Why won't my engine won't go above 1700 RPM?

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May 1, 2011
4,189
Pearson 37 Lusby MD
Took the boat out yesterday - glorious 50NM on the water. Going out and coming in, engine easily got up to 2100 RPM. Today, on a solo run, engine cranked and started normally. Got out of the slip and gave it fairly high throttle and it wouldn't go above 1700 RPM. The engine didn't sound like it was firing on all three cylinders (Universal 3-banger in a Pearson 37). Looked over the stern and saw black crap in the water. Returned to the slip.

After tying up, opened the throttle wide (no load) - got about 2000 RPM, still putting black crap in the water. Shut the engine down, checked the oil level. Normal. Any ideas? Tks.
 
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Sep 25, 2008
7,077
Alden 50 Sarasota, Florida
You know it's difficult to diagnose these type problems on the internet as the possibilities are almost endless from something interfering with the prop, a bad injector, bad fuel, clogged filter, restricted or bad mixing elbow, etc...

All I can suggest is start with the easy (cheap) possibilities by checking the running gear, fuel and filter(s) and inspect the mixing elbow to eliminate those possibilities before digging into the engine or replacing injectors.
 

zeehag

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Mar 26, 2009
3,198
1976 formosa 41 yankee clipper santa barbara. ca.(not there)
if your engine has a mixing elbow--check it for blockage or corrosion. check injectors. .
 
Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Unless you changed something I'd say the fuel system is OK as it worked fine previously. checnk air cleaner, exhaust elbow for restrictions. If it is not firing on all 3 then do a compression test to see which cylinder is having issues.
 
May 11, 2005
3,431
Seidelman S37 Slidell, La.
First thing

The first and easiest thing to check is the air filter. Don't know what that engine has, but the foam filters can deteriorate and cause this kind of problem. The usual list of suspects to check, but start with the easiest first.
 
Mar 20, 2012
3,983
Cal 34-III, MacGregor 25 Salem, Oregon
I assume its a diesel engine your having trouble with..... but either way the black stuff is unburnt fuel on the water... an air restriction will cause this or an injector tip that has blown off.... usually if there is more serious problems like a hole in the piston, you will get whitish smoke..... although the injector tip going thru a valve can be serious too if it hangs up or bends a valve....
you can tell which is the suspect cylinder by loosening the injector lines individually while the engine is running.... the bad cylinder will have no change in how it runs when its loose..... i would bet its an injector gone bad.
if its a gas engine, you have a fouled or otherwise failed spark plug..... good luck.
 
May 1, 2011
4,189
Pearson 37 Lusby MD
Turns out the exhaust elbow (mixing elbow, exhaust riser - whatever name you prefer) was plugged up. The scaling constricted the exhaust, leading to insufficient air flow through the engine and a rich fuel mixture which in turn started clogging the exhaust further with carbon ("black crap"). So it's been cleaned for now and will probably be replaced during winter refit. Thanks all for your inputs.
 
May 1, 2011
4,189
Pearson 37 Lusby MD
Took the boat out with the Scouts on an overnight training sail - it was like having a new engine! Lots of RPM, lots of acceleration, etc. The engine was quieter and sounded much lest strained. Guess this has been working for a while.
 

jtm

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Jun 14, 2004
312
Hunter 28.5 Dataw Island, SC
I would have added to check that your compression levers are FULLY engaged - sometimes they look like position engaged but have't "clicked"into position.
 
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