RPM drops suddenly on OMC I/O

John Harting

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Nov 21, 2005
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I have a 1985 2.5L 4 banger on my houseboat. I will be crusing at 3K RPM, and the motor drops to idle, does not stall or run rough. Put the shifter into neutral, engage forward and it goes back to 3K RPM with no problem. It does the same thing when holding the houseboat to shore when we are staking it down, which is when it can get exciting (drops into idle and the wind starts pushing it to shore). There was a similar entry mid last year with some suggestions but no real answer. The engine did the same thing for the previous owner 8+ years ago. I've installed a new fuel pump, rebuilt carb, copper fuel pickup in a custom 120Gal tank with 90+ gallons of fuel, checked timing, coil, etc. I consider myself a pretty good mechanic but I cannot figure out how it goes from 3K RPM to 600 RPM with no starving or ignition symptoms and recovers every time. Any suggestions would be appreciated.
 

Don S

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Aug 31, 2004
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Re: RPM drops suddenly on OMC I/O

Have you checked the trottle linkage to see if it's still got the throttle at open? You might try disconnecting the throttle cable from the carb and giving a good pull on the inner cable. I'll bet the inner strands are broken and one is holding by friction alone and sometimes give way.
 

John Harting

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Nov 21, 2005
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Re: RPM drops suddenly on OMC I/O

That is a thought, the cable is 20 years old and is 40+ ft long. When I have someone work the throttle while I'm in the motorbox it always seems to return to normal idle position. I'll give it a try.<br /><br />Any other thoughts out there?<br /><br />Houseboat John
 

rodbolt

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Sep 1, 2003
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Re: RPM drops suddenly on OMC I/O

next time it does it use a squirt can and spray fuel down the carb, if it picks up it was lean, if it does not its on to the next issue. do you know what ign system is on the motor?<br /> if you spray fuel wear eye protection and keep the old fire bottle handy.
 

Richard Petersen

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Dec 17, 2004
Messages
778
Re: RPM drops suddenly on OMC I/O

I do not see how a normal running 3000 rpm engine can drop to 600 rpm with the throttle plate in the same position and not die or be so rough at idle. I vote for A loose flex cable clamp at either end of it. That will cause similar symptons. Maybe even no clamp at 1 end.---edit. I am talking about the clamp that holds the outside sleeve from moving at all.
 

John Harting

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Nov 21, 2005
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Re: RPM drops suddenly on OMC I/O

I certainly haven't seen something loose on the motor side, but I haven't checked the throttle control side. I agree that the fact that it drops RPM and idles smoothly cannot be independent from the throttle being closed somehow. I'll check out the suggestions and report back with what I find.
 

Bondo

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Re: RPM drops suddenly on OMC I/O

Ayuh,.......<br /><br />I like Don's Idea,..... I sure can see That Happening,.............<br />Like Don says,...... 10 minutes,+ a couple of 7/16ths" wrenches will Diagnois that...........<br />Or,........<br />Set the Throttle to WOT,+ go to the motorbox,+ Manually Try to return the Carb. to Idle, from that end of the Cable...........
 

tony_56

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Dec 21, 2005
Messages
14
Re: RPM drops suddenly on OMC I/O

yea i would have someone personally get in the hull and when it drops to 300 rpm take a look at it, see if the throttle plate moves at all and get a fuel pressure tester and see what it does when it drops down, you might have a electronic issue with the fuel pump??? very small chance but you never know?
 
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