Engine cuts out and warning horn at mid to full throttle

tablesaw?

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Mar 1, 2014
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Just bought my first boat last spring. What started as an intermittent problem has become a full time problem. Motor cranks and idles perfectly. It used to run full throttle perfectly. If I reduced throttle at cruise even a little it would start stuttering and jerking. Now I can't go above 1/3 throttle before it starts and the horn sounds. I installed a new water pump and fuel pump. It's been tricky as there is no serial# tag and the silver medallion on starboard side of motor seems to be from a mercury. There is an M at the top and G2595843. I have it guessed as a 1988 Johnson 20 or 25 hp. I drained the float bowl and it looked like clear water coming out. Any help would be greatly appreciated.
 

racerone

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Dec 28, 2013
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That silver medallion is NOT from a Mercury at all !!!!-------Likely the M is inside a diamond and an OMC identification.-----Salt water motor ??------Have you removed cylinder head and inspected the water " deflecters ??---Was the old waterpump bad ??
 

tablesaw?

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Mar 1, 2014
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Yes it's a diamond. Don't know about the saltwater. Last owner implied it had 30 hp carb. The engine cowl was repainted with new 20 hp stickers because of local lakes with hp restriction. Haven't pulled the head, have been focused on possible fuel restriction. Ran it yesterday after changing fuel pump and it ran like a demon until I stopped and stared again. Then it was same old problem.
 

bob johnson

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Feb 25, 2009
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I think OMC used the slanted M to designate system MATCHED components...

there should be a TAG on the engine clamp bracket.....for some years it was a sticker.

bob
 

tablesaw?

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Mar 1, 2014
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The serial # is gone. The silver medallion on the engine is G2595843.The old impellor looked as good as the new one I put in. Tomorrow I'm going to test the spark and compression. When going over the motor, I found some of the bolts on the head cover(?) were loose and I snugged them down until I can get hold of an in/lb torque wrench. Don't know if this helps but I drained the carb bowl and it looked like clear water. I have replaced the fuel tank, fuel line,and fuel pump. I can't find a fuel filter but plan to install one and a water separator. Any help would be greatly appreciated.
 

Bosunsmate

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Apr 7, 2012
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Just bought my first boat last spring. What started as an intermittent problem has become a full time problem. Motor cranks and idles perfectly. It used to run full throttle perfectly. If I reduced throttle at cruise even a little it would start stuttering and jerking. Now I can't go above 1/3 throttle before it starts and the horn sounds..

Whats the horn sound like? Full time or beeping?
Have you a VRO?
Is it pissing water ok?
Hows your plugs look? Same colour or some a different

Idling perfectly is a good start as that means the reeds and many other things are working normally.
 

Bosunsmate

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Apr 7, 2012
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. When going over the motor, I found some of the bolts on the head cover(?) were loose and I snugged them down until I can get hold of an in/lb torque wrench. \.
You may of had water pressurising the cylinders at higher speed pressure as the water pump increases, snugging it down may have helped that. Replacing them and cleaning out the passageways, shaving the heads on gass with sandpaper would help even more but comes at a cost of the parts
 

racerone

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Dec 28, 2013
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VRO was never used on the 20 / 25 / 30 HP 2 cylinder motors !----The block has " water deflectors " installed.----These guide water flow around the block.----If they are out of place it can result in reduced flow.-----Look into this possibility.----Do some research on the subject.-----Requires removing cylinder head and a new gasket.
 
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tablesaw?

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Mar 1, 2014
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The horn is steady but stops when I back off the throttle. No vro. Water flows good even at idle. Plugs look the same, I did check the gap and one was.045 the other .040 I set them both at .040. Motor has always cranked, idled, and run full throttle perfectly. The stutter and jump problem was intermittent and only when I tried to back off full power for cruising. It has evolved over the summer to this. The symptoms now started about 5 or 6 last trips to the lake. If water was getting in there should there be some sign of rust in the cylinder?
 

Big Fish Billy

Petty Officer 1st Class
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Dec 31, 2009
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301
If water was getting in the plugs would be white or steam cleaned. If there is water in the fuel system, sitting in the carb bowl, when you throttle back, you may be sucking in in. I'd try new gas in a remote tank to rule that out.
 

tablesaw?

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Mar 1, 2014
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Both plugs look the same and are pretty black. I check the spark yesterday and they both had good blue spark dancing around the tester. Still haven't gotten my hands on a comp gauge but that's next. Did find a head cover bolt I had missed when I was tightening them, it was really loose. Searching old post I found people with very similar symptoms but none were answered or resolved. Hope I have better luck. I will definitely post the cure if its ever found.
 

tablesaw?

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Mar 1, 2014
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Ok the compression was 120 on both:) and good spark on both. Took it to the lake yesterday and same ole problem. Starts and idles perfect, runs good to about mid throttle and the horn and cut out start. If I leave it at full throttle it acts like its trying to smooth out but the horn sounds steady. The horn sounds as soon as I push it past mid within a minute of leaving the dock cold. Could it be overheating that quick? When it stops raining I'm going to pull the carb and run a wire through the jets. As many old posts that I've found with similar problems I can't believed it hasn't been solved before. Thanks for any help.
 

boobie

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Nov 5, 2009
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I'm going along with possibly bad water deflectors. These motors were great for that. Rather then messing around, pull the head and look.
 

tablesaw?

Cadet
Joined
Mar 1, 2014
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14
Fixed it! There was a chafed wire going from I think the thermostat to I think a power pack under the flywheel. Motor runs smoother and faster than ever before. Guess tightening a few bolts didn't hurt. Thanks for the replies and suggestions.
 

Jimdsi

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Feb 22, 2007
Messages
29
That makes perfect sense. Some engines have the "S.L.O.W" system that limits RPM when an overheat occurs. Yours was artificial due to a chafed temp wire causing a false overheat condition. Congrats on the fix.
 
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