I've got a 1956 F100 with a 312 engine. I got it running several weeks ago after a prolonged period in a garage. I changed out the condenser and points for a Pertronix electronic ignition which has made idling very smooth until recently. Lately while idling the engine briefly stutters or misses. If I accelerate slowly you can hear it and feel it too. If I accelerate rapidly it's not noticable. I have checked all my plug wire connections (they're new) and looked into the carb which was rebuilt and don't find anything amiss. It seems to be more common after driving it for 20-30 min and not just after starting. I did have a loose plug wire previously and the engine would miss fire, this isn't quite as impressive as that but similar. Any suggestions?
Chris

 

Chris,

It sounds like a bad plug or plug wire, especially if its at a light load situation like you describe, just coming off of idle. Maybe one of the plugs cracked while you were installing them. Either that or one of the plug wires has a nick or cut in it and is shorting out to the engine or a bracket, etc.
The best way to check for a cracked sparkplug is to pull them out and visually check them. Look at the ceramic portion for a fine hairline crack (sometimes with carbon tracking) While you have the plug wires off shine a flashlight down into the spark plug boot...if its bad it will look black and burnt, if its good you should be able to see the end of the plug wire crimped over to the little metal doohickey that fits on the plug.

Years ago I would check my plug wires by runnignmy hands down each one with the engine running, you would know when you had a bad one. (I quit using that method with the advent of electronic ignition....ouch).

Sometimes you can mist water onto the plugs and plug wires from a spray bottle and find a shorting wire (listen and look for the snapping of the arc)

Good luck
Bobby

 

Bobby, The other day I had the engine running and when it was sputtering I pushed on one of the plug wires and it "bit" me. Does that mean that that one is bad, can you feel a shock through the rubber or is that supposed to be adequate insulation?

 

Chris,
You should not feel a shock. There must be a crack or rubbed through area in the wire (or at least through the insulation) I'm betting that you found your problem.

How old are the plug wires? If they are fairly old then you might as well replace them all. If they are new then that one wire might have either ubbed up against something or got pinched, nicked, etc. and you could get away with just replacing it.

When replacing them take the time to shine a flashlight down into the boot at the spark plug end to check for burnt ends. If there is one or more with a burnt end then you should replace the spark plug also. The spark will have cause some carbon tracking on the ceramic of the plug and may cause the new plug wire to fail prematurely.

I hope its something simple
Bobby

 

That "bite" can also happen if the plug isn't firing; your body has a lower resistance path to ground than the plug.

 

The plug wires are all new. I took the advice of an old posting which said to replace all the electrical wires and points etc. if the truck hadn't run in a while. That wire could just be defective. I will have to pull it off and check it. If the insulation has a crack will the plug end still show the carbon burning or is that only if the plug is bad?

 

Chris,

Since the plug wires, etc are all new I would inspect the wire carefully to make sure it didn't get damaged or nicked during installation. If you can't find anything wrong with the wires then chances are that you cracked a plug putting one in. Is the spark plug wire that bit you going to a spark plug that is hard to get in and out, hard to reach, etc? Its the ones that are hard to get to that usually get cracked.

Pull the spark plug and carefully inspect the ceramic. Hairline cracks in the ceramic are very hard to see. Sometimes you can see carbon tracking sometimes you can't. Since everything is new chances are that the end of the plug wire is not burned out and should be ok.

Bobby

 

Well I pulled the plugs and looked at the wires. I couldn't find anything wrong with either. I might buy an extra long plug wire and move it from position to position while idling to see if the sputtering stops and then change out that wire and/or plug. I'm open to other suggestions. Where can I find just one plug wire?

 

Are these new or recent plugs? How does the one that's sputtering look? If it isn't firing by the looks of it, just replace it. If it got oil soaked or gas fouled before you replaced the wires, it may be ruined.

I hate to ask, but is it a Champion? If so and only 1 out of 8 was bad, you're ahead of the game IMO.

 

Ross, I see we share the same feelings about Champion plugs (personally I prefer AC Delco plugs)

Chris, I would have bet big bucks on you finding a cracked plug. A hairline crack in a spark plug is very hard to see (the newer the plug the harder it is to see it as well) Sometimes it helps to rub your greasy ol' thumb around the ceramic area and then wipe it clean (hoping that some of the grease will stay in the crack so you can see it)
Did any of the plugs look like they hadn't been firing well?

A light missfire like what you describe is always a pain to track down. Sometimes it won't miss at idle, or it will only miss at idle in gear (light load) and sometimes you have to just give it a slight amount of gas while in gear(light load). These can be the worst as you have to set the brake, chock the wheels really well (Do Not Stand In Front Of The Truck) and give it a bit of fuel while with it running in gear. Sometimes it helps to use a squirt bottle full of water and lightly mist each spark plug (the water will help induce the miss) You should be able to hear it, feel it, and see it when it starts arcing.

This is where one of those old Sun engine analyzers with the scope would really come in handy.

Bobby

 

The plugs are new but they are Champions. They all look about the same with regards to firing residue. I will try the dirty finger technique and see if I have any better luck. It only seems to intermittently misfire like every 3-5secs not with each engine rotation.
Chris

 

This is a Known problem w/ the Pertronix Conversions, take it out, send it back & either get another one or Purchase the Mallory conversion, as it's a much Better design.

 

I'm not surprised to hear from the Colonel that it's a Pertronix problem, but like I said, out of the box, 1 out of 8 Champions is par for the course... before you go to the trouble of packing off the Pertonix, try swapping that plug and another.

Colonel, any details on the problem?

If the dizzy shaft bushings are worn I could see it causing a similar issue.

 

Isuues w/ quality control of the product & worn distributors.

 

I don't have any experience with Pertronix ignitions what so ever. Before I condemned it I would definately try swapping out a plug or two. Worst case scenario is that you are out a couple of bucks for a decent "test" spark plug.

If after all of this you still can't find the cause of the miss I'd be tempted to find a shop that still has an engine analyser (oscilliscope). And have them hook it up. That way you will be able to "see" the problem and determine which cylinder, if its plug, plug wire, or in the distributor.

The hard part about it will be finding a place that still has a scope for the old style distributors, has a tech that knows what he is looking at on the scope, and is willing to have you standing there with him to show you whats going on.

Bobby