misses on one side but fires with air gap

[burns]

I had a plug cap come loose from its coil wire - not compleely off but loose. The motor would run on the other jug and when I touched the "bad" wire I got a pretty good shock. The connector prong in the cap was broken so I replaced the cap (NGK). The cap that I put on reads 4.9 ohms on my meter. My ignition is PAMCO (dual tower coil). NGK BP7ES plugs.

The new cap didn't fix the problem. The motor will start and run on the other jug but the "bad" side will fire only if I hold the plug cap a little away from the plug. There is a fat blue spark.

I'm scratching my head. Do I need a new coil?

 

[Grinder]

Did you try changing the plug on that side? They used to make a plug with an air gap like that for motors that burned oil to keep the plug from fouling.

 

[burns]

Did you try changing the plug on that side? They used to make a plug with an air gap like that for motors that burned oil to keep the plug from fouling.

Yes I did. The new plug fired for a short while then the condition (air-gap needed) returned.

 

[5twins]

Since the Pamco fires both plugs at the same time, you could try switching the plug wires from side to side and see if the problem follows. That would tell you if it was ignition or motor/carb related.

A dual fire coil like this usually doesn't go bad on just one side. If one plug fires, they both should. If one doesn't, neither usually will. About the only things that would affect one side only are the plug wire and plug cap for that side. You changed the cap, maybe you should check the wire.

 

[burns]

Well, the plug wires are intregal to the coil so if a wire is bad the coil is shot. Functionally my question comes down to do I need a coil AND a PAMCO.

And I don't think so.

I am obviously getting the break in the ground side of the coil that is the PAMCO's role in this movie.

As I understand it, in a twin tower coil the high voltage discharge ("spark") originates on one side of the coil, passes through that coil's high tension lead ("spark plug wire'") meets resistence at the cap that raises the voltage, jumps the plug gap on that side, passes through the metal of the head to the other plug's threads, jumps that plug gap and goes up that side's high tension lead and "home" to the other side of the coil.

So, if either spark plug wire is out of the circuit I wouldn't get any spark at all.

But I am running on one side and getting a fat blue spark on the other side when I up the resistence on that side by putting some atmosphere in the circuit.

My best guess is that the coil windings on the " bad" side were damaged by the cap failure and it is not putting out enough voltage to jump the plug gap. The voltage is increased enough to arc that gap by putting the extra resistence of the air-gap.

I'm not totally happy with that guess, since it seems to me that if a winding fried there should be nada coming out of that coil.

I don't have any grasp at all of what in the PAMCO innards could mess with this reasoning.

I'd like to understand this, but I think I'll be ordering a new coil if nobody here can enlighten me further.

 

[burns]

would it hurt anything if I put a higher resistence plug in that hole? just a thought.

 

[mrriggs]

The coil sounds fine. In a waste spark system, the plugs are wired in series. The energy travels from the coil to the plug, jumps the gap, travels through the head to the other plug, jumps the gap, then travels back to the coil. The only way you can have one plug fire and not the other is if one of the plugs or leads are grounded and allows the energy to bypass the spark gap.

The first thing I would check is the boot. Just because it's new doesn't mean it's good. Fire it up at night in a dark place and see if it is arcing on the outside of the spark plug (from the boot to the hex part of the plug). The boot and plug need to be clean so there is nothing to cary the spark away. A dab of dielectric grease on the rubber part may also help to hold the spark in.

Normally I would also suggest checking the coil and spark plug wires for shorts but in your case they seem to be fine. Pulling the boot back increases the demand of the coil and wires. If they work then, they will work with the boot on.

This reminds me of a time when I was working at a lawnmower shop. Customer brought in a mower that had a bad miss. I discovered that the spark was jumping on the outside of the plug. Put in a new, properly gapped, plug and it was still doing it. Put on a new boot, still doing it. Finally, I grabbed a plug with same heat-range and reach but a longer porcelain, problem solved.

 

[burns]

The coil sounds fine. In a waste spark system, the plugs are wired in series. The energy travels from the coil to the plug, jumps the gap, travels through the head to the other plug, jumps the gap, then travels back to the coil. The only way you can have one plug fire and not the other is if one of the plugs or leads are grounded and allows the energy to bypass the spark gap.

The first thing I would check is the boot. Just because it's new doesn't mean it's good. Fire it up at night in a dark place and see if it is arcing on the outside of the spark plug (from the boot to the hex part of the plug). The boot and plug need to be clean so there is nothing to cary the spark away. A dab of dielectric grease on the rubber part may also help to hold the spark in.

Normally I would also suggest checking the coil and spark plug wires for shorts but in your case they seem to be fine. Pulling the boot back increases the demand of the coil and wires. If they work then, they will work with the boot on.

This reminds me of a time when I was working at a lawnmower shop. Customer brought in a mower that had a bad miss. I discovered that the spark was jumping on the outside of the plug. Put in a new, properly gapped, plug and it was still doing it. Put on a new boot, still doing it. Finally, I grabbed a plug with same heat-range and reach but a longer porcelain, problem solved.

Thanks Mr. R (I'm still running your stator bracket BTW) That will save me the price of a coil (or two).

 

[burns]

the rest of the story (lesson learned that may help somebody some time)

The boot (plug cap) comes with a rubber insert. That insert was missing from this cap. The insert's purpose (as I discovered in this discussion) is to prevent the arcing condition that Mr. Riiggs described by putting an insulator between the cap end and the metal of the head/spark plug base.

Sometimes just a nudge in the right direction will crystalize a problem
Thanks again Mr. R.