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Author Topic:   OMC V4 Misses, Tachometer Jumps

rukkus posted 07-15-2008 08:37 PM ET (US)
My 1970 Johnson V4 starts to miss at 4,900-RPM, and the tachometer begins to drop and jump. What might be causing this?
jimh posted 07-15-2008 08:57 PM ET (US)
The tachometer signal is derived from the permanent magnet alternator winding and the rectifier-regulator assembly. The ignition coil primary voltage is derived from the charging coils. The timing of the spark firing is derived from the timer base. The control of the spark is accomplished in the Power Pack.
These are four separate components. I do not see a common link between them.
rukkus posted 07-15-2008 10:02 PM ET (US)
You are speaking of the CD ignition, my ignition system consists of points, distributor, rotor and ignition module, also know as a pulsepack.
jimh posted 07-15-2008 10:04 PM ET (US)
I did not notice that your motor was almost 40 years old. Yes, it probably does not have a capacitor discharge ignition system. Disregard my comments.
WHALETEX posted 07-16-2008 11:23 AM ET (US)
I grew up with a '69 V-4.
IF it seems like the engine is completely cutting out and the tach is bouncing violently I would look for a wire with internal breakage in the wires that go to the points plate under the fly wheel. These are the wires that connect the points with the rest of the ignition. They have a couple of bends in them as they near the points plate and when the points plate rotates with the spark advance these bends get flexed and in our case the metal part of the wire eventually broke inside the insulation. We found this by checking wire continuity while flexing the wires. I our case this condition was intermittant and on-off nature of the break would send a false signal like the points opening and in some cases would cause the engine to reverse rotation while idleing. It would also cause the engine to randomly cutout with violent tach action as we advanced the throttle.
If the tack bounce does not seem violent and it always happens at the same RPM another possibily is one set of your points is too far open or the spring is weak. If your engine is like ours it will have two sets of points with each firing a pair of cylinders. As you RPM is going up one or both sets of points may be floating out where they are bouncing so fast that they don't get a chance to close. We had this a number of times usually with just one point. If the point was too open at a specific high RPM the engine would start to die off but as the RPM came down it would catch again then it would die off are repeat until we pulled the throttle back.
A similar thing would happen when one point was too closed. The motor would run bad at idle but once it got to a certain RPM the other two cylinders would instantly start firing and it would run like a champ.

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Author	Topic: OMC V4 Misses, Tachometer Jumps
rukkus	posted 07-15-2008 08:37 PM ET (US) My 1970 Johnson V4 starts to miss at 4,900-RPM, and the tachometer begins to drop and jump. What might be causing this?
jimh	posted 07-15-2008 08:57 PM ET (US) The tachometer signal is derived from the permanent magnet alternator winding and the rectifier-regulator assembly. The ignition coil primary voltage is derived from the charging coils. The timing of the spark firing is derived from the timer base. The control of the spark is accomplished in the Power Pack. These are four separate components. I do not see a common link between them.
rukkus	posted 07-15-2008 10:02 PM ET (US) You are speaking of the CD ignition, my ignition system consists of points, distributor, rotor and ignition module, also know as a pulsepack.
jimh	posted 07-15-2008 10:04 PM ET (US) I did not notice that your motor was almost 40 years old. Yes, it probably does not have a capacitor discharge ignition system. Disregard my comments.
WHALETEX	posted 07-16-2008 11:23 AM ET (US) I grew up with a '69 V-4. IF it seems like the engine is completely cutting out and the tach is bouncing violently I would look for a wire with internal breakage in the wires that go to the points plate under the fly wheel. These are the wires that connect the points with the rest of the ignition. They have a couple of bends in them as they near the points plate and when the points plate rotates with the spark advance these bends get flexed and in our case the metal part of the wire eventually broke inside the insulation. We found this by checking wire continuity while flexing the wires. I our case this condition was intermittant and on-off nature of the break would send a false signal like the points opening and in some cases would cause the engine to reverse rotation while idleing. It would also cause the engine to randomly cutout with violent tach action as we advanced the throttle. If the tack bounce does not seem violent and it always happens at the same RPM another possibily is one set of your points is too far open or the spring is weak. If your engine is like ours it will have two sets of points with each firing a pair of cylinders. As you RPM is going up one or both sets of points may be floating out where they are bouncing so fast that they don't get a chance to close. We had this a number of times usually with just one point. If the point was too open at a specific high RPM the engine would start to die off but as the RPM came down it would catch again then it would die off are repeat until we pulled the throttle back. A similar thing would happen when one point was too closed. The motor would run bad at idle but once it got to a certain RPM the other two cylinders would instantly start firing and it would run like a champ.