Yanmar sour spot

[Dubo]

I am trying to smooth out my 2GM20F's idling roughness. I find between 1200 and 1400 RPM the engine idles very rough; to the point where my binnacle gets the shakes. I find once I get her up to 1600 and beyond she idles smoothly. 800-1100 is also smooth idling. Some things I have done is replace the fuel, adjusted valve clearance(thanks for those who offered advice) and changed all filters(yearly).

I am wondering if this is an injector issue??? Could carbon have formed at the nozzle? Am I expecting too much? Is this a Yanmar feature?

Any thought appreciated.

Dubo

 

[navigator97]

I had the very same thing on my 2GM20F engine in a 1990 H-33.5. Except that my rough spot was at around 1400 t0 1800. I had it for a lot of years and would just routinely accelerate thru that RPM range. This winter I had the yard replace my cutlass bearing. I no longer have that vibration in ANY RPM range. It is very smooth all the way up to full throttle. I believe that the vibration from the worn cutlass bearing reached a "self resonance frequency" at that RPM range and magnified the vibration.

 

[Nom De Guerre]

I've found similar behavior with my own (and read about from other's) 2GMF. Not binnacle-shaking, but definitely hull-rumbling. I believe that the various sweet and sour RPM vs. vibration spots are the nature of the old Yanmar GMs. You may also have, and might want to consider replacing, original worn-out engine mounts.

I simply avoid the sour spots (or more accurately, stick to the sweet spots).

 

[Benny17441]

I second the suggestion of checking the motor mounts. All engines tend to rock at idle but it is the restraining capacity of the motor mounts that keeps those vibrations in check. Check for soft or separated mounts.

 

[Dubo]

Mounts seem fine. Keep in mind this is neutral so there is no shaft/prop variable involved.

 

[Benny17441]

Evaluating motor mounts can be deceiving. They may feel relatively stiff and the rubber may look good but they may not be doing the job. These engines as they age the components wear down and fall off specs giving way to the development of rough spots in the RPM range. You indicate the engine idles well between 800 and 1100 RPM so if you set your idle at around 800 you should be OK. The problem range seems to be between 1200 and 1400 RPM but this is to high to be considered idle range. If you throtle the engine up in neutral with no load and perhaps a little cold it may not behave as if under normal operation. A better indicator would be to establish how the engine behaves at this RPM range under load and with the boat underway. The roughness may not go away and perhaps just move to a different RPM range but it would indicate the ranges to quickly by-pass during normal operation. Short of a costly complete rebuild the alternative would be to learn how to live with the engine and this is where a new set of engine mounts may reduce the amount of vibration making it easier to live with. Identifying and replacing engine components to reduce vibration may be rather difficult and costly.