1986 Yamaha 115 V4 Looses power at full throttle

johntking1981

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Feb 26, 2015
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I have a 1986 115hp V4 Yamaha and when I take it out on the water it will idle fine and when I put throttle to it the boat comes on plane and then the motor looses power. It sounds to me like it floods. I changed the spark plugs and put 15gal of new gas in inboard tank, Carburetor is new so I do not suspect it. I went to check compression and was very upset. The motor had not been run (it was cold) and I disconnected all the spark plugs, installed my compression gauge and turned it over about 4-8 times. I got the following data. 60# 90# 60# 55#. I am not sure if I messed up the test or need to go look for a new motor. Seems to me it would be the piston rings, pistons, or really bad blown head gasket. Any thoughts on this would be awesome.
 

Capt Ken

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Jul 30, 2002
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2,269
Rebuild your fuel pump. Run a heavy dose of ring free thru it and de carbon it
 
Last edited:

99yam40

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Sep 7, 2008
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8,851
try a different gauge to see if your other one was not reading properly

Putting new carbs on and adding gas to the tank does not mean you did not get a bunch of crud into the carbs from tank, lines, pump, etc. when you fired it up
 

johntking1981

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Feb 26, 2015
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I think I will get it running and warm up the engine and reperform the compression test.

Second, what do yall think about running some seafoam threw it from a separate gas tank? It may clean it out.
 

robert graham

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Apr 16, 2009
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6,908
A wide open throttle issue like you describe makes me think of a fuel flow/restriction/venting problem.....check fuel pump, filters, fuel lines and especially fuel tank venting....I'm doubting it's a compression problem since you mentioned it "comes on plane"......maybe consider installing a fuel/water separator type filter to help prevent plugged carb jets in the future
 
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