75hp Mecury

[lyncraft_mechanic]

I have a late model 75hp mercury 3 cylinder outboard probably from the early 80's, it has been stalling at high speed so i cleaned all the carbs and blew them out with compressed air, did a compression test and spark test all were up to specs. after the carb cleaning I saw an improvement but the motor still wants to die at anything above 3/4 throttle(like it's running out of gas) and has been idleing rough. does any one know what the correct air adjustment on the carbs are? is it 1 turn or 1 1/2 turns? does anyone have any suggestions on what could cause this problem?

 

[Barnacle_Bill]

Re: 75hp Mecury

If it has the square type fuel pump, rebuild it. Be sure to get the kit from Merc as it will contain new type check valves and instructions for installing them.

 

[BoobieBouncer]

Re: 75hp Mecury

My 85hp had the same problem.

I replaced the fuel pump diaphragm and it fixed the problem.

http://forums.iboats.com/showthread.php?t=311655

 

[Silvertip]

Re: 75hp Mecury

There is no one special setting for the idle mixture screws. 1-1/2 turns out from lightly seated is a "starting point" for final adjustment. You need to adjust them for best idle with the engine in the water - not on muffs as there is no back pressure when running on muffs.

 

[lyncraft_mechanic]

Re: 75hp Mecury

okay i will give it a try, another thing would surface gap spark plugs have something to do with the high speed problem?

 

[verado7]

Re: 75hp Mecury

by surface gap if you mean gapless plugs then likely not unless they are really old or have cracks in the porcelan - but you change yours yearly though right - lol do th efuel pump like suggested the diaphrams do get weak and even leak thru to each other . post back on the outcome .

 

[lyncraft_mechanic]

Re: 75hp Mecury

well I recently discovered that gas was getting in to the oil tank so i did replaced the fuel pump, spark plugs, oil pump and 2 coils. I drained all the old oil and put new in. i had been expierencing very low idle and the motor would stall while in the water so i advanced the throttle cable adjustment but the motor is still running very poor. I noticed that it is producing little to no smoke while running. does anybody have any suggestion where to go from here?

 

[Moody Blue]

Re: 75hp Mecury

Take the motor for a hard run if you can and do a high speed shut down. Don't throttle back, just kill the ignition while in gear at speed.

Pull the plugs and see what they look like. If they are really clean like they have been steam cleaned then you are getting water intrusion into the cyl(s).

Most likely to occur at higher rpms because the water pressure is higher.

 

[lyncraft_mechanic]

Re: 75hp Mecury

it could be the head gasket because today i noticed that the middle spark plug was unusually clean, the motor does not want to idle and when i took it out today the motor produces white smoke and had excessive vibration. where is the most common entry point for water to get into the cylinders? is it thru the head gasket?

 

[lyncraft_mechanic]

Re: 75hp Mecury

Bump

 

[lyncraft_mechanic]

Re: 75hp Mecury

well i took off the cover,that the spark plugs thread into, on the back of the motor thinking it was the head but found it was only the water cover. does this motor have a head gasket? would i have to split the block to install a head gasket? how else would water be getting into the cylinders, i don't think the block is cracked?

 

[lyncraft_mechanic]

Re: 75hp Mecury

i talked to local mechanic and i guess the only way water could enter the cylinders would be a crack in the cylinder wall or possible a spark plug not being tight enough. before i put it back together I'm going to have to make sure nothing is cracked and maybe perform a leak down test on the cylinders.