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Old 04-07-2021, 07:07   #1
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Low RPM Help

Hi there - Hoping for some troubleshooting help here. I have a Yanmar 3YM20 on a Hunter 31. Changed the oil/filter before storing on the hard over winter. Had no trouble starting once back in the water. Forward and reverse both fine, but RPMs won’t go over 2K—so she puts along slowly at only half hull speed. Water is discharging from the exhaust, impeller looks good, no smoke to speak of. Any thoughts on what might be limiting the RPMs? Thanks!
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Old 04-07-2021, 07:27   #2
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Re: Low RPM Help

Assume your prop is clean. (yes?)

Change your primary and secondary fuel filters and see if that does the trick.
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Old 04-07-2021, 07:28   #3
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Re: Low RPM Help

Lack of fuel would be the next thing to check. Did you change fuel filter?
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Old 04-07-2021, 07:29   #4
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Re: Low RPM Help

Next would be air intake
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Old 04-07-2021, 08:06   #5
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Re: Low RPM Help

I would be checking the fuel system. Replace your filters, the one on the engine too. They may be full of growth. Fill the filter bowls with fuel if you can.... to avoid air in the system when you do this. You may have to bleed the engine although Yanmar's are usually pretty forgiving.

-Is a fuel vent clogged with insects? This would only be a problem after you've run the engine for a while though.
-Is it possible the shaft is corroded inside the stuffing box so much it's dragging on the engine? SS closed up with moisture for a long time corrodes badly sometimes. Again, probably not super likely , but feel your stuffing box after running a while...is it hot?
- Do you have a feathering or folding prop? Is it maybe stuck/ not free to move?
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Old 04-07-2021, 08:22   #6
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Re: Low RPM Help

Check if the speed control cable from the cockpit has not slipped through the nut of the speed control lever on the engine. Keep it simple.
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Old 04-07-2021, 11:23   #7
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Re: Low RPM Help

Quote:
Originally Posted by Bill O View Post
Assume your prop is clean. (yes?)
The RPMs are normal when in neutral, it’s only under load that I suffer the performance issue. (Just did a test run and did notice excess smoking.) I haven’t take it from the marina in the month since it was put back in the water, so I didn’t expect a fouled prop. Does this sound more likely then the fuel issue, given the “under load” behavior?

Thanks for all the input.
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Old 04-07-2021, 12:03   #8
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Re: Low RPM Help

Exhaust elbow should be checked as well.
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Old 04-07-2021, 12:34   #9
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Re: Low RPM Help

Quote:
Originally Posted by CruisingDave View Post
The RPMs are normal when in neutral, it’s only under load that I suffer the performance issue. (Just did a test run and did notice excess smoking.) I haven’t take it from the marina in the month since it was put back in the water, so I didn’t expect a fouled prop. Does this sound more likely then the fuel issue, given the “under load” behavior?

Thanks for all the input.
What color was the smoke? Did it persist the whole time you tried to give it more throttle or just in the beginning?

Do not know your location so impossible for me to know if your prop might foul out in a month. I'm thinking probably not unless you are in very warm, fertile water.

Have you ever changed your fuel filter on the motor (secondary)? If not, would change both the primary and secondary, then see if that fixes it.

Another quick fix/check. Is the tank vent clear? Mud wasps, etc. can clog it from outside/thru hull causing similar symptoms to fuel starvation.

Also mentioned is the possibility your mixing elbow is getting clogged/rusting out. (would do the others first).

Let us know how things work out.
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