Re: service manual for mid 80s inline 6 Merc 90hp

[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!