Yanmar 2 Gm not reaching 3800 rpm

weinie · Jul 15, 2012

Change your fuel filters lately?

kloudie1 · Jul 15, 2012

Coupla things, Mark.. The original tachometers for those engines are notoriously inaccurate.. It would be good to borrow or buy an optical tachometer to take a real RPM reading.. What are you using for speed reading? If GPS, you could be seeing a current that is slowing you down.. Has the engine ever gone to 3800?? First is to make sure you have good clean fuel and clean filters and no leaks in the fuel lines between the tank and the suction side of the "lift" pump on the engine.. (especially the banjo fitting on the suction side of the pump) Bleed the fuel lines .. first to the HP pump, then to the injectors.. There was a recent post that had the procedure and pictures .. If you search on bleeding fuel lines, you'll find it.

Steve Dion · Jul 15, 2012

The GM engines are rated at 3600 max. There can be any number of issues. You should be checking your speed over the ground to deterimine how fast you are going.

Does the engine rev to anywhere near the rated RPM when it is in neutral? You may also want to check to see if the throttle is opening to it's max position at the engine. You cable housing may not be locked down correctly.

Filters can obviously restrict the flow of fuel.

Has this engine ever reved properly?

Ron20324 · Jul 15, 2012

Mark Nooney said:
...I noticed that at
full throttle I went from 4 knts to about 3 knts after about an hour. What might be some of the issues that I should look into?...

Slip neighbor came in on Saturday, complained of loss of speed. Dove on the hull and found he had dragged a crab pot- with float- for several miles and into his slip.

nutheastah · Jul 16, 2012

Mark Nooney said:
Thanks for the responses.
The tachometer reading was done with a laser optical - I was concerned about
the gauge at the helm. That seems to read 500 rpm low from the laser reading.
I had a issue with air in the diesel line so I changed the Racor filter and
bled the lines and changed the air filter.

I think that the cable is installed correctly - I just replaced it when the
filter was replaced.
I will make sure that the governor hits the WOP stop.

I will confirm the WOT rpm in neutral.

Is there a way of testing for flow of diesel to the engine? I read that everything
works fine but the engine is just not getting enough fuel to produce the
projected RPM's

Thanks
Mark

I'm new with big sailing stuff, so only things I can help you with is general however I have a 2 cyl Yanmar in my lawnmower. maybe they are similar, i'm not sure and wouldn't want to be misleading. when I used to fix diesels ( all types) first rules.Diesels need 2 things fuel and air. if you got them it should run, BUT here is where i would start. Old fuel(even last years can develop algie), if you suspect this change out your fuel and also any fuel filters. next check air intale, no mouse nests in air filter box and air filter clean. . any bad fuel lines (even return lines) and any check valves to return MUST be 100%> any, any, any air that can get into your fuel system from pickup line to check valve for return WILL cause hard starting and loss of power. That being said ther are a couple of quick things to do. once running disconnect your throttle linkage from injection pump and throttle up, if you get your RPM's doing this adjust your linkage and enjoy your good fortune., You can also disconnect your intake pipe at the closest rubber joint to the intake manifold and see if this causes an improvment, if so suspect blockage upstrean such as air filter or mouse nest. But be careful I have had the pest that caused the blockage fall into the intake. on a 800 Cubic inch CAT the mouse will go through and come out the exhaust but I doubt your little unit could handle that. One other thing, check all intake hoses and make sure they are not soft and oil soaked as they can collapse and restrict air flow while running and when shut down look perfectly normal.
I hope this helps
Bruce/nutheastah

nutheastah · Jul 16, 2012

I used to carry sight glasses made for automotive air conditioning. I suspect that you could get something on line for your yanmar. All that being said I wouln't trust myself until I had looked at a manual to see what was suggested. I'v been out of the trade a while. and dont want to give info that would muddy your problem more.
Bruce

nutheastah · Jul 16, 2012

One other thing to look for is a rag in your fuel tank, a rag in your tank will become inert and just float around near the bottom of the tank at Idle it will not be affected but under higher RPM it will be sucked up to the fuel pipe and add just enough restriction to starve the system but not stall it
Bruce

kloudie1 · Jul 16, 2012

Ohhhhh.. the other one not mentioned yet is the little screen on the tip of the fuel pick-up in the tank.. Not all have them but it is fairly easy to get out. Take the fuel inlet off and the dip tube and little screen might be on the tip. Take it out and don't put it back if it is there.. let your primary filter handle whatever the tube picks up..

MikePz · Jul 16, 2012

Mixing Elbow

Mark Nooney said:
Hi
I have a 1984 Yanmar 2 gm, The top RPM should be 3800 rpm.
At full throttle I get 2205 rpm. Also, today I noticed that at
full throttle I went from 4 knts to about 3 knts after about an hour
of running.
The bottom was just cleaned. I feel that I should be able to
get 5 to 6 knots.

What might be some of the issues that I should look into?

Is this something a mechanic could solve?
Any thoughts would be appreciated.
Thanks
Mark

Mark, I would check your Mixing Elbow, when it clogs up with soot and carbon it retstricts engine RPM's ...I have replaced two on a 3 GM, worth the time and effort, should be done every three years or 7oo hrs I've been told. R/Mike Pz

Blitz · Jul 16, 2012

Mark Nooney said:
Thanks for the responses.
The tachometer reading was done with a laser optical - I was concerned about
the gauge at the helm. That seems to read 500 rpm low from the laser reading.

This exactly what I determined, reads about 500 rpms low on cockpit tach. Less error at idle.

abaragones · Jul 16, 2012

I think the key here is what you mentioned in your second post. From what I hear, everything was ok then you had air in your fuel system (why, how?), you changed the filter (primary/racor), bled the system and continue to have low rpm/performance issues.

Did you fix the air issue? Meaning where the air was getting in? If you did, I would bleed the engine again, hard! At the secondary filter and then at the hose that gets to the injector pump. Bleed it until you get bored of doing so...then take the boat out right away for a test.
I had a very similar issue jn my 2GM20...tiny amount of air was the problem....if you get better or ok performance but after sitting for a while you get the issue again, then more air is getting into the system.1

Try above mentioned issues as well (mixing elbow etc), those are common issues that affect performance.

Keep us posted

Abe

rasselas · Jul 17, 2012

First, I am no expert these are my issues and my fixes so don't bombard me with your critical emails. My problem was the same as you described and the problem was fixed when I changed my fuel filter. If you have not checked or replaced your mixing elbow lately you might want to do it because if you have been burning bad fuel for any length of time chances are you have a build up inside your mixing elbow that can lead to more problems later.

Steve Dion · Jul 17, 2012

Mark:

If you have opened your fuel system there are some washers (banjo/crush) that should be replaced. You may want to be sure that you replace these too. Do not forget to check your secondary filter on the engine and be sure that bowl is full of fuel. These fuel/air leaks can be difficult to diagnosis.

kloudie1 · Jul 17, 2012

Great news, Mark.. and relatively inexpensive !

the_alpine · Jul 19, 2012

Had this same problem. It was the mixing elbow. She was clogged bad. Replaced with new and the motor is working great. Much more quick to respond now too.