I'm running an MSD 6A box with a TFI coil, along with a DS 2 distributor and Taylor Thunder Bolt wires.

Today I forgot to replace the metal breather oil cap and it fell atop the distributor while driving. Right away it started missing and running rough. I had spent the afternoon dialing in the carb, so thought the carb was acting up. I drove for 4 miles like that.

Got home and opened the hood to see the cap sitting atop the dizzy. The engine was running and I thought about possibly getting zapped, but thought that with such nice plug wires that couldn't happen. I thought wrong. It shocked the heck out of me. I tasted the fillings in my ... teeth. My bare stomach must of been touching the grill making ground, because it bit me there too.

Anyway, my question is if I fried something in the dizzy. It still runs as though on 5 cylinders (it is an inline 6). I was quite surprised to see the cap contacts look like welding slag, bumpy and white. The rotor the same. Any chance I toasted something in the dizzy?

 

It's certainly possible; the high-voltage spark from the secondary winding of the coil is meant to be a very low-current sink into an isolated area (the plug gap). The human body is actually a pretty good conductor, so if you sank the secondary current to ground through your person, it will try and go back to the coil winding through any path it can. That path may have included the ignition module.

I'd actually be more concerned about the ignition module than the distributor - there's really not much to the distributor. The only sensitive component (the magnetic pickup) wouldn't have been in any sort of ground return path. If this event really did cause any damage, it would more likely be isolated to the ignition module.

Are you sure this problem actually coincided with the oil cap hitting the distributor? Your story makes it sound like the engine started acting up after some carb adjustments; when you popped the hood you found the oil cap on top of the distributor. That doesn't actually mean the problem started the instant the oil cap fell off. This all could just be a red herring and your problem could be elsewhere.

 

Yes, of course, the problem may be elsewhere. But the carb adjustments were only jetting and tuning. It was not missing b/f the oil cap landed. It was running fine, and I went up a driveway and pulled into a parking space when I noticed it. However, tomorrow I will surely check the carb again.

Please give me more info regarding the ignition module. Thx.

 

Sounds like first step is a new cap and rotor to fix the damage. If it runs right after that your question is at least partially answered.

I am wondering why the metal cap sitting on the rotor/plug wires caused the shock and damage. Is the plug wiring "leaky"? Are the wires/terminals OK?

 

yeah, if you got shocked and it was arching,, either the cap is bad or one/more of the wires are bad. It's a sealed system when right.

 

The more I think about this, the less I think this could have actually damaged the ignition module. The coil secondary winding is extremely high output impedance (thousands of ohms). Any "short" to ground through your person would pretty much end there; I don't see how it could harm anything upstream. But strange things do happen when re-directing tens of thousands of volts, so anything is possible. Maybe a first step as a sanity check would be to check the primary and secondary winding resistances of the coil using a multimeter. If anything would have taken direct damage, it would have been the secondary winding of the ignition coil. The primary side can handle much more current (several amps) and wouldn't have likely taken any damage.

I agree with the others. While I certainly wouldn't advocate others try this, I'm a little surprised you got shocked that hard by grabbing something off the top of insulated plug wires. That almost suggests that something is exposed, your wires are too high resistance, or your plugs are gapped too wide. As the others have said, if your cap is that chewed up, it needs replaced anyway.

 

Thank you for the helpful replies. Electrical is my shortcoming, but I will try to read up a bit and test the coil.

The plugs may be adding to the problem. I am running original plugs that are gaped at .044 in the book. However, I am running them at .055. Was I wrong to think that with a higher power ignition they could run at a wider gap? Would it be better to go with an efi plug, designed for more voltage and a wider gap, given there is no valve/piston issue? (The engine is a '75 300 inline with some hp mods.)

 

Not necessarily; your thinking is correct. Plug gap is a function of secondary voltage - the higher the output, the wider the plug gap for an ideal burn (too narrow and the plugs will burn prematurely). Personally I would run the gap that's specified for an engine running a TFI coil. If the manual for your MSD module has a recommendation, that would supersede mine. It does make sense to run wider than the 0.044 typical of the stock Duraspark-only setup.

 

I am using a similar ignition set-up, F-250 restorer. 1986 4.9L converted to Duraspark with MSD ignition firing through copper Motorcraft plugs for the FI engines. My sparkplug gap is .050" and the plug wires are stock Motorcraft.

Perhaps your ignition system had an undetected fault and when you changed jetting [made leaner?], the spark was not strong enough to efficiently fire the leaner mixture? Is the misfiring worse when under a load?

 

On the plug gap... a MSD coil and ignition box can power the wide gaps. Wide gaps increase secondary voltage and effectively "load up" the rest of the system more. Is the OE cap/rotor rated for the higher voltage? NO. Are the (stock replacement?) wires rated for the higher voltage? Probably not.

Even though MSD says you can increase plug gap with their parts, the rest of the ignition system may not be able to handle it. I'd put plug gap back to stock for acceptable reliability and durability. Maybe replace plug wires also. You still get the benefits of multi-strike spark with the MSD... the whole system just lasts longer with more moderate gaps.

 

Thanks for all the good info and helpful responses. I tested the MSD unit with their method as shown on youtube. Evidently it is functioning fine. Since the coil was under warranty I replaced it too. Now I am in holding pattern because I have one motorcraft wire mixed with a set of Taylor Thunder wires. I have checked in the dark and don't have any sparks, but I'm going with a new set of wires when the bank is ready.

The miss only seems to be at idle. I get on it and it performs great. I only notice it at idle. Since it started when the cap fell onto the dizzy and I got zapped, I am assuming that is the source of the problem.

According to my AFR gauge the carb is not running lean. I did notice a slight surging at idle when I finished making air bleed changes, from leaner to richer. I am back to the point where the carb ran smooth and well before, so I'm assuming it is not the carb.

Once the wires are changed to another high energy brand, I'll look closer at the ignition if new wires don't take care of the problem.