Suspect coil, Ballasted or not?

[irco]

My recently purchased 7V8 starts to develop a misfire at higher revs the longer it runs. I suspect the coil is starting to misbehave the hotter it gets,
After starting car and travelling 2 - 3 miles to give it a chance to warm the engine & oil a bit it will rev cleanly to over 5000rpm.
As distance/heat increases it starts to misfire in the region of 4000 - 4500rpm. Not a major issue as acceleration is brisk through the gears and 4000 plus is well into illegal territory. Car starts 1st turn of the key hot or (cold - with choke).
So as stated I suspect the coil. Don't think it's a fuel issue but could be wrong. Malpassi glass bowl filter king is clean and clear with no residue at all in bowl.

Coil Primary resistance cold = 2.7ohms, hot = 3ohms
Coil Secondary resistance cold = 9000ohms, hot = 10000ohms
Ballast resistance 1.87 ohms from connection at white ignition wire to coil, resistance of test leads & wire from ballast included, so ballast probably ok in region of 1.5 ohms.
Voltage at coil:-7.8v Ign on, not running
10v Cranking
11.9v Running
Battery 12.6v (14.1v running)
Coil is drawing 2.7 amps.

So from all the above readings it appears clear ballast wire is still in circuit + I'm presuming it's a 12v coil, going by the above coil resistance readings, there are no identification markings on the coil. It would appear car has wrong coil (I think should be 6v with 1.5ohms primary resistance) Car has Newtronic electronic ignition fitted, all mods carried out by previous owner/s

Looks like new, correct, coil required.

Are my conclusions correct or have I missed something?

[HDRider]

You did not state whether you had electronic ignition or not. If you have points it could be the condenser.

You can run a direct wire from the battery to the coil to eliminate the existing wiring to see if that makes a difference as well.

Edward Hamer
Petaluma CA

[saabfast]

Car has Newtronic electronic ignition fitted, all mods carried out by previous owner/s

Irco says it has Newtronic electronic ignition. I don't know anything about that system so can't really help.

Something looks a bit odd with the voltages. IIRC if the Eureka wire is still there should it not crank/start at 12V and then drop to 6-8V when running?
If it is a 12V coil the Eureka wire could be bypassed to run as a 12V system at all times?

[irco]

Thanks for replies.

I assumed low cranking voltage at coil is caused by high current draw by starter motor & corresponding volt drop. I believe this was one of reasons manufacturers went to ballasted systems, so even 10v, rather than 6v for starting, to a 1.5ohm coil is giving a substantial boost. Maybe some slightly dirty connections contributing.

So I'd rather keep the ballasted system for more reliable starting.

Ign on, voltage at coil of 7.8v is spot on for operation. The 11.9v with engine actually running is probably some sort of average due to optical pickups continuous opening & closing of the signal, effectively shorting & open circuiting the coil. A problem with Digital Voltmeters, an old analogue dial meter would probably show the fluctuations & an oscilloscope definitely.

I'm unsure how the internals of Newtronic unit actually work.

Have emailed the Newtronic Electronic Ignition manufacturers for confirmation that 1.5ohm coil will be ok with their unit. I await their reply.

[Hasbeen]

I had a similar, but opposite problem with the 7, which plagued me for over 5 years. The car had a slight hesitancy at low revs low throttle opening when cold. After about 3 or 4 kilometres it would disappear.

Every time I did something to the fuel or ignition systems it would disappear for a while. By the time I got to my auto electrician it was warm, running fine, & tested well. Cleaning everything meticulously, changing bayonet connectors to rings, going right through the fuel system, everything appeared to have cured the thing, but only for a few months.

Finally the hesitation became a miss, & then a cut for a second or more. Enough to let me see the rev counter dropping towards zero. I found a mobile auto electrician who came & tested it when fully cold & found the module in the Lumenition ignition system was failing. A very difficult fault to find, but easily fixed when found.

Hasbeen

[irco]

Thanks for that information.
I'll bear that in mind if I don't get problem resolved by changing the coil.
Don't know how to go about testing the Newtronic unit though. May be more investigation required.

[irco]

I have now bypassed the Eureka wire with a direct switched, non fused feed to the coil.
The Newtronic unit requires a 12v feed, unfortunately a previous owner had taken this feed from the coils positive. This meant that while running the Newtronic feed was ballasted, therefore 7.8v, not good.
So bypassing Eureka wire has removed that issue as well.
How long car had been like that I have no idea, but possibly since installation of Newtronic unit & coil & probably with the erratic performance I experienced. Both look like they've been installed for a while.
Test drive & car would rev cleanly to 5500rpm. RESULT
Thanks to those who replied.

[irco]

Reply from Newtronic Electronic Ignition supplier.

"The Newtronic system is designed to be used with a 3 ohm coil which you’ve got."

So bypassing ballast is way to go.

[busheytrader]

IIRC, the piranha brand became newtronic or vice versa.

I had a similar issue with a misfire / hesitation at higher revs. It was the distributor’s vacuum advance module. It held a vacuum but the diaphragm had hardened with age, which restricted its overall movement. When I rotated the distributor to enable high revs, I lost the bottom end tractability.

Replacing the vacuum advance instantly restored the engine’s response across the whole Rev range. On the plus side they’re relatively cheap.

[irco]

Replacing the vacuum advance instantly restored the engine’s response across the whole Rev range. On the plus side they’re relatively cheap.

I'll bear that in mind for future reference. In this instance the ballast wire bypass has solved my immediate issue. I'm surprised the previous owner/s hadn't noticed the issue, although performance was adequate & maybe they didn't utilise it's potential.
My initial drives of the car didn't show up the issue, as it only happened once fully up to temperature & coil got hot, although it's now running more cleanly all through the rev range.
Better now annotate the wiring diagram to reflect change.
The joys of 40 year old modified car ownership