[bill2000]

Last June I posted a thread about the difficulties I was having starting my '73 carbed GTV. After checking everything I could think of, I went away for 2 weeks and when I came back it started perfectly. Now I'm having troubles again with an intermittent miss, particularly on #2 and #4. The last 2 days it's run very unevenly, on 3 or even 2 cylinders. So I went out tonight and started it in the dark. There was no visible electric leakage from the plug wires, but I discovered that when I pulled the #4 wire and held it to the end of the plug forcing to arc, the cylinder would run smoothly. Put it back on and it doesn't. What does this mean? The plugs, wires, coil and electronic distributor are all new from Centerline and I've verified that the timing is correct. When it's not giving me problems, it starts instantly and runs well (though it has a slight hesitation just off idle). As I say, the wires are only a couple of months old.

Thanks in advance,
Bill

 

[MrT]

You've caught my attention. That's something I saw on a car years ago and I can't remember what the problem was.

But...maybe the plug itself is cracked and has a leakage path internally. If you kept the plug wire from applying voltage until it avalanches over and gives a very sudden rising electrical pulse, you might swamp the leakage path and cause the main plug electrode to fire. If this is the case, you want to either clean the outside of the plug (but you said it wasn't giving a visible leakage path so that's a very slim chance) or replace the plug. You might swap plugs around to see whether the problem "goes away" or goes with the plug. but I think I'd be inclined to replace that suspect plug outright. They're not that expensive and it's easy.

I hope this helps. It's at least plausible.

 

[bill2000]

Thanks, MrT. I'll try moving the plugs around tonight and see if the problem follows them. The plugs are NGKs, less than a year old.

Bill

 

[green 69 gtv]

firing order?

Hi
Long shot but any chance the wiring from plugs to distributor is in wrong order? The newer caps are numbered incorrectly.

Cylinder # Dist. cap. #
1 1
2 3
3 4
4 2

 

[bill2000]

Thanks, no, they are correct. As I say, it's been alternately running fine and giving me trouble. Also, incorrect wiring wouldn't explain the weird plug/arc business.

Bill

 

[5yearplan]

My money is on the plug, as good as they are, every now and then you get a bad one.

 

[papajam]

...but I discovered that when I pulled the #4 wire and held it to the end of the plug forcing to arc, the cylinder would run smoothly. Put it back on and it doesn't. What does this mean?

This is a (fairly) common diagnostic technique called gapping the coil (or wire gapping the plug in this case). It forces the coil to output maximum voltage as opposed to the low voltage requirment needed to fire the plug at idle. Your symptom could indicate either a bad plug or an excessively lean/rich mixture in that cylinder.

 

[bill2000]

This is a (fairly) common diagnostic technique called gapping the coil (or wire gapping the plug in this case). It forces the coil to output maximum voltage as opposed to the low voltage requirment needed to fire the plug at idle. Your symptom could indicate either a bad plug or an excessively lean/rich mixture in that cylinder.

Thanks for the explanation. It really had me scratching my head. I just got back in from going through the idle mixture settings again with the colortune. #4 might have been set a little rich, so I reset it to where the flame is steadily blue, though it does occasionally flash a little yellow. One thing I did notice is that when I open the throttle just a crack, the flame goes very rich for a moment and it almost stalls then recovers (it does this on all 4 cylinders). The idle/transition circuits have new 50F8s. Should I drop back to a F14 or F11? It came with 50F11s (the intake and carbs were probably taken from a 1600).

Thanks,
Bill