service manual for mid 80s inline 6 Merc 90hp

[fleetwin]

OMC goes by model year when it comes to service manuals, but I don’t think Merc does it this way. How does one go about finding the proper service manual for a mid 80s inline 6cyl 90hp. My friend bought one, and has it running, but I told him he should at least pull the high speed jets and clean them with a pipe cleaner.
Does this engine have three single barrel carbs? Where are the high speed jets located, and how do you get at them? I’m asssssuming the jets are behind the bowl drain plugs like on OMCs, but really have no idea.
Any guidance would be appreciated.

[20mercman]

Yes, three single carburetors. The high speed jet is behind a plug. These carburetors need to be synced to make sure that the throttle plates all are opening at the same time. This is a wonderful motor, and was one of Mercury’s best.

Steve

[jeff-register]

Don,
They started adding stuff like start enrichiners, Should be the extreme hi V ign. Yes they all had 3 carbs except the racing 6’s, Be careful of the ign, Something to do with a different spark plug too. Remove if so equiped a top block plug & run it out the bottom, always know the top of the motor is getting cooled, Compression might be a suprize, bet they are over 10% diff. Whats the gear oil look like? No bushings, just roller & needles. My books go to 1965 is all, so sorry,
Jeff
Change the wires 4 the plugs & check voltage rating. Joe Poole has carb parts. I would do the complete fuel system, pumps & hoses too. Those old hoses screw up the tiny jets flaking raho to start problems like a burned piston runnning dry. That is just me I can enjoy a carb job for weeks & Mag too. Try to get them better than new. I was the only one in the hood that was running a Mac 6 on my mini bike @ 14.
Regards,

[fleetwin]

Thanks guys, I know these are fine engines, and just want to be sure this one doesn’t get wrecked due to a lean condition from dirty carbs…
So, can the high speed jets be easily removed with a regular screwdriver, or is a special tool needed like the OMC jet screwdriver?

[jeff-register]

20 Merc man,
When we built Hi po Corvair engines reverse rotation. We had vacuum ports to get perfect balance which Mercury did not provide. More to tuning carbs than mechanical adjustments. That has always bothered me & sloppy linkage. Gets you in the ballpark I guess.

[jeff-register]

Takes a wide flat screwdriver & maybe heat. If you can get it apart without damage your doing good. Make a good fit driver.

[billw]

quote 20mercman:

Yes, three single carburetors. The high speed jet is behind a plug. These carburetors need to be synced to make sure that the throttle plates all are opening at the same time. This is a wonderful motor, and was one of Mercury’s best.

Steve

Plus one on the carb synch. The top carb throttle valve is driven by the advance mechanism, which drives the middle, which in turn drives the bottom. The plastic links get a little out of shape over time; so the bottom carb will open really late. On these inline sixes, that linkage adjustment is EVERYTHING to how smooth these motors can really be. It is very easy to check and make right, too.

Sometimes they would run rich at idle. If you can pump the primer bulb and overflow a carb, a set of floats, needles and seats will be a big help.

If you set the low speed carb needles for best idle quality, you may end up with acceleration bog. If this happens, just richen the low speed needles an eighth turn at a time until the bog is gone.

Depending on what year the last impeller was installed, the hub may spin in the rubber, causing a cook. Brass hub=bad; will only be safe to use for a year or two. OEM Mercury current production hubs are fiber-wound and are very reliable.

Jeff isn’t wrong about compression. They were known to stick piston rings, much like the cross-flow OMC V engines. Some Ring Free in the fuel might be a good idea.

Service manuals are zeroed in on by serial number, like all things Mercury. I have a list someplace that will give a Mercury part number for a manual, if you give the serial #.

Long live American manufacturing!

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