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Old 15-06-2015, 16:12   #1
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3GM30F Power loss

Background 1997 Beneteau 321
3GM30F
Just got back from a sail ,on return to moring we were powering into a 25 MPH head wind and falling tide (strong current)
After aprox 45 minutes RPM droped to 1800 for a few seconds then returned to cruse RPM 2500 (engine smoothe)repeated this sequence 2500 down to 2000 RPMs another 4 times
before we were back on mooring,engine runs smoothe in and out of gear ,
Took a few minutes and checked vent line to fuel tank OK
Day 2--Motor cold on mooring engine started on first roll of starter,ran engine ran normally (no load) took RPMs up to 2500 no issues
Took boat off mooring after about ten minutes RPMs droped from 2500 to
1800 then back to 2500 and kept cycling like this , returned to mooring engine stalled during pick up of mooring ,restared without any problem
Any suggestions? Thanks in advance
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Old 15-06-2015, 16:24   #2
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Re: 3GM30F Power loss

Dirty fuel filter(s)? Air leak?

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Old 16-06-2015, 07:11   #3
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Re: 3GM30F Power loss

Sounds like classic dirty fuel filter behavior. I would suspect this first.

Could be a vaccum leak, but not likely unless you've worked on some part of the fuel system recently.

Could be an intermittently clogged vent. This has happened to me on a few monos in very rough weather. Their vent hoses had a dip in them that would fill with fuel when heeled hard in rough weather...then drain when the boat stood up straight.

Could be the pick up tube sucking air...particularly on monos with irregularly shaped tanks in rough conditions with lots of heel.

But, I would bet its just a clogged fuel filter.
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Old 16-06-2015, 07:17   #4
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Re: 3GM30F Power loss

Filters always clog at the worst moment. A minor enhancement that can save your butt in these circumstances is to install a pump or bulb upstream of the filter. Then you can apply some positive assist pressure upstream of the filter to keep the engine running those last few feet. Also handy for maintenance use, like priming the system after fuel filter changes.

A vacuum gauge is also handy so you can see the problem developing.
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Old 16-06-2015, 07:46   #5
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Re: 3GM30F Power loss

Parallel fuel filters, with the vacuum gauge. Mine,also, has a Racor vacuum switch in line with the vacuum gauge. When the vacuum reaches 3 (the yellow zone), it sets off a buzzer and an LED light above the vacuum gauge. Then I pop down and flip the A-B fuel switch to a fresh filter. You then have plenty of time to change the filter. I learned this from making deliveries where the tanks got stirred up and we had picked up crappy fuel. It gives me a great deal of confidence entering a difficult harbor channel in bad weather.
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