3GMD Yanmar Vibrates near max RPM

[tomgrigsby]

I have a 3GMD Yanmar in a 79 Hunter 30. Two blade prop. My propulsion system has always vibrated at high rpms but I thought it was a worn/bent shaft. Had the shaft replaced and now can definitely say I have vibration only under high load. In full reverse the engine is smooth as butter, but anything above 3/4 throttle forward (or jamming full throttle from stop) and it vibrates. The only troubleshooting I've done so far is to read the manual and start familiarizing myself with the fuel injection system. Anyone have a similar problem and/or recommendations? I have zero experience with this fuel injection system. I have some experience with automotive TBI, and some experience with carbs (balancing 6 on my Honda CBX) and tuning SUs on my MG, but it's not really germane, of course. This has been going on for as long as I've owned the boat (8 years) (yeah, 8 years, I know...) and I'd like to run it to ground. I'd appreciate any pointers.

 

[Ted]

Have your prop balanced. Also worth investigating is the diameter of your prop shaft. Some manufacturers cut corners when sizing the diameter of the prop shaft. A 1/8" undersied diameter shaft can cause it to whip at high rpm's. Check to see if the unsupported length is excessive for the diameter and material used. Another possible cause may be that your shaft anodes are placed too far away from your strut.

 

[Sanfelice]

Yanmar engine mounts, if they are indeed Yanmar, are known to break down easily. They are designed to smooth out the engine. . I would check for soft or broken mounts. The engines run smooth until put under load at increased RPM. If they need replacing, the Yanmar mounts are expensive and may use more than one part number per engine based upon the location of the mount on the engine.

 

[tomgrigsby]

Ah, engine mounts - they're at least a decade old and certainly weren't new when I bought the boat. They need to be changed whether they are the problem or not. Excellent advice, thanks. Regarding the shaft, the problem existed prior to putting in a new shaft last summer. The new shaft improved things, but still bad vibration - shakes the lifelines. No vibration in reverse, so propeller balance is most likely ok. Good thoughts though.

 

[Franklin]

I had that problem and it was the engine was out of alignment causing the shaft to rub hard on spots of the cutlass bearing.