1984 mercury 75 hp extreem case of carbon build up in one cyclinder

hello fellow boaters. im haveing a problem with one cyclinder. i have carbon build up. every time i test the boat i run it at 4200 rpms for 5 to 10 min. and then all of a sudden the motor drops 1000 rpms. then i pull to the side and take off spark plug number one and see that its full of carbon. then i clean it put back on and run the boat for another 10 min and starts the same problem all over again. note that all other spark plugs are great. ive also used power tune spray and still continues the same problem. what eles can i do to fix this problem.

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

falcon305 said:

hello fellow boaters. im haveing a problem with one cyclinder. i have carbon build up. every time i test the boat i run it at 4200 rpms for 5 to 10 min. and then all of a sudden the motor drops 1000 rpms. then i pull to the side and take off spark plug number one and see that its full of carbon. then i clean it put back on and run the boat for another 10 min and starts the same problem all over again. note that all other spark plugs are great. ive also used power tune spray and still continues the same problem. what eles can i do to fix this problem.

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The first thing you need to do is a compression test on all cylinders. I think that's going to give you the answer your looking for.

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

Are you sure it's carbon? and not aluminium?

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

Extreme carbon comes from somewhere... ring lands or cylinder head. Do a sync and link but before any of that... the compression test is where you need to go first.

Carbon is unburned fuel and oil. Look at your timing, bad spark or weak spark, poor choice of 2 cycle oil or possibly a rich or even a poor mixture will do it.

My bet is the timing/sync and link is way off. If so, you may have toasted your motor. That compression test will tell you.

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

backyard mechanic said:

...Carbon is unburned fuel and oil. ....

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No it is not. Carbon is the produce of combustion...

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

hello guys. ive checked all cyclinders mutlipul times and all four cyclinders are over 120 psi. i belive its carbon build up because its only happening on one cyclinder that keeps fouling my sparkplug. dirty peace"s of grayish materail keep clogging up the sparkplug. pressure is great! now when i look into the cyclinder when i take of the sparkplug i see the head of the piston and see peaces of the grayish stuff chipping off. it looks alot like a hard crust. also when i test the boat every few min the sparkplug fouls and i quickly pull over trim the motor up take off sparkplug clean it up. put it back on. and then the motor runs great like nothing ever happen. i do this multiplu times for about 3 hours to see if it will soon clean up. (any info would be gratefull!!!!)

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

That dirty grayish material is NOT carbon (which is why I asked in the first place). Unfortunately for you it's aluminium, and it's off the crown of the piston. You have what we call a detonated piston... Which requires two things. Firstly, a strip of the engine to replace the detonated piston, and secondly, a thorough investigation as to why it did it... If you don't find out why it detonated the piston in the first place, it'll do it again, that's a guarantee.

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

Possibility of reed valve malfunction

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

crem1 said:

Possibility of reed valve malfunction

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Faulty reeds wouldn't cause detonation...

Causes of detonation include bad/old fuel (low octane number), lean fuel mixture (caused by, low float level, air leak in the manifold, air leak in the carb), too far advanced timing. There is a more detailed list in the manual.

Chris.......

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

how do i fix a detonated piston. how much would that cost me?

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

how do i fix a detonated piston. how much would that cost me?

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You would need to pull the powerhead and split the crankcase to gain access to piston and if you do it yourself you can save quite a bit of money. You will need a FACTORY service manual right off the bat. You will also need the recommended following parts:
A) Upper/lower crankshaft seals....................$23
B) Misc gaskets..........................................$25
C) 3 ring sets.............................................$36
D) piston...................................................$80
E) misc sealers & cleaners............................$15
D) impeller kit.............................................$35
E) machine work as cylinder is likely scuffed.....$100-200

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

thanks alot guys for giving me alot of info! but one more question. how hard is it to replace a lower unit. what are the things that i have to be carefull with. (the does and the dont does) there selling me the excat same motor. the only problem it has is the lower unit is shot. and i have my motor to use for parts. there selling me a 1984 75hp merc with a shot lower unit for 300 bucks should i buy it and replace the lower unit?

Re: 1984 mercury 75 hp extreem case of carbon build up in one cyclinder

Lower unit is not hard ,7 bolts and its off. Check the replacement motors compression before buying. It wil likely need carbs rebuilt from setting ...