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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 10-12-2008
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more RPM issues - yanmar

Earlier (early August) I posted an issue where my fairly new (2004) 1GM10 would not rev past 2100 RPM - turned out the fouled prop was an issue. (have a martec folding prop)

So here's what is happening now (yesterday) 8 weeks later...(this happens both at the dock, and when underway)
  • Motor revs in neutral to 3000+ rpm no problem.
  • Motor runs in gear (REVERSE) to 3000+ rpm - no problem.
  • Motor runs in gear (FORWARD) to 2500rpm - will then rev up to 3000 rpm maybe once every 10-15 min or so - definite vibration - not severe but more than what should be expected..
  • Regarding smoke - no smoke per se - just some very very light haze - light grey - very light.
Diver checking bottom later this week - i thought at first it was the prop - but why would it run fine in Reverse, but be an issue in Forward? Also checked position of throttle control arm on engine - in forward and reverse it appears move same amount so don't think it has anything to do with the throttle cable. I'm hoping there isn't an issue with the governor or the transmission...

Will let you all know what diver has to say. Day wasn't all bad though- had a nice sail.

Thx

Rick
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Old 10-12-2008
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Off the cuff I would say that your prop remains the problem. Remember a prop reacts differently in reverse and forward, both from the perspective of how it unfolds and it's torque characteristics.

I can't see any reason what would cause an engine to behave differently in one direction from the other.
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Old 10-12-2008
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That bit about every 10 to 15 minutes it reaches 3000 rpm in forward is a bit odd. It suggests the prop is not stopping it reaching 3000. I would suspect the throttle control or the gearbox - what sort is it?
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Old 10-13-2008
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Any change in speed when the RPM increases to 3,000?

Rick
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Old 10-14-2008
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My 1GM

My Marshall 18 catboat, GALA VI, has a 1GM (an earlier version of the 1GM-10). This engine is rated at 3400 r.p.m. continuous and 3600 r.p.m. intermittent ("1-hour" rating). A propeller improperly matched to the hull and the reduction gear ratio can cause the engine to fail to rev up to specs, though this usually results in black or brown smoke. In fact, any factor external to the engine which would reduce r.p.m.'s would cause this, and also cause overheating after a while. The increase from 2500 to 3000+ is puzzling, but could be due to cavitation, I suppose, which would also cause vibration. The lack of excessive smoke tells me that the issue may be INTERNAL to the engine, probably the centrifugal governor, keeping the throttle from opening all the way. In neutral, the engine should max out at 3600 (don't leave it there long when out of gear!). If it doesn't get all the way to 3600 in neutral, you have a governor issue.
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Old 10-19-2008
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Problem solved.

Diver cleaned prop - she powers up in forward just as she did in reverse prior to the cleaning. As such Omatako's suggestion of hydrodynamic difference in FWD v REV was the issue.

Rick
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