1974 Johnson 50 hp...no fire on #1

ronward

Petty Officer 1st Class
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Jun 24, 2013
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Hello again. My Johnson 50ESL74M has intermittent, but mostly no fire on #1 cylinder. I had a blown head gasket and fixed that but this problem was already occurring so I know I didn't cause this while working on it. Here's what I know and what I think... I have fire at #2 so I know there is fire to the pack. I switched the coils at the pack and still have fire on #2 which was originally the #1 coil. That tells me the pack is failing causing no fire at the #1 cylinder terminal. Am I right in thinking that? Is this a common situation or does it not work that way? Is there just a single power source coming into the pack and the pack decides which cylinder it goes to? Or are there two signals coming in at separate times, one for each cylinder? If that it the case I suppose there could be something bad before the pack. By the way, I have the factory manual and have done the CDI checks but am still not completely sure. I should say I have done what cdi checks I can do with limited knowledge and test equipment.
 
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Joe Reeves

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Feb 24, 2002
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13,262
Re: 1974 Johnson 50 hp...no fire on #1

Obviously both coils are okay.

There is only one timing sensor in the timing base under the flywheel. Center magnets (2) being one south and one north send a signal to a electronic switch within the powerpack to fire the individual coils. It's possible that you have a poor connection at some point between the timer base and the pack or the pack and the #1 coil... or a failing powerpack.

With both spark plugs removed, the spark should jump a 7/16" gap on that model with a strong blue lightning like flame... a real SNAP! Is that what you're getting when the coils fire properly?
 

ronward

Petty Officer 1st Class
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Jun 24, 2013
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346
Re: 1974 Johnson 50 hp...no fire on #1

Hey Joe, thanks for the reply. Just came in from working on it. I made a spark tester out of clear tubing and 2 pieces of 1/4" threaded rod. The spark will jump 7/16" but it's not as blue and snappy as you say it should be. I'd say it's barely blue, and sounds more like tic-tic-tic-tic-tic. Does that sound like a weak pack. Checked all connections, including grounds and still no spark on #1.
 

F_R

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28,195
Re: 1974 Johnson 50 hp...no fire on #1

How it works: Magnets passing by coils under the flywheel create electricity that is fed to a capacitor (just one) within the power pack. The capacitor stores that electricity until time to fire (either cylinder). There also are a pair of magnets in the center of the flywheel that pass by a sensor coil. As one passes by, it generates electricity of positive polarity, which is fed to a SCR (electronic switch) within the power pack. That turns the SCR on, which allows the storage capacitor to discharge to one of the spark coils. The spark coil increases that pulse of electricity from the power pack to whatever it takes to jump the spark plug gap. Then it is all over till the next time.

Comes the "next time", the same thing happens, except this time the other magnet passes by the sensor coil, generating electricity of negative polarity, which is fed to the other SCR, turning it on and discharging the storage capacitor to the other spark coil.

Note that:

1. The same charge coils generate electricity to the same storage capacitor, no matter which cylinder is about to fire. So it can't fire one but not the other.

2. There is only one sensor, it's output polarity determined by whichever magnet is passing by. The one sensor triggers both SCR's. The wires connecting it all together are also common to both cylinders. So, the sensor cannot trigger only one SCR. (Unless one magnet was bad, which is unheard of).

2. There are two SCR's, one for each cylinder. So, one SCR, if it is bad, will not fire it's associated cylinder.

See a pattern here? Only the one bad SCR within the power pack, or it's associated coil, can cause loss of fire on just one cylinder. You pretty much eliminated the coil possibility by swapping them. So, logical conclusion is a bad SCR. Entire power pack must be replaced, since you cannot get in it to replace the bad SCR.

Judging the spark by color or sound can be interpreted differently by different people. The spark from those things is very fast and does not have all the harmonics of other systems. Compared to a conventional spark from other ignition systems, it is not as blue. In fact, I would judge it as being white. Ambient light also affects the perceived color of the spark. If you could get a conventional system to make a 7/16" spark, it would be a lot bluer, fatter, and noisier. With all that in mind, I just say it should jump a 7/16" gap with ease and let it go at that.

