evinrude 140 losing power

supmat

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I have a late 70s evinrude 140 horse that started taking longer to get on plane and lost about 7mph in top speed. So far I have replaced the plugs,drained the tank and replaced the fuel, and replaced the throttle cables (they were sticky anyway). I am a novice and at a loss for what to try next, any suggestions?
 

oldboat1

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Picking up weight in hull (water/water logged floatation). Poor or diminished motor operation (need to describe).
 

jimmbo

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How long has it been since you decarbonized it? When winterized, did you add fuel stabilizer or drain the carbs?
 

supmat

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I did a compression test and all seem to be about 105, one of the spark plugs however looks brand new. I performed a spark test and it is sparking, what does that mean?

I use my boat pretty much year round so no winterization. The boat does take water but seems to act the same whether the bilge is pumped or not.
 

racerone

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Sounds like the motor is running on 3 cylinders.----Check the carburetor and the reed valves for the cylinder that has the new looking plug.-----Could be a bad plug.------Does the spark jump a gap of 3/8" on that cylinder with the new plug.
 

supmat

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These are the plugs in my boat, how can I test if this is sparking hot enough?
 

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racerone

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The sparkplug is NOT used in doing that test.-----Will the spark jump a gap of 3/8" or more on that cylinder ?
 

supmat

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How do i test that....fwiw this problem was happening before these plugs were installed.
 

emdsapmgr

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Once you find the final problem, I'd suggest you get the correct Champion plugs for that engine. OMC's just seem to run better with Champions. A late 70's 140 hp crossflow should have come from the factory with over 130 lbs compression on each hole. If the 105 is an accurate reading, the engine will not make normal hp. It may run, but will be anemic.
 

jimmbo

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Run a good engine cleaner thru it. it just might be sticky rings. Don't be too worried about the compression numbers, there can be a bit of variance between gauges. The numbers are all the same. As compression goes up it take more voltage to jump the spark plug gap. That's why a spark tester has a gap of about 5/16 in. min. As for spark plugs, in the 70s we always had the best results from AC plugs. In the 80s and 90s I use NGKs with good results. I never had good luck with brand "C"


"I use my boat pretty much year round so no winterization. The boat does take water but seems to act the same whether the bilge is pumped or not. "

perhaps the water is not accumulating in the bilge but is saturating the floatation foam
 
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emdsapmgr

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Possible the fuel pump is getting weak. You can try a test. When you get the engine running on plane, have a buddy constantly prime the fuel hose bulb. This will force extra fuel into the carbs. If it runs normal rpm's as long as you constantly prime the fuel hose bulb, the pump either has a restriction or may need replacement.
 

supmat

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I tested the plugs by cranking the motor and seeing if the spark would jump 3/8 or so and all did. They new looking one was actually the strongest. These plugs are the oem plugs for this motor I have been told by a dealer when i bought them. I replaced the fuel pump/diaphram about a year ago, but i will try the bulb prime test the next time I'm on the water. Another odd thing I didn't think was related but I now wonder is that there is oil running from the power head dripping on the floor, not much but some. Is this coming from unused fuel?
 

supmat

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It is on the bottom left if viewed from behind the boat. I'm trying to figure out what number that is.
 

emdsapmgr

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All these old two strokes have a problem with exhaust residue. Unburned fuel/oil coats the inside of the exhaust tube. Over time, the black, tarry, residue builds up, and it starts to drip out of the prop. Normal for these old crossflows.
 
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