1984 90hp

[aarongreen123]

Hey guys, looking at a used boat with a 1984 merc on it. i am waiting on model info, dude is sending a pic of the plate with serial etc. The motor runs very well, in the driveway with the mouse ears on. I have no way to do a compression test. How much of a risk am i taking by just going by how the motor starts and runs? I realize its not new, and that this question could be totally retarded.

 

[aarongreen123]

Re: 1984 90hp

the serial number on the plate says 6571836

 

[Chris1956]

Re: 1984 90hp

OK, Serial number fits an '84 Merc inline 6 cylinder 90HP motor. Without testing compression and inspecting overall condition and checking gearoil for metal/water/other damage, you are taking a big chance on an old motor. I doubt you could tell if it is running on all six, five or even four cylinders, just by listening, which means it could need hundreds in repairs.

I think you need to touch it yourself...just my .02

 

[aarongreen123]

Re: 1984 90hp

Chris
Thanks a lot for the response, i agree with you. I am going to do a compression test, and check the lower unit oil, on that do i need to drain it all or just a little bitty?

For the comression test, is there a problem with me doing the following:
1. warm the motor up
2. remove the plug wires, labeled
3. remove first plug, turn motor over, test compression by turning over motor 4-5 spins
4. replace first plug, remove second, test compression


what should the compression be roughly?

 

[Chris1956]

Re: 1984 90hp

I would drain a little gear oil. if it looks brand new, ask when it was changed last. If it looks dark brown, the gears are probably fine.

You can do the compression test cold. Spark plug wires are obvious where they go, so labeling is unnecessary. You can pull all the plugs at once. I would jumper the starter solenoid, with the ign off. Wait for compression to max out and then write it down.

Compression needs to be close between cylinders, 15% variation. Any compression over 100PSI is acceptable.

 

[aarongreen123]

Re: 1984 90hp

ok thanks for the explanation and info, if i am unable to do the starter jumper what could happen?

 

[Chris1956]

Re: 1984 90hp

You only need a simple clip lead or short piece of wire to jumper the starter solenoid. If you cannot do that, tape the spark plugs bases together and ground them to the block. There is a small chance of ruining the ign system if the spark cannot get to ground, and you crank the motor with the key to test compression.

jumpering the starter solenid with the ign off, eliminates this risk.

 

[aarongreen123]

Re: 1984 90hp

Understood, thanks for explaining that too me