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1984 Johnson 90 HP Compression numbers

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MitchB

New member
Looking for some diagnostic expertise on some compression test numbers on a 1984 Johnson 90 HP V4. Done after running for a few minutes. Starboard side top - 65 / bottom - 85. Port side top - 70+ / bottom - 80-. Definitely outside the 10% and seem low but can't find what the normal compression numbers should be. Including a picture of the spark plugs too. 3 of 4 look like the one on the left, coated with the black stuff. One looks like the ground wire is charred. Do these numbers and plug inspection tell me anything definitively...worth fixing it up or forget it. Also, an already questionable starter went out while cranking the last cylinder, hope I didn't do something stupid. spark plug.jpg
 
Thanks for the advice and oops, I didn't open the throttle up but had all four plugs removed. Learning the hard way kind of guy. I did check that each cylinder is sparking with an in line tester, by pulling each plug wire one at a time while running...how does that let me know if the cylinder is actually firing. The starter just clicks now and felt very hot after this deal. I am willing to replace the starter and possibly starter solenoid too because prior to this was firing right up.
The mix is the recommended 50/1 and known clean. I will look into the prop but the one that is on should be ok, but who knows in the past. I'll get the starter back in order and follow your steps. Will repost the results. Appreciate your reply.
 
Pull the small bypass cover to inspect the pistons and rings.----A cold running cylinder will have a black plug.---Pulling the bypass covers will only cost you for 4 new gaskets at about $1.00 each.----Throttle does not need to be open to test compression on a 2 stroke like yours.
 
I assume by post #3 that you have converted from the VRO setup to a premix setup.

Compression in those ranges..... you've either got a defective gauge, a starter that is cranking the engine over too slow, or a engine that is in need of a major overhaul.

The throttle does not need to be open when checking compression.

The condition of the spark plugs (black) indicates weak ignition or an extremely rich mixture.

The spark (with plugs removed) can be tested as follows:

(Spark Tester - Home Made)
(J. Reeves)

You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:

A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.


Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:




..........X1..........X2


.................X..(grd)


..........X3..........X4
 
Thanks for all the helpful input. Got to get the starter replaced so I can get her running. Praying for just a weak starter and battery combo, even though battery tests ok it is old.
 
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