Yanmar 3gmf - Cant reach 3000 rpms

Maine Sail · Jan 25, 2012

palmettobum said:
Hello,

I have a 1983 Hunter 34 with a Yanmar 3GMF. At full throttle, I can't get the RPMs to reach higher than 2600. The fuel filters are clean. Does my throttle cable need adjusting? What else can I do to get her running full speed? I'm also afraid of carbon build-up. Thanks for the help!

- Justin

Other potential issues, dirty bottom, fouled prop, barnacles on prop or over propped. Check to make sure the throttle is moving the full stop.

kloudie1 · Jan 25, 2012

Be sure to use a good digital hand held tach to get readings. That tachometer on the 34 is notorious for being inaccurate. Mine reads 2000 at a real RPM of 2800..

Steve Dion · Jan 25, 2012

Mixing elbow?

Just remove it for inspection.

Endurance2010 · Jan 26, 2012

I just replaced my mixing elbow and I get max 3100 rpm under full load. I too am wondering about my throttle cable.

--Dan

Bill Roosa · Jan 26, 2012

Is this with the engine in neutral or while underway?
There are stops in both direction for the throttle. Just have someone work the throttle while you observe wither the lever on the injector pump actually moves stop to stop. Adjust as needed.

Richard Bryer · Jan 26, 2012

What speed are you going (calm sea, no current) when you are running at 2400? You could be over-propped. I just changed to a CS 3 blade 14X9 last spring( from the original 2 blade 15X12) I found I couldn't get up to my cruising rpms of 2700. WOT would only give me 2500-2600 underway. Norm is sending me back the prop after changing dia, pitch and cupping.

Dan Johnson · Jan 26, 2012

Justin...

As Steve pointed out, a clogged mixing elbow will restrict the amount of exhaust gas that can pass through it, thereby restricting RPM that can be achieved. It's not uncommon for these elbows to clog up over a half dozen years of running at less than optimum RPM. Steady state cruising RPM should be at 2,600-2,800 RPM (about 70-75% of max RPM).

At full throttle, in neutral or in forward, I can reach 3,400 RPM on my engine tach. The true RPM is 3,600 RPM, determined as Claude pointed out, with a digital laser tach reading the mainshaft pulley RPM. This is with the original 2 blade 15x12 right hand prop. The engine tachs are notorious in their inaccurate readings as Claude said.

You can check the throttle's range of movement easily with the engine off to see if it is moving the injector pump lever to its full setting with throttle setting at the pedestal at its fullest setting as Bill Roosa suggests. The lock nut on the throttle cable could have come loose and allowed the cable connection position on the lever to have changed.

MaineSail pointed the other possibilities which must be considered too.

gpdno · Jan 27, 2012

Clean your prop. You'll be surprised at how few barnacles it takes to cause a problem. Had the same thing early this month. Normally run at 2800 rpm, was having trouble maintaining 2500 rpm. Checked mixing elbow and replaced both fuel filters and air filter. No joy.

Water was cold so I had not been down on the bottom for two months. Sucked it up, got out the wetsuit and jumped in. Honestly prop didn't look that bad. It was covered with a bunch of very small barnacles 1-2 mm at best, looked like a light dusting of snow. Took maybe 10 min to brush them off.

Started her up. Success. I was back to cruising at 2800 rpm with power to spare.
Keep your prop clean

Joe Mullee · Jan 27, 2012

As Claude & Dan Said........

I have the same boat with the same engine. My tach has never read more than 2200 RPM since I purchased her eleven years ago. I did have the engine checked with a digital laser tach reader while at idle and I reached 3300 RPM a full throttle. I also regularly hit 7.1 knots each spring on calm waters right after I clean and paint the bottom (measured with GPS). By the end of the season in December I still get 6.7 knots with a dirty bottom and no doubt a few barnacles on the prop. I have tried fixing the tach problem with no luck. Also had Conllyn Marine at Herrington Harbor give me an estimate of $700 to replace the tach display. I decided I could live with it.

Maybe you just have the same problem as me and Claude. It's easy to check it out. Good luck.

Capt Fernando Martinez · Jan 27, 2012

Good suggestions by all. I had a problem once that all seemed perfect but after further inspection of the exhaust hose inside I found that one section of exhaust hose had come loose and was preventing the engine from exhaling properly. Replaced that section of hose and the engine performance increased. About the tach being off, that may certainly happen however ask yourself if the engine sounds different than before when it used to run at desired rpm.