1979 Merc 90 Specs needed

[sjj623]

I have had a really difficult time finding correct specs for this motor. It is a 1979 1/2 (per the sticker on the motor) Mercury 90 hp serial 52999XX. It has a chrome and blue stripe around the cowling. It has a distributor-less ignition. I am looking for WOT rpm range, compression test psi range, and lower unit gear ratio.

I read in a clymer manual the WOT rpm range should be 4500-5000 but I have been told it should be 5000-5500. Since neither spec was supplied by a Mercury manual, I am not sure which it should be.

The comp test shows all cylinders within 4 psi of each other (100-96) but I can't seem to find a spec as to the psi range. Since they are all within 4 psi of each other this would seem to indicate decent engine health. But, it doesn't tell me if my psi reading is on the high side, low side, or really low side and I should do something about it.

Lastly, by turning the flywheel and counting the prop revolution the lower unit gear ratio appears to be 2:1. Can anyone verify this with a manual?

I am aware of the Classic Mercury hotline but would rather not pay per question.

So, if anyone can help me with this info from a real live Mercury manual I would really appreciate it.
Does anyone know where I can find a real live Mercury manual for this engine?

Thank You

 

[444]

Re: 1979 Merc 90 Specs needed

I've got an 84 merc 90hp 6cyl. Should be the same motor as yours. I read somewhere else too that the max rpm should be somewhere near 5300 or so. My tach doesn't work so I don't know for sure. Mine's on a 15' speedboat so I can go plenty fast without maxing out the throttle until I get my new tach put in.

I recently did a compression test on mine and I got all 6 cylinders between 130-135psi. I did the compression test with all spark plugs removed and throttle wide open. I've only ever had one 2-stroke that ran well with less than 100psi of compression and that was an old ski-doo with a 1 banger. And usually 100psi is considered the operational limit. However typically the emphasis is on all cylinders being within 10% of each other being ideal and you have no problem there. Several things can affect the outcome of a compression test including having the spark plugs/throttle like I did, altitude, engine temperature at time of the test(I like to do a cold test and a hot test and compare), condition of battery/starter and how accurate your compression tester is. If it starts up well, runs fine and all compression test numbers close like yours, I would not be too concerned with the actual compression #'s. With mine I usually start it up and run it in neutral for a minute or so to warm it up before I pull away from the dock. These old mercs don't have thermostats so they run quite cool during normal operation, which does make for an easier 'hot' restart. And I say that loosely because the water comming out of the pee hole never gets hot to the touch.

 

[Chris1956]

Re: 1979 Merc 90 Specs needed

Gear ratio is indeed 2::1 as are all inline sixes. Your compression gauge likely reads low. Compression should be 120PSI or so, however, since all your cylinders are nearly exact, the compression is fine. Prop it to run 5500 at WOT.