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casey1999 10-18-2010 12:47 PM

Yanmar 3gmd Max RPM
I have a Yanmar 3gmD and have gone through the engine replaceing fuel lift pump, fuel and air filter, cleaned fuel tank, added new fuel, added Racor filter/water seperator. Engine starts easily and runs great. Only problem at full throttle only can get to 2400 rpm. Engine is rated for 3400 rpm. Engine is in an S&S 34 sail boat. At 2400 boat moves at about hull speed in 5 foot seas and 20 knot head wind. Power seams ok for boat at 2400 rpm, but manual says at this rpm hp is about 14 hp. At 3400 rpm should get 20 rpm. Manual says injector pump could be the problem.

Any other ideas for low RPM? Could the throttle be adjusted to get full rpm?
Thanks, Casey

Faster 10-18-2010 01:13 PM

Are you sure the prop is clean - and properly sized/pitched? Are you starting to develop black smoke at that rpm?? Is this a new phenomenon?

casey1999 10-18-2010 01:40 PM

I recently cleaned and bottom painted hull and cleaned the prop. I bought the boat 4 months ago and from what I know the prop is original. Boat was built in Austrailia in 1978. Engine looks to have been installed in about 1980. Prop might be wrong but if it is it has been in the boat for 30 years. The engine may have always run like this, the last owner could not give me much information on the boat. The prop rotates freely by hand (no binding) and I have no abnormal vibration. The boat has been around the world recently (via panama, and cape of good hope) so the system is workable- I just want to make sure it is not going to hurt the engine long term. The engine does produce a white / grey smoke at all rpm's both high and idle. I have replace the thermostat (which was totally corroded) but this had no effect on the engine running. The engine does produce a kind of burning odor from the exhaust but the engine temperature seems ok (no high temp light and engine and exhaust can all be touched with the hand for several seconds without being too hot. Engine buns no oil and at 2400 rpm burns about 1/3 gallon per hour (at 6 knots) which seems normal.

mitiempo 10-18-2010 03:20 PM

Could it be overpropped? Does it rev higher than 2400 in neutral?

Vasco 10-18-2010 03:28 PM

Check your mixing elbow.

casey1999 10-18-2010 03:57 PM

I did look into the mixing elbow- it looked clear. Also, all cooling water lines are new. and the inlet strainer is clean. Looks like adequate cooling water is coming out the exhuast outlet.

I imagine the fuel injectors are original and never been serviced. Could white / gray smoke be from un burned diesel coming from a poor spray pattern from worn fuel injectors?

casey1999 10-18-2010 03:58 PM

I have not tried to go to max rpm at in neutral. Could I damage the engine by going to max rpm with no load?

JohnRPollard 10-18-2010 04:25 PM


Originally Posted by casey1999 (Post 655773)
I have not tried to go to max rpm at in neutral. Could I damage the engine by going to max rpm with no load?

There is no problem revving the engine in neutral. I think most of us assumed the rpms you were reporting were in neutral.

If it revs right up to 3200-3400 in neutral (I suspect it will), you have all the signs/symptoms of being over-propped. That is not good for the engine, so you should change it even if somebody else went around the world like that.

casey1999 10-18-2010 04:31 PM

I will check max rpm in netral and let you know. It will be a few days till I get back to the boat. Could white / grey smoke be unburned fuel from bad fuel injectors? would bad fuel injectors lead to below spec horse power output?
Thanks, Casey

mitiempo 10-18-2010 04:41 PM

I think it could be unburned fuel because the throttle is asking for more rpm than the prop is letting it get to.

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