Johnson 1979 100hp cylinder not firing

[Ffarrell]

Hi guys, this is really wierd. The lower left cyl(standing behind the motor) does not fire, unless I remove the top plug wire. Compression is 118 on that cyl, 125 in max cyl. Carbs rebuilt last year. Pull lower cyl plug wire, no drop in rpm. While lower wire is pulled, pull top and motor almost stalls. Reconnect all wires and restart. Pull top wire and no drop in rpms, meaning bottom starts firing. Pull bottom wire and motor almost stalls. Certified Johnson mechanic was stumped. Checked the stator, timer base, power packs, coils, no problem. Anyone ever see this before? Could it be mechanical - exhaust from top cylinder after it fires somehow leaking into bottom cyl, perhaps a cracked block? Perhaps some form of leakage or short in a connection to the powerpack or coil? Im comletely lost. When both cyls fire(rarely) the motor is very strong.

This is cross posted in another forum.

Help!

[Joe Reeves]

I've seen the other post and have held off as what you're saying is taking place makes no sense... never come across such a happening. However..........

Standing in back of the engine, facing the spark plugs, LEFT is "PORT"... right is "STARBOARD". The cylinders are number as follows:

2....1
4....3

So you are talking about #4 cylinder.

The ignition is completely solid state (no points) and has two timing sensors in its timer base. One sensor fires one bank, the other sensor fires the remaining bank. Which cylinder either sensor fires is taken care of by the center magnets of the flywheel. The south pole of the magnet may fire the top port cylinder as it passes a particular timing sensor with the north pole firing the bottom cylinder as it passes that same sensor etc.

There are no moving parts to go out of whack excepting the whirling flywheel.

I ask myself... why would removing the top plug wire (you don't say which cylinder) cause #4 cylinder to fire? I've never encountered this problem but you're obviously causing a electronic imbalance somewhere.

Check the continuity of "all" the coils as follows (below), then break open your service manual to obtain the readings of the coils, timing sensor, anything in that ignition circuit.
- - - - -

(Magneto Capacitance Discharge Coils - Continuity Test))
(J. Reeves)

Check the continuity of the ignition coils. Remove the primary orange wire from whatever it's connected to. It may be connected to a powerpack screw type terminal, a rubber plug connector, or it may simply plug onto a small boss terminal of the coil itself.

Connect the black lead of a ohm meter to the spark plug boot terminal, then with the red ohm meter lead, touch the ground of the coil or the powerhead itself if the coil is still installed.

Then touch (still with the red lead) the orange wire if it's attached to the coil, or if it's not attached, touch the primary stud of the coil. You should get a reading on both touches (contacts). If not, check the spring terminal inside the rubber boots of the spark plug wire. Poor or no continuity of a coil is one reason for s/plug fouling.
- - - - -
What spark plugs are you using and set at what gap?

[Ffarrell]

Yea, I agree

The wire I pull to make #4 fire is #2. Both attached, 4 doesnt fire and can be pulled wth no drop in rpm. Pull 2, motor almost stalls. Quickly put #4 wire back on it fires. Replug 2 and no rpm increase, meaning 4 stops. The shop was stumped and gave the motor back to me after sinking about 8 hours into it, no charge.

They checked the entire ignition system out. Btw, my orange wires from the powerpack go into a 2 prong plug. I wonder if thats somehow sorted inside.

[Joe Reeves]

The sockets and pins within those rubber plugs have been known to back out and result in erratic contact... check them.

Also... don't forget to check the coils continuity, including the plug wire.

[Ffarrell]

The coil has been replaced on the #4 cyl. The spark is snappy. Im thinking of rewiring that 2 prong plug to eliminate it, but that would force me to resolder it.