Max RPM

[PaulM]

Hi All,
This is my first post here. Looks like a great site. I have a Yanmar 2GMF in a 30' sailboat. The engine runs great no smoke and plenty of power, but it will not reach above 3000 rpm with no load. The specs indicate it should reach ~3600 RPM. Does anyone have any suggestions on where to look? Is the tach suspect?

Thanks
PaulM

[camaraderie]

Paul...Welcome. I would first suspect the tach which is driven from the Alternator. If OEM alternator has been swapped out, then the different sheave size can give a false reading on the tach. The easiest way to determine this is to get a mechanic to use a laser gun to read actual shaft rotation and compare it to the tach readouts. When I switched to a Balmar alternator, I found the tach readings were reduced by over 1000 rpms at cruising speed. Of course, a bad tach or something wrong with the engine are also possibilities, but first things first!

[Alan Wheeler]

Tach as stated above, or.... You could simply have the lock screww for the max throttle set down low. There should be two set screws. One to rest the throttle at an Idle RPM and the other to stop the throttle going to far in high RPM. It could be that someone has wound it down to stop the throttle being opened wide.

[PaulM]

Thanks for the replies. This is an older engine (85’). After looking in the manual I believe that the tach is run mechanically off a gear in the clutch housing. I also looked for the high RPM but can only find the idle adjustment screw. There is what looks like a knob in the top of the injector pump.

PaulM

[Hud3]

Paul,

Has the engine ever been able to reach higher rpms? Check out the simple stuff first, but if this is a new phenomenon, you may have a restriction in the fuel supply.

My Yanmar 4JH3E all of a sudden couldn't do over 2800 rpm under load. I tracked the problem down to some stray turnings from a plastic fitting that had probably fallen into the tank during the manufacturing process, and had found their way into the fuel pickup tube.

[Alan Wheeler]

The higher RPM may also be the adjustment of your throttle cable. What you have to check is if the throttle is opening all the way up or if there is still some way to go when you push the Control fwd.

[PaulM]

Hud3 and Alan,
Thanks. The engine has done this a long as I have had the boat. I've checked the linkage and the throttle seems to be opening all the way (hits the timing cover). I have not measure the gap at the end of the linkage nor do I know if there are any stops at the pedestal end of the cable. I believe that someone must of played with the max rpm or fuel as I can't find any adjustments that are wired off. I also noticed that the idle screw is screwed in most of it's length.

As I understand there are three adjustments on the governor and please correct me if I'm wrong. The idle, the no load max rpm (limits the max. amount of fuel) and the fuel adjustment (adjusts the amount of fuel for a given throttle setting or span).

I already removed the exhaust riser and checked for blockage. I also need change the fuel filters. I'm going to see if I can find an external tach to verify the rpm before I start playing with any settings.

Paul

[Alan Wheeler]

No don't go looking for things that may not be there. Keep it simple. First off, you reach 3000RPM no load. Second, the 3600 is Max intermitent RPM. You don't want to be going there on a regular bases.
OK, now what do you reach when in gear? It should be close to the same. It doesn't matter what the engine actually is set to idle at. My engine for instance has a max continuous RPM of 2800RPM. I have the max unloaded throttle set to 2400, which is the max continuous RPM. I usually run the engine at about 1700 for best economical cruising. DON'T go adjusting the Governor.
I am a little worried about the linkage hitting the cover plate. It shouldn't and am wondering if someone has extended the lever, so that may actually be the problem.

[Ex-Calif]

I think there's a hint with the idle screw turned all the way in and the throttle arm hitting the timing cover.

Is the throttle arm mis-indexed?

It may be possible to reindex the throttle arm screw out the idle screw and get more "push" at the top end.

The real check for this is whether the butterfly valve is all the way open.

[PaulM]

Thanks again guys. I like the idea of a misindexed throttle arm. Where’s the butterfly valve? Alan, don't worry I'm not going to play with the govenor........yet. I like the KISS principal and need to understand exactly what's wrong first. Easy stuff first.

Paul

[Ex-Calif]

The butterfly is a plate in the inlet of the engine. You should be able to remove the airbox cover and the filter and look into the inlet of the engine.

With the throttle closed the butterfly should be almost closed. With the throttle pushed all the way the butterfly should be almost all the way open.

If the butterfly is opening all the way as the throttle arm hits the timing cover you problem lies elsewhere. But if the butterfly is not opening all the way I would suspect the throttle arm index.

[colemj]

Paul is talking about a diesel. The butterfly is found in the carburetor of a gasoline engine.

Mark

[Alan Wheeler]

Butterflies can also be found in Diesels, but not common. It is found when a Road Diesel is marinised for Boat use. The Road Diesel is governed in a slightly different way and some used a Butterfly to shut the air off to the engine along with the fuel. I may be wrong, but I do not know of any Marine engine with a butterfly.

[PaulM]

I just got the bottom painted. What a difference!!!. But the Max RPM at no load has not changed. On the way back to the slip I found that the throttle lever would spring back at the higher speeds. At the slip I looked at the throttle cable and found a small clamp pinching the cable. I removed the clamp, now the throttle springs back to idle and you have to hold the throttle open much like the gas pedal on a car. I could put the clamp back, but I thought the throttle on a marine diesel stayed where it was set. There is no spring on the engine that I can see and have not looked at the pedestal end. Any thoughts?

PaulM

[Alan Wheeler]

Normally you have a "clutch" in the morse control unit. A simple screw that you tighten to set the desired amount of friction on the lever. You are right that the throttle lever on the pump does not need a spring. However, some like myself fit one for the sole purpose that should the cable come disconnected, the throttle pulls back to idle and not slip to full open. That could be a little embarrassing.