Yanmar 3GMF can’t get rpms over 2200 I believe is soon to be resolved !

[Capt Robbie]

little history of my issue,
Under load in forward gear, not able to get engine rpms to get above 2200 rpm . Hull speed at 2200 rpm is easily achieved 6.0 knots and verified by gps which is normal at 2200 rpms. In past years I have always been able to throttle up to 2500-3000 and reach approximately 7 plus knots. My issue started a few seasons back and I’ve been trying to figure out what is wrong with my engine by systematically looking for what was wrong with the help of everyone here on the forum. Also need to note that my engine is original 38 years old and to the best of my knowledge has about 2000-3500 hours and regular maintenance done over the years.

engine has absolutely no issues other then the rpms. No smoke, runs smooth, starts up always on the first crank, looks like it did the day it rolled off the assembly line.

everyone including myself thought it is a fuel issue.
1. Changed both primary and secondary fuel filters.
2. Replaced all fuel lines 38 years old need to be replaced anyway
3. Pickup tube replaced
4. Fuel tested in tank “ tank fuel was good “ and ran on separate container just to eliminate anything to link to fuel contamination.
5. replaced lift pump
6. Replaced mixing elbow approximately 6 years ago and new one has at best 20-30 hours on it.
7. Replaced all three injectors. And old injectors looked to be in pretty good condition after 38 years.

Nothing has worked.

So now hired the big guns to look at the engine. Showed the marina technician everything that has been done. First thing he saw was that the throttle cable didn’t look right and I had to rig the cable to attach it to the throttle body. I explained that the cable is not my problem, because even if I manually push the throttle body to max in gear the max rpm is only 2200.

we tested this while the boat was tied to the dock. Then one of the old timers said that this is not going to work!!! He said we have to take the boat out for a sea trial and then force the throttle body to max. He said we will never be able to get to max rpms tied to the dock!!!

I thought about his statement and totally disagreed. What it the difference weather the boat is tied to the dock or not, the propeller is still turning. The engine and propeller is under load regardless if the boat is tied to the dock. He insisted that because the boat is not moving and tied up that the engine will never increase the rpms and he was also very surprised that I was able to get the rpms up to 22,000 tied to the dock.

after going back and forth about our two completely different theories, I gave in and we went out for a sea trial.

I can’t believe it !! He was right !! As soon as he push that throttle body to max the engine throttled up to 3000 rpms and we instantly were moving at 7+ knots.

The entire time I’ve been thinking that the engine is malfunctioning, it’s been working perfectly fine. It was my testing method that has been the issue the entire time. I still don’t quite understand what the difference was between being tied to the dock vs on the open water is?

I believe it has to do with maybe the propeller is caveating and not allowing the governor to open properly.
So now as it seems all I have to do is replace a bad throttle cable and all will be right in my world.

 

[Benny17441]

The operational turning range of a Yanmar engine is between 750 to 3,600 Revolutions per Minute. You say you attain 6 knots of boat speed under power and do not mention any other issues. Check the Tachometer for accuracy.

 

[Capt Robbie]

If $39 photo tachometer from harbor freight is accurate the yanmar tachometer seems to be off by approximately 175 rpm. I can live with that considering it’s the original and 38 years old. Spoke with technician at Mac boring and they said engine is able to race to 3600 rpm not recommend, can also be run at 3400 rpm as per yanmar mfg plate rating also not recommend. They said most sailboats with this engine typically run between 2600 and 3000 rpm. They said that realistically anything about 2500 rpm will drastically effecte fuel economy vs typically what the average hull speed of a 30 -35 foot boat can achieve under power. They basically said that at that high rpm the engine is just burning more fuel for the little bit more speed.
I totally agree with them as I typically run engine at 2000 to 2400 and cruise at about 5.5 to 6.5 knots partially due to the age of my engine of 38 years. I don’t want to put to much strain on the old girl. That being said I always like knowing that I have power in reserve if ever I need it.

Unfortunately that is what started me down this road. I never thought that having the boat tied to the dock could ever effect how the engine would preform. I still don’t quite understand how being tied to the dock effected the performance other then the possibly of cavitation.

 

[Hayden Watson]

I think you have an extra "0" in all of your numbers.

 

[senang]

Yeah.....you were warned, inflation is rising....

