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Mariner 40hp 1986 revving problem

Poor performance can be a number of things.-----Up to you to do a compression test.---Then a spark check.-----Spark must jump a gap of 3/8" or more.----If those check out OK then look for fuel issues.-----Too much or too little fuel.-------Do not run the motor if you suspect fuel blockage as it can damage the motor !
 
thank you for your reply, the compression on each cylinder was 120psi, I wonder is there any chance that it could be the fuel pump?? any thoughts or experience????
 
speeds up a tiny bit sometimes and other times it doesnt so its tricky, carborettors are fine and floats are set, its loosing its smoothness at high revs trying to rev high and failing, im out of theories now
 
Well now----------Have you tested the spark yet ?--------How about running with a timing light hooked up to see how spark behaves going to full throttle.-------Up to you to do the work / testing.
 
I did squeeze trhe primer bulb once and it did speed up considerably, have not used a timing light yet, its like it is starved of fuel, maybe one of the cylinders, interestingly it revs very high when it is in neautral but when you rev it high and go down the river at full throttle it bigs down and shakes and then you have to throttle it down to half rpm to smoothing out the engine again
 
Most motors will rev up in nuetral, even running on 2 of 4 cylinders.--That sort of assumption / testing means nothing !-----Do some real testing to see what is going on !
 
It's obviously missing. Bad plugs/ wires/ coils/ possibly stator. Start with plugs, then wires and coils.

Jeff
 
Thank you for all your help. I took out the thermostat and found it to be broke in two ways, 1 that it was not closing and 2 the top part/collar on the thermostat was broke off or broke off, i have ordered a new thermostat today, what was happening was that the engine was running cold and there was oil building up on the spark plugs resultig in poor performance due to the spark plugs being covered in dark black oil , the engine was never warming up so hopefully in a week when i get the thermostat it should run properly thanks
 
I hope so, but.... The video sounded to me that one or both cylinders were "missing" (meaning not firing continuously).

Jeff
 
Thanks for your reply, I ran the engine with the new thernostat and it ran far hotter and smoother, it never got as hot before, there is warm water coming out of the water hole port, I got an inspection camera and had a look into the bottom cylinder through where the spark plug is and there is a thin layer of carbon on the piston head, I suspect that was something to do with it never warming up due to a faulty thermostat and idling too much, I usually use a 100:1 mixture of oil in the boat but i dont know what the previous owner used, should I run the engine hard to clean the engine out? There are some products for cleaning engines out that maybe I should try
 
Got a mechanic to check the fuel pump today and he found a hole on the diaphragm inside the fuel pump, hopefully this is the problem, i was also told that you leave out a plate and spring because if you put it back i, it will damage the new modified diaphragm
 
dual coil motor, After replacing the fuel pump diaphrams and gaskets on the fuel pump, took the boat out today and it rain at full throttle nice for 15-20 seconds and then it started to stutter at high revs, about two minutes later the engine completely cut out on me but started easily again and failed, it never turned off before i installed a thermostat in in that works because the previous one was broke open and therefore the engine was always cool, could the coils be breaking down, it is a dual coil motor with a mitsubishi logo on the coil and is about 35 years old, would the coil heat up from the engine and break down, the fuel pump and carboretor have been completely ruled out so it must be something else now but what i dont know.... does anyonew have any suggestions that they could add thanks
 
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