Can’t get yanmar 3GMF above 2200 rpm under load question

Jan 24, 2017
666
Hunter 34 Toms River Nj
have been looking into this issue for two seasons and still have not been able to resolved it.
A little background: I have a 1983 hunter 34 powered by a yanmar 3GMF Diesel 25 Hp and I am the original owner. It is now 38 years old and all original. Engine is in excellent condition and has been well maintained over the years. No engine hour gauge but I estimate engine hours to be approximately 3000 - 3800 hrs.
Fuel pump and injectors replaced 2003, all regular routine yearly engine maintenance done, fuel/oil filters changed, fuel inspected and stabilized etc. replace mixing elbow 4 years ago, and throttle cable replace two years ago.

Engine runs absolutely perfectly, starts on the fist crank, no smoke, excellent water flow, no issues at all except the rpm issue under load. The fuel filters both have less then 20 hours on them and have always been clean.

Normally I would be able to reach hull speed of 7 - 7.5 knots any time of the season at full throttle and now 5.5 - 6.2 knots with no wind or current. Every season the boat is hauled no barnacles and very little slime if any. Prop is the same I have had for thirty years and always clean.

No apparent fuel issues, engine doesn’t appear to be starved of fuel and tank was emptied and new diesel added to rule out bad fuel. Manually able to engage the throttle to max throttle and cable appears to be functioning properly. Just cannot throttle up past 2200 rpm.

I read that the tech gauge on these boats are problematic and sometimes inaccurate however if this is an instrumentation issue then ground speed would not be effected.
Other post have reported rpms of 3200 - 3800 and ground speeds of 7.5 knots which is about what I remember it normally running at.

any suggestions would be appreciated.
 

DArcy

.
Feb 11, 2017
1,691
Islander Freeport 36 Ottawa
If everything is running fine, the first thing I'd check is the throttle linkage. Perhaps something slipped and the engine is not seeing full throttle.
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
Don,
I believe it’s the same, however I never tried it in neutral, tied at the dock or out on the water I only had it in gear.
why do you ask?
 
Feb 14, 2014
7,400
Hunter 430 Waveland, MS
Don is asking since if in neutral you get > 2200 RPM it shows what your engine can do while not under load.

This will help you trouble shoot your issue.
Good idea by Don.
Jim...
 
Feb 26, 2009
716
Oday 30 Anchor Yacht Club, Bristol PA
I have the same boat
7.5 knots???? 3800 rpm???
I don't think so lol

Since I've been playing with the engine & recently replaced the throttle cable I have discovered that a throttle cable with more travel would be in order 2" is standard but it looks like 3" inches would move the throttle lever even more, I've also discovered that apparently, all tachometers are "off" a few hundred RPM.
I'm not a big believer in wot operation but if push on the lever it seems to kick up around 26-3,000 which tells me it's probably up around 36,
I'll be getting one of those handheld tachometers eventually maybe play more with it in the spring.

I don't quite get all these posts about trying to get their sailboats to motor fast, I mean why don't they get a power boat if they need speed lol
 
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Apex

.
Jun 19, 2013
1,197
C&C 30 Elk Rapids
timing of replacing the cable and stated problem are BOTH 2 years. Perhaps the cable is bottoming out, limiting throttle.
WTO in neutral would then show the same thing. So would simply moving the throttle to full with the engine off. Check the linkage at the engine, to see if fully open.
 
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Oct 29, 2016
1,915
Hunter 41 DS Port Huron
I don't quite get all these posts about trying to get their sailboats to motor fast, I mean why don't they get a power boat if they need speed lol
I don't believe these post have to deal with wanting to make the boat go faster under the iron jenny, its about trying to figure out why his engine isn't developing the RPM it used to.
 
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Sep 25, 2008
7,077
Alden 50 Sarasota, Florida
Don,
I believe it’s the same, however I never tried it in neutral, tied at the dock or out on the water I only had it in gear.
why do you ask?
Because if the RPM goes up to 3000+ in neutral, it isnt limited by linkage, cable, restricted mixing elbow or any other guess people made here. If it reaches max RPM in neutral, the cause for your problem can only be load related.
 
Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
If the engine revs to 3500-3600 RPM in neutral and won't go above 2200 under load, that can be a symptom of a sticky or restricted fuel delivery "rack" in the injection pump.. or restricted fuel flow (strainer on tip of fuel pickup) (very small air leak between tank and lift pump, partly collapsed fuel line, filter gasket not sealing completely) .
The "throttle" on a diesel is really a "rpm set point control" so if the engine is able to get to 35-3600 RPM in neutral, then the set point is increasing and the cable is probably ok. The fuel rack in the injector pump would be required to move a lot more to get to 3000+ under load than unloaded.
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
as apex mentioned, about replacing the throttle cable 2 years ago, unfortunately the issue started before I changed the cable, that was one of the reasons I changed it. I thought maybe there was possibly some slack inside the cable sleeve that I could not see. Anyway when I tried to increase the throttle manually without the cable attached
And the governor wide open, I still could not get rpms above 2200.
 
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Feb 26, 2009
716
Oday 30 Anchor Yacht Club, Bristol PA
Well maybe it's not about making their boats go faster.. but, for those trying to determine RPM with the a Yanmar tach... it's still a guessing game apparently, so y'all need to establish the actual engine RPM is with a separate tachometer, (which I plan to get) before I start playing with anything else,

When we replaced the throttle cable it was very evident that the throttle lever can move a lot more then cable provides.
Experience of others and myself proves time and time again the things that are important for diesels to run properly are air fuel and water.
But those three items can become very complicated because fuel pressure has to be proper fuel delivery has to be at the proper volume and clean
Air seems to be the easiest because most diesels have a simple air intake, water of course is going through a couple strainers an impeller pump a heat exchanger and then an exhaust elbow, just the nature of cooling exhaust in the space of an exhaust elbow is enough to start making soot almost instantly and if it's let go for years and years...

I've seen what happens, the carbon can build up so much it can actually split the elbow.


 
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Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
No apparent fuel issues, engine doesn’t appear to be starved of fuel and tank was emptied and new diesel added to rule out bad fuel. Manually able to engage the throttle to max throttle and cable appears to be functioning properly. Just cannot throttle up past 2200 rpm.
1. Have you tried disconnecting the governor controller cable (incorrectly referred to as the throttle) and moved the lever on the engine by hand while under load. This will eliminate the cable as a source of the problem.

2. I know the mixing elbow was replaced 4 years ago, but now may be the time to check it again before you start spending any real money and effort.
 
Jan 24, 2017
666
Hunter 34 Toms River Nj
Yard tech suggest to change both fuel filters again, even though they only have less then 20 hours on them. He said number one cause of power loss is fuel filters clogged and not allowing sufficient fuel to pump. I agree with his theory about clogged filters causing low power issues, but I’m having a hard time wrapping my head around both filters clogging with less then 20 hours, and when I emptied my fuel tank and refilled it the old fuel and new fuel was clean. He believes that I might have some sediment in the tank then has gotten into the filters. I get that because the tank is original and 38 years old. I would think that the raycor fuel separator would catch that and even if it were partially clogged would still allow enough fuel to reach 2200 rpm which I feel I just a little over half throttle.
Im going to pull the fuel line off and see what kind of flow I have, check the rpm in neutral, then change both fuel filters again. Hopefully this corrects the issue.
 
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Oct 29, 2016
1,915
Hunter 41 DS Port Huron
You should check the fuel pickup tube for pinholes as well, the old pickup tubes were made of copper which over time developes small perforations which will allow are to be entrained into your fuel line.
 
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Feb 26, 2009
716
Oday 30 Anchor Yacht Club, Bristol PA
38 year old fuel tank! Almost guaranteed you have an inch of sludge in the bottom, clogged fuel pickup tube and every time you start moving, sludge gets stirred up!
Filters are expensive, labor is cheap when you do it yourself. Fuel tanks (most) just don't get inspected because they don't come with inspection ports!
 
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Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
Kloudie is on the mark - know that the injector pump rack can go full on and the throttle cable will make the rack go full on.

Maybe you won't know about the capacity of the pump but these "box" type pumps tend to develop wear but there are no symptoms of this. (A full redo of the Yanmar injector pump will be about $600 to put it back to factory specs.)

But - you need to verify there is NO restriction whatsoever on the exhaust side. Means clear tubes from the exhaust valves all the way to the overboard fitting. The most common cause of your symptoms (where you have hull and prop clean) is strangulation on the exhale side of the motor.

Ralph has some good strangulation pictures - some place.

Charles
 
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