 

[SabreToothedEngineer]

I think you have an extra "0" in all of your numbers.

If he doesn't I wouldn't stand too close to the engine...

 

[Hayden Watson]

Yep. a small RC plane engines can turn in the tens of thousands of rpm's but anything bigger would be terrifying. I had a Detroit Diesel run away on me and about the time that it hit 4000rpm's it sounded like the gates of hell had opened. Fortunately, it had emergency dampers and I was able to pull the ripcord and choke it to a stop. Without that, it is just "RUN!"

 

[Tom J]

If $39 photo tachometer from harbor freight is accurate the yanmar tachometer seems to be off by approximately 175 rpm.

Depending on the tach, you can usually calibrate it to the photo tach by turning a calibration screw in the back of your tach.

 

[Capt Robbie]

sorry about the zero, still recovering from hand surgery. Hand still has a way to go, should have seen that

 

[Alan Gomes]

Depending on the tach, you can usually calibrate it to the photo tach by turning a calibration screw in the back of your tach.

Not on that tach.

Also, in my experience with two Yanmar tachs on to different 1GM engines, they were not merely off, but the error was not linear; it can be more or less depending on the RPM in question. So, at very low rpms my tach were accurate. Then, there was a range of RPM where it was off by about what you are seeing. And finally, at the top RPM, the difference between tach and true actually narrowed. Not sure why this is, but it was so on two Yanmar tachs on two different engines. (I have a spare 1GM in my garage, just because.)

What I did is make a deviation table, so I could see what the true RPM is for a given tach reading. But really, there are only a few different RPM numbers I'm interesting in anyway, such as operating it within its optimal cruising rpm.

 

[Benny17441]

Yes, tied to the dock will affect performance. The fuel governor responds to loads on the engine, and when tied to the dock the prop is transferring maximum load, just as if the boat was so heavy and with so much drag that it would not move under throttle.

 

[Capt Robbie]

pulled the old throttle cable that I replaced a few years ago and confirmed that it was either manufactured defective or shipped incorrectly. The length of the cable was 7 feet however the ends where it attaches were incorrect. The the old cable the throw of the threaded ends was 7-1/2 inches and the new cable is 8-1/2. Fit perfectly into the attachment clip, tested it before attaching at the pedestal, and works just fine.
Few hours of being a contortionist and everything reattached, took boat out for sea trial, everything working perfectly again. Engine purring like a kitten.
Problem finally resolved

 

[Tally Ho]

pulled the old throttle cable that I replaced a few years ago and confirmed that it was either manufactured defective or shipped incorrectly. The length of the cable was 7 feet however the ends where it attaches were incorrect. The the old cable the throw of the threaded ends was 7-1/2 inches and the new cable is 8-1/2. Fit perfectly into the attachment clip, tested it before attaching at the pedestal, and works just fine.
Few hours of being a contortionist and everything reattached, took boat out for sea trial, everything working perfectly again. Engine purring like a kitten.
Problem finally resolved

Did you notice that the throttle lever didn’t move all the way to “fast” with the bad cable?

Greg

 

[Terry Cox]

I still don’t quite understand what the difference was between being tied to the dock vs on the open water is?

Hi Robbie, think of it as resistance. With a stationary boat, the prop requires more effort to overcome the resistance to move water past the blades. As the boat picks up speed the prop requires less effort to move water past the blades because the faster you go, the less resistance placed on the prop. Much like a jet engine. When stationary the engine requires more energy to draw air into the compressor blades, but as the plane picks up speed, the flow of air increases requiring less energy to bring air into the engine for combustion. That is why they put large fans on the front of the jet engine, to reduce the effort required by the engine to bring in air for combustion. As the plane picks up speed the faster moving air reduces the amount of energy required by the engine to bring air in for combustion. Same principle.

 

[Capt Robbie]

Greg,
I never really noticed any issue, because my throttle lever has always had a little bit of play in it due to the round brass shaft set screw notch is a bit worn out and doesn't line up perfectly. So unfortunately I've been dealing with it for 15 years. I never bothered to try to get it welded and re tapped partially because I couldn't find anyone in my area to work on it, and most of the slack in the lever is at idle. The idler adjustment on the throttle body stops the arm from going to far anyway. So I have just left it alone.