Low RPMs on Yanmar 2GM20 - SailNet Community
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post #1 of 16 Old 11-16-2010 Thread Starter
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Low RPMs on Yanmar 2GM20

My father and I have a Yanmar 2GM20 diesel on a C&C 29 (1983 or 84?). I calculated the max hull displacement speed around 6.33 kts. This past summer the engine worked great, no problems starting or other major issues. However, we just canít seem to get that engine up to 3400 or 3600 rpms. It is rated for 3600. The best I can get out of the engine 2000 to 2200, then I see a little bit white smoke from the exhaust and the engine is loud as heck! According to the GPS, at best I did 5.5 kts. Iím planning to ticker with the throttle cable this weekend and see if it needs adjustment. Also planning on changing all the fluids, zincs, filters, etc. since it is close to winter. Any suggestions on how to increase the RPMs?

Here are some other specs for your enjoyment : LOA 28í6Ē, LWL 22í4, BEAM 9í5Ē, DRAFT 5í3Ē, DISPLACEMT: 6,700 lbs. I calculated I need 13.4 HP at 3200 rpms for 6kts., unless I misinterpreted the performance curves for the engine.

Thanks!
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post #2 of 16 Old 11-16-2010
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I would initially suspect a clogged fuel filter. The increased noise from the engine is puzzling though.

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post #3 of 16 Old 11-16-2010
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There are many factors to slow you down. Certainly your acvtion plan is aiming some of those and should be conducted carefully. White smoke generally is not bad signal, it would indicate steam out of exhaust due to low engine temperature. Can you get higher revs if your propeler is not engaged ? Have you checked for botton fouling, specially propeller fouling ?

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post #4 of 16 Old 11-16-2010
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Quote:
Originally Posted by negrini View Post
Can you get higher revs if your propeler is not engaged ? Have you checked for botton fouling, specially propeller fouling ?
What Negrini said.

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post #5 of 16 Old 11-16-2010
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Even with a clogged filter you can usually rev the engine to full rpm, but under a load it just pukes out. If the bottom was fouled terribly, or the prop, I would expect to see a lot of black smoke at full throttle but these internet engine discussions are ... let's just say interesting.

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post #6 of 16 Old 11-16-2010
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I'm with jrd, it doesn't sound like a load condition.
Let's start with the basics:
greg, is it hard to start?
when was the last time the injectors were serviced?
When was the last time the valves were adjusted?
What rpms is the engine idling at?
What is the prop size and pitch?
Can you get the engine up to 3600 in neutral? If so, that eliminates the throttle cable.
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post #7 of 16 Old 11-16-2010
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Fuel filter or bottom fouling (possibly explains the loud engine, struggling) would be places to start.

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post #8 of 16 Old 11-17-2010
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Have you ever been able to hit hull speed or get the engine to 3500-3600 under load?

If yes, then dirty bottom or prop, throttle cable slipped,or perhaps a clogged filter, perhaps the tach is off.

If no, then the prop is likely over sized/pitched for the engine and the prop will need to be addressed.



Check tach with a digital photo tach to confirm accuracy

Check bottom and prop to be sure it is clean

Check throttle linkage

Check engine in neutral and be sure you can get to 3600

Check for fuel starvation. It will generally not be smooth and it will surge at higher RPM. Unless it's surging it may not be a fuel restriction.

Check your speed confirmation. If using STW then make sure your paddle wheel is clean. If using GPS SOG make sure you are not fighting a current.

Check air flow to air intake.



About 7 times out of 10 I find this same exact complaint to be an issue of being over propped..

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post #9 of 16 Old 11-17-2010 Thread Starter
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I just read some of the comments from people; let me try to answer some of your questions. By the way thanks for the quick responses!
-This problem has always been around, even after the boatís hull was just painted and the prop/shaft cleaned. The boatís performance and handling of course improved but just couldnít get the higher RPMs.
-I have tried to increase the RPMs while at dock and the engine in neutral, no improvement.
-About a year and a half ago my father hired a diesel mechanic to remove the fuel tank and clean in out. He also added a fuel/water separator filter system before the lift pump. I believed there was some improvement at that time, but even the mechanic commented on how the engine just didnít get up to the rated RPMs.
-A co-worker suggested it could be the fuel injectors and need to be inspected and even clean. He recommended getting a five-gallon can of fuel and add diesel fuel injector cleaner and run it through the system and look for improvement. But he also suggested using an aerosol fuel injector cleaner, if I didnít want to bother using the liquid form, any suggestions on that part?
-Before I do anything else, I do plan to change out the fuel filter and of course bleed the lines. Also, disconnect the throttle linkage and see if I can manually push it up to the rated RPMs. I also plan to replace the air filter/screen assembley.
-As for the prop size, I donít know for sure. Itís a simple two-blade prop, but I will have to look up the diameter and pitch.

Keep the questions coming...given me a lot of ideas!

Thanks again.
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post #10 of 16 Old 11-17-2010
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If the rpm in neutral and in gear under a load are app. the same then I would suspect that you are not getting full throttle either because of a linkage/cable problem or an internal problem in the inj. pump. Obviously the linkage/cable is the place to start checking.

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