Misfire when warm

[Tim Ward]

1973 Mk 2.Starts as normal, then after a few miles occasional hunting and stutter. Noticeable when under load. Eventually very poor running.Have a suspicion its the ballast resistor. Checked the voltage to the + side of the coil when ignition is in the on position, it's showing 12 volts. Am I correct in assuming it should be around 6-9 volts ? I again am assuming if the coil is getting a continuous 12 volts its overheating. If it is the ballast and I want to keep the existing coil is the only option a MK1 type ballast ? Or are the resistored white and pink wires still available ?
Thanks for your help
Tim

PS Just been out again checked with multi meter when running. Its showing just over 9 volts. which I assume is correct. Looks like it might be ok and not the problem I thought it was .

[StagJonno]

If you measure the voltage when no current is flowing, you'll just see battery voltage. When you checked with the car running, current would be flowing for some of the time, hence you observing the reduced voltage. There's another thread currently running on an issue with symptoms similar to yours, so worth checking out.

[Tim Ward]

If you measure the voltage when no current is flowing, you'll just see battery voltage. When you checked with the car running, current would be flowing for some of the time, hence you observing the reduced voltage. There's another thread currently running on an issue with symptoms similar to yours, so worth checking out.

Thanks for reply, yes have been following that one as well.

[richardthestag]

I had exactly this issue with my mk2 stag a few years back, the ballast feed had failed. In the short term I took a feed from one of the white wires on the back of the fuseboard and ran it to a mk1 ballast resistor and then to Coil -ive.

I just insulated the old pink ballast wire and left it dangling

[Tim Ward]

I had exactly this issue with my mk2 stag a few years back, the ballast feed had failed. In the short term I took a feed from one of the white wires on the back of the fuseboard and ran it to a mk1 ballast resistor and then to Coil -ive.

I just insulated the old pink ballast wire and left it dangling

That's what I thought I might have to do. I intend to carry my meter with me when next out to test things when the fault occurs. Thanks for thoughts

[richardthestag]

If you have a jump / test lead and connect from battery +ive to coil +ive and the fault goes away you have root cause

beware that running a ballasted coil at 12v will cause it to overheat and fail. also the engine will not turn off until you disconnect the lead at the battery end

[tvrgeek]

Frequently it is a failing coil. Of course, that is after you verify all the connections are 100%. It can also be the condenser, or the thermal compensators on the carbs.

FWIW, the ballast wire in my car had fried. PO put in a Pertronix so does not use it. Yea, a 1.5 Ohm coil with full voltage will run great, briefly.