Re: 1986 Johnson GT 150 Low RPMs
That 1986 150hp model hits its hp at 5000 rpm. The top rated rpm, not to be exceeded, is 5500 rpm.
Yes, that compression is low BUT that may simply be due to a gauge malfunction and/or the speed at which the engine is turning over. Best to check the compression and the spark with all of the spark plugs removed in order to obtain the highest cranking speed possible.
Normally on that model, you would be reading approximately 95 psi on the starboard bank and 90 psi on the port bank, the difference being due to the engine's design.
There is a 10 psi difference between the lowest and highest reading, not necessarily a real bad thing but not good either.
Using a spark tester whereas you can set a gap to 7/16", the spark should jump that gap with a strong blue lightning like flame... a real SNAP! Does it? (No tester... available at auto part stores, or build the following)
(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