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Old 20-06-2019, 02:10   #1
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Strange Yanmar RPM issue.

My Yanmar engine recently did a 'run away'.
It turned out that "Govenerlever 2" had broken and allowed the fuel rack to go to the max open position.
It has been replaced but a new problem has developed where the motor in gear will rev to 2450 plus in reverse but only 1850 if in forward!
Anyone have a clue how this could be? The drive train is free to spin in both directions and my mechanic is at a loss and so far so am I to 3K...
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Old 20-06-2019, 04:04   #2
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Re: Strange Yanmar RPM issue.

Welcome aboard CF, Mick of 42.
It might help us to know what model the engine is, how many revs could you previously achieve and how many can you get now in neutral?
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Old 20-06-2019, 04:13   #3
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Re: Strange Yanmar RPM issue.

Greetings and welcome aboard the CF, Mick.
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Old 20-06-2019, 05:37   #4
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Re: Strange Yanmar RPM issue.

Greetings, Mick.

Could the difference in forward versus reverse be as simple as your propeller not biting as well in reverse? Wotname's question probes that.
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Old 20-06-2019, 05:57   #5
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Strange Yanmar RPM issue.

You most likely have restricted fuel. Most probably from the governor, cause it was replaced.
The rev in neutral will tell you if its restricted fuel or not, if it revs to proper rev line then itís most likely restricted fuel supply. If it doesnít then that also points to the part that was replaced, the governor.
It may just be adjusted incorrectly?

The reason it will rev higher in reverse is due to first the gear reduction being different and also because the prop is being spun backwards itís less efficient and therefore puts less load on the engine. Exception is the flat plate bladed feathering props, they are equally inefficient both forward and reverse, but you still have the difference in gear ratio
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Old 20-06-2019, 10:04   #6
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Re: Strange Yanmar RPM issue.

The explanation is almost certainly the difference in gear ratios between ahead and astern. I found exactly the same with a Yanmar 2GM20 that I had on a 31ft Miura: the max revs in forward gear was just over 2000 but in reverse (or neutral) we measured well over 3000.
Andrew MacKenzie
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Old 20-06-2019, 12:36   #7
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Re: Strange Yanmar RPM issue.

Adjust the throttle cables in your remotes. Especially if it ia a single lever type. Stating the obvious--but it might just be the problem.
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Old 20-06-2019, 18:04   #8
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Re: Strange Yanmar RPM issue.

thanks for letting me join this group.
more info and background... the engine is a Yanmar 3GMF. I have owned the boat for over 2 years and on the occasions I run the engine whilst at the marina in the past there was never an issue with getting 2450RPM in forward or reverse.
In neutral 3000RPM is reached and the engine sounds very smooth with no knocking etc..
The fault was the 'governor 2 level' broke allowing the fuel control rack to go to the maximum fuel rate. The 'governor 2 level' has a fork like arrangement and one of the fork fingers swapped off.


If you need more detail just ask and thanks again for any comments.


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Old 20-06-2019, 19:08   #9
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Re: Strange Yanmar RPM issue.

It sounds like you are just not getting enough travel on the throttle lever on the injector pump - maybe not correctly adjusted after the aforementioned repair.

As Mike Banks posted in post #7.

Easy enough to check, disconnected the cable from the cockpit lever at the injector pump and try it by hand. If you get back to the original RPM by hand operating the fuel lever, then all you need to do is readjust the cable arrangement.
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Old 20-06-2019, 19:11   #10
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Re: Strange Yanmar RPM issue.

By the way, you should be getting at least 3,000 in forward with the proper prop pitch and a clean bottom but this is a seperate issue and not to be confused with your current problem.
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Old 20-06-2019, 21:46   #11
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Re: Strange Yanmar RPM issue.

Quote:
Originally Posted by Wotname View Post
It sounds like you are just not getting enough travel on the throttle lever on the injector pump - maybe not correctly adjusted after the aforementioned repair.

As Mike Banks posted in post #7.

Easy enough to check, disconnected the cable from the cockpit lever at the injector pump and try it by hand. If you get back to the original RPM by hand operating the fuel lever, then all you need to do is readjust the cable arrangement.
been there done that no difference its 'internal'!
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Old 20-06-2019, 21:58   #12
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Re: Strange Yanmar RPM issue.

Quote:
Originally Posted by Andrew MacKenzie View Post
The explanation is almost certainly the difference in gear ratios between ahead and astern. I found exactly the same with a Yanmar 2GM20 that I had on a 31ft Miura: the max revs in forward gear was just over 2000 but in reverse (or neutral) we measured well over 3000.
Andrew MacKenzie
Cape Town


Yep, that's the most likely answer and your prop most probably needs cleaning and if it's already clean you're slightly over propped.
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Old 21-06-2019, 00:26   #13
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Re: Strange Yanmar RPM issue.

before the 'runaway' it gave the same RPM's either way... thanks
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Old 21-06-2019, 02:16   #14
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Re: Strange Yanmar RPM issue.

To recap, before the run away issue, you could reach 2450 in forward and 3000 in neutral at WOT.

After the repair you can only get 1850 in forward but still get 3000 in neutral at WOT.

The prop remains the same and bottom is just as clean now as before.

If so, then your governor is still working (by evidence of reaching 3000 in neutral currently) but you are not making enough power to reach 2450 in forward. As nothing else has been changed except the repair fuel system, this is the place to start.

Timing shims incorrectly fitted during repair?
Maximum fuel adjustment has altered during repair?

Normally I would suggest easier things first like valve lash or blocked fuel filters etc but it does seem the problem only surfaced after the run away repair.

Do you have the service manual?
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Old 21-06-2019, 03:33   #15
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Re: Strange Yanmar RPM issue.

Quote:
Originally Posted by Wotname View Post
To recap, before the run away issue, you could reach 2450 in forward and 3000 in neutral at WOT.

After the repair you can only get 1850 in forward but still get 3000 in neutral at WOT.

The prop remains the same and bottom is just as clean now as before.

If so, then your governor is still working (by evidence of reaching 3000 in neutral currently) but you are not making enough power to reach 2450 in forward. As nothing else has been changed except the repair fuel system, this is the place to start.

Timing shims incorrectly fitted during repair?
Maximum fuel adjustment has altered during repair?

Normally I would suggest easier things first like valve lash or blocked fuel filters etc but it does seem the problem only surfaced after the run away repair.

Do you have the service manual?
Thanks,
I think the timing shims thing is so steampunk but you may be right. The same shims were refitted but I wonder if they are the problem. Such a dumb way to effect timing adjustment.
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