So...............is your system producing a 7/16" spark off both coils or not? If not, I'd say you have a bad power pack.

EDIT: Let me clarify. There are two SCR's one is turned on by a positive pulse from the sensor, the other one is turned on by a negative pulse. The both are "seeing" both pulses, but only react to the one that has the correct polarity, and ignoring the other polarity pulse.
 
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ronward

Petty Officer 1st Class
Joined
Jun 24, 2013
Messages
346
Re: 1974 Johnson 50 hp...no fire on #1

How it works: Magnets passing by coils under the flywheel create electricity that is fed to a capacitor (just one) within the power pack. The capacitor stores that electricity until time to fire (either cylinder). There also are a pair of magnets in the center of the flywheel that pass by a sensor coil. As one passes by, it generates electricity of positive polarity, which is fed to a SCR (electronic switch) within the power pack. That turns the SCR on, which allows the storage capacitor to discharge to one of the spark coils. The spark coil increases that pulse of electricity from the power pack to whatever it takes to jump the spark plug gap. Then it is all over till the next time.

Comes the "next time", the same thing happens, except this time the other magnet passes by the sensor coil, generating electricity of negative polarity, which is fed to the other SCR, turning it on and discharging the storage capacitor to the other spark coil.

Note that:

1. The same charge coils generate electricity to the same storage capacitor, no matter which cylinder is about to fire. So it can't fire one but not the other.

2. There is only one sensor, it's output polarity determined by whichever magnet is passing by. The one sensor triggers both SCR's. The wires connecting it all together are also common to both cylinders. So, the sensor cannot trigger only one SCR. (Unless one magnet was bad, which is unheard of).

2. There are two SCR's, one for each cylinder. So, one SCR, if it is bad, will not fire it's associated cylinder.

See a pattern here? Only the one bad SCR within the power pack, or it's associated coil, can cause loss of fire on just one cylinder. You pretty much eliminated the coil possibility by swapping them. So, logical conclusion is a bad SCR. Entire power pack must be replaced, since you cannot get in it to replace the bad SCR.

Judging the spark by color or sound can be interpreted differently by different people. The spark from those things is very fast and does not have all the harmonics of other systems. Compared to a conventional spark from other ignition systems, it is not as blue. In fact, I would judge it as being white. Ambient light also affects the perceived color of the spark. If you could get a conventional system to make a 7/16" spark, it would be a lot bluer, fatter, and noisier. With all that in mind, I just say it should jump a 7/16" gap with ease and let it go at that.

So...............is your system producing a 7/16" spark off both coils or not? If not, I'd say you have a bad power pack.

EDIT: Let me clarify. There are two SCR's one is turned on by a positive pulse from the sensor, the other one is turned on by a negative pulse. The both are "seeing" both pulses, but only react to the one that has the correct polarity, and ignoring the other polarity pulse.

Thanks F-R. From what I have read that is exactly how I understood a power pack worked. I have good spark to #2 and none to #1 so I figure the trigger circuit in the power pack is bad. Furthering that conclusion, every once in a while I get a spark on #1 but mostly nothing...like 99% of the time. I'm gonna throw a new pack at it and see what happens. Almost everything else has been done...new coils, plugs, wires, rectifier, head gasket. I have great compression, I have fuel, but only firing one cylinder. Gotta be the pack...I can't afford much more, lol. I would probably be better off with a newer motor but I got this thing so cheap I can replace everything for less than a newer motor would cost me. I recently came across an 85 hp in immaculate condition that was advertised as a 1976. Drove 5 hours to get it only to find out it was a 1969 with electric shift...they scare the he-- outta me. Turned it down but did leave with an almost new ss prop perfect for my 50 hp for 50 bucks and a galvanized loadrite trailer for 150 bucks. Not a total waste of time.
 

boobie

Supreme Mariner
Joined
Nov 5, 2009
Messages
20,826
Re: 1974 Johnson 50 hp...no fire on #1

Try a new pack and see what happens.
 

ronward

Petty Officer 1st Class
Joined
Jun 24, 2013
Messages
346
Re: 1974 Johnson 50 hp...no fire on #1

New power pack and it runs like a champ. Thanks everyone
 
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