Yanmar SB 12 wont reach maximum RPM in neutral - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 02-28-2010
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Yanmar SB 12 wont reach maximum RPM in neutral

I have a 1976 Cal 29 with a single cylinder 12 horse Yanmar. The engine has about 1400 hours on it with no visible leaks or or maintenance issues. It appears to have lots of compression as it starts easily even in cold weather. The problem I'm having is that it will only rev up to about 1900 RPM. In gear I get black smoke and oily residue at 1900 and I understand that it may be over proped but this dooesn't explain why it wont rev up in neutral. In neutral there is no smoke but it will only rev to 1900. There no restrictions in fuel flow as it has a new Racor filter and clean tank. Also it doesn't feel like a fuel issue as it will pull hard right up to 1900 rpm. I have also checked the linkage and it appears to have full range of motion right back to the stopper.
The previous owner said he only ran it at 1200-1400 rpm. I can only make about 4-4.5 knots at that rpm. If anyone has any ideas I would appreciate it. Thanks.

Kevin
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Old 02-28-2010
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Black smoke is unburned fuel carboning up. A diesel needs the right amount of fuel as well as unrestricted air both in and out to burn cleanly. If the air intake is clear I would check the exhaust system for blockages and restrictions, especially the elbow where the water is injected into the exhaust. If the previous owner ran the engine at low rpm for a long time as you say it could be very carboned up.
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Check the prop, barnacles, a ball of line etc.
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I've checked for a fouled prop but it wouldn't explain not reving in neutral would it?. The intake has a large silencer type unit but I don't really see an air cleaner. I will check the exhaust manifold for carbon. Thanks all.
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The problem could be exhaust back pressure. It could also be a tiny air leak on the fuel lines.
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I've checked for a fouled prop but it wouldn't explain not reving in neutral would it?. The intake has a large silencer type unit but I don't really see an air cleaner. I will check the exhaust manifold for carbon. Thanks all.
I should read closer before I post. It does sound like she starving for air. Pull that cleaner off and see if things changes.
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Sailingdog can you eplain what you mean by exhaust backpressue. Thanks
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If the exhaust mixing elbow or other parts of the exhaust are clogged or restricted, the exhaust gases can not escape the engine quickly enough. The exhaust which builds up ends up preventing the engine from breathing properly since the exhaust doesn't end up leaving the cylinders the way it is supposed to, and the symptoms can be very similar to that of a clogged airfilter.

Basically, if you think of the engine as an animal... if it can't exhale properly, it can't inhale properly...and it can't run very well if that is the case.

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Sailingdog can you eplain what you mean by exhaust backpressue. Thanks
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If run at low rpm for a long time I would bet on a restricted exhaust elbow near the water injection point. The air intake would be obvious if clogged and I think this is unlikely.
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