Re: johnson 120 with one cylinder burning rich why ?
(Compression & Spark Test)
(J. Reeves)
1st - Regardless of what problem one might encounter, always start with the basic troubleshooting procedure. Check the compression which should be in the range of 100+ psi (somewhat less on the smaller hp engines) and even on all cylinders.
2nd - With the spark plugs removed, rig up some type of spark tester whereas you can set a gap to be jumped. On magneto ignition systems.... and also the Battery Capacitance Ignition system OMC engines from 1968 to 1972, set the gap to 1/4". On the solid state OMC ignition systems from 1973 to the present, set the gap to 7/16". The spark should jump the gap with a strong blue lightning like flame..... a real strong blue snap!
NOTE: Checking the spark via using the spark plugs is a waste of time and proves nothing. The above stated gaps must be used for a proper test.
If there was no spark, on engines from 1973 up (If the electrical plug exists), disconnect the RED main electrical plug at the engine. Remove the s/plugs. crank the engine via the starter solenoid (jumper bat term to small 3/8" term nut, not the ground nut) and observe spark. If spark is now okay as stated above, the usual cause is a shorted ignition switch BUT that black/yellow wire could also.be shorted to ground at some point wthin the wiring harness.
For the later model engines where that large RED electrical plug was discontinued for updated electrical connections, find the black/yellow wire leading to the powerpack and disconnect it to run the above ignition switch/wiring harness test.
If the above checks out as it should, and the engine dies out when throttle is applied or won't idle, it is usually due to one or both of two things. The carburetors are fouled, in which case the cure would be to remove, clean, and rebuild them..... or the timer base under the flywheel is sticking which would result in a retarded spark. If the engine will not fire/start at all even though you have the proper spark and compression, it's usually due to fouled carburetors.
To check the timer base for sticking (Engine Not Running), spin the prop to align the shifter dog with forward gear and while doing so, put the engine into forward gear. Now, while watching the timer base, slowly apply throttle clear up to full throttle. The timer base should move smoothly all the way up against the black rubber cap that is atached to the end of the full advance spark setting screw. If the timer base sticks, find out why and correct that problem.
Usually any sticking of the timer base is caused by one of the four retaining clamps being slightly too tight. Putting a very thin washer under the clamp cures that problem. Some boaters use a thin screwdriver to bend the clamp upwards slightly but I prefer the thin washer. The sticking can also be caused by having a faulty stator start to melt down, resulting in a sticky substance dripping down on the timer base nylon retaining ring.
Let us know what you find, especially in the compression and spark department.
