough to see what they attain.
Write it down. Your governor will stop the engines from going past the mfgrs. absolute maximum rpm they have determined.

I also suggest that you purchase a handheld laser pointer digital photo tachometer for about $20.00 from Ebay or Amazon. THese inexpensive tachs are accurate to +/- 2 rpm. NOt robust but more than good enough to check the actual revs. Many dash tachs can be way off making things look better or worse than they are.

Plugged , fouled fuel filters can also cause lack of revs but there will not be the dark smoking. Change the filters to be sure. Without fuel the engine cannot make HP. You will need to reprime the engine afterwards. Shut of the tank feeds while this filters are changed so you loos as little fuel from the lines as possible.

Others more competent than I can talk you through your priming effort. My engine is quite different.
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Old 05-29-2023, 06:17 PM   #7
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Before i go too nuts. You already said she got a dirty bottom. But also check calibration on RPM gauge. Just because it says 1800rpm doestn mean shes turning 1800
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Old 05-29-2023, 06:27 PM   #8
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I second the photo tach idea. When I bough a 73 GB 36 with twin lehmans on it the port engine had about 200 rpm less than the stbd at same throttle positions. The engines sounded like they are at equal RPM so I bought one on amazon and set it up. They were both at same RPM and it was the tach that was off.

Priming a FL120 is not too bad. Here is the section on bleeding from the manual:

If any part of the fuel system is disconnected or air has entered the system, it will be necessary to remove all air from the fuel and to prime the injection pump by bleeding.
Bleed the Minimec Fuel System asfollows:-
1. Ensure that all fuel pipe connections are tight and that there is sufficient fuel in the tank.
2. Loosen the two bleed screws on the top oft h e filter two or three turns (see Fig. 38) and operate the priming lever on thefuel lift pump. Continue pumping until a stream of fuel, free of air bubbles, issues from the filter.
NOTE.-If the eccentric on the injection pump
camshaft is on maximum lift, it will render the fuel lift pump priming lever inoperative. If this occurs, r o t a t ethe engine until the priming lever can be operated.
3. Tighten first the inlet and then the outlet bleed screw on the filter as the priming lever returns to its downward position.
4. Loosen thebleed screws on the injection pump body approximatelytwo or threeturns (see Fig. 37), operate the lift pump as before.
NOTE.-Under no circumstances should the
injection pump plungers be levered up and down
to prime the injector pipes or test the injectors, as the plunger arms may becomeseriously damaged.
5. When fuel free from air bubbles issues from the bleed screws, tighten the screw furthest from the inlet connections first and then the other one.
6. Wipe all surplus fuel oil from the exterior o ft h e filter and fuel injection pump.

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If that process does not work, then you might have air in the lines past the pump. In that case, loosen the fuel lines from the top of the engine block one by one and crank the engine as the lifting pump cant push fuel on the other side of the injector pump. The cranking will drive the injector pump to push out the air. As each one spits fuel, close it up fast and move on to the next one until the engine cranks.
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Old 05-29-2023, 08:02 PM   #9
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You guys are fantastically helpful. Thanks so much. Will digest all of this and do some testing after haulout and a clean bottom and polished prop.
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Old 05-30-2023, 12:21 AM   #10
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You guys are fantastically helpful. Thanks so much. Will digest all of this and do some testing after haulout and a clean bottom and polished prop.

Before going crazy on what it "could" be, get the KNOWN issues taken care of first. Take some pics of your bottom and post em.

Your boat bottom!
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Old 05-30-2023, 02:05 AM   #11
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Hi,

if the bottom is very dirty, it's like over proping, not good for the engine or the wallet. Read the lesson on the link, what the condition of the bottom affects the boat.

https://forums.ybw.com/threads/pair-of-very-smokey-ad41s.595899/

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Old 05-30-2023, 08:42 AM   #12
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There could be a number of reasons you are not hitting the numbers. As a startingpoint you should know the F.L 120 should hit 2500 at WOT.

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Old 06-01-2023, 07:29 AM   #13
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Photo tachometer on Amazon is about $20.
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Old 06-01-2023, 08:49 AM   #14
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Yes you need to calibrate your tachometers with a photo tach. Then adjust the boat racha to be within 25 ish rpm if you can because the difference might not stay linear.
Otherwise you’re just guessing and so are we.
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Old 06-05-2023, 01:49 PM   #15
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My GB32 Ford Lehman 120 year 1976 gives me at full throttle between 2250 to 2350 RPM. Cruising speed 6.5 to 7.5 knots at 1800, burns about 5.5 liters/hour, temp. 70 to 80įC, oil pressure ~3, voltage 13 to 14. I'll check it out if I were you. Not normal. Could be air in the injectors fuel pump lines, bleed them.
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Old 06-05-2023, 04:22 PM   #16
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It’s probably overpropped.
Overpropping was very popular in the 60’s when your boat started life.

You should IMO get w somebody w a GB32 w same engine that gets 25-2600 rpm at WOT in gear. Make sure that boat has the same gear ratio in the trans. Doing this after cleaning the hull bottom, prop and rudder you should get 25-2600 rpm. 2550 = perfect. Then every several months go to WOT just for 10 seconds or so to insure your rpm hasn’t changed. This as an engine test to insure all’s well below. Cruise rpm can be 2000 then. Most skippers run less but 500 rpm down from max is safe if all else is right and engine is 100% good. Most probably run 700 down as in 1800 or less as their choice. All my opinion.

Did you have the engine surveyed?
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Old 06-07-2023, 11:09 PM   #17
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As others have said, no point in considering this until the bottom, and especially the prop, are clean. Even a couple of barnacles on a prop blade will drop the revs a few hundred. If not slipping for a while, get a diver to just clean the prop & try her again
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Old 06-12-2023, 02:19 PM   #18
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I went through such worries when we bought SNOWBIRD in 2017, concerned that overpropping the engine would be harmful. Brian at American Marine, the guru on the subject, assured me that overpropping was not an issue with these engines (mine is a 2001 American Marine 140, the last non-turbo model) and not to worry about the prevailing story that all diesels should be run hard. He said that turbocharged diesels are another matter.

Here's what I get with my 1977 GB 32 with a 24Ē dia x 18Ē pitch, 3-blade prop and a clean hull in flat water. I think the boat originally had a 4-blade prop, which is a bit less efficient but smoother:

Displacement hull speed 7.41 knots (8.53 statute mph) @ 1.34 S/L ratio

9.0 knots (10.36 smph) @ 1.63 S/L = 2600 rpm (WOT)
7.4 knots (8.27 smph) @ 1.34 S/L = 1800 rpm
6.91 knots (7.95 smph) @ 1.25 S/L = 1700 rpm
6.64 knots (7.64 smph) @ 1.2 S/L = 1600 rpm, 1.6 gallons/hr (3/2017) = 740 nm range
6.36 knots (7.32 smph) @ 1.15 S/L = 1550 rpm
6.08 knots (6.99 smph) @ 1.1 S/L
5.8 knots (6.67 smph) @ 1.05 S/L
5.53 knots (6.36 smph) @ 1.0 S/L

I worked this out on a 1500 mile ICW trip.

Based on this, my normal cruise is 1700-1750 rpm for about 7 knots and that appears to be the boat's "Happy place". A long range economy cruise would be 1600 rpm.

These measurements have held up in the subsequent years with the boat. The GB 32 is a wonder for efficiency if you follow the numbers.

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Old 06-18-2023, 02:39 AM   #19
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once against thanks everyone for very high-quality answers & comments!
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