Hot running and misfire - cool rad hot head

[Mrmci]

I will check plug types and report back.

Plug colours all look good, nice light brown
Timing was re-set after the electronic ignition went in. I can't remember the setting but it was done and checked.
Fuel pressure was also checked.

The only part of the ignition system not changed is the coil.

[mbell]

Coils can" fail" with heat, so might be worth swapping it out.

There also been a lot of problems with new rotor arms failing when getting hot. Where did you get the rotor arm you fitted?

There's also a lot of fake sparks plugs around, so need to ensure you got them from a trustworthy source.

[Mrmci]

I'm on NGK BP7ES plugs

Just rechecked the colours and they're slightly sooty, apart from #2 which is grey. The specialist who fitted the electronic ignition setup the carbs at the same time and the #2 mixture screw was about 3/4 turn less open than all of the others. I've set it roughly the same as the others, it was about 1 1/4turns open, the others are 2 to 2 1/4 turns open and I'll check the plug colours after another drive.

[mbell]

I'm on NGK BP7ES plugs

I'd try bp6es, the 7 can foul if you idle for longer periods.

[Mrmci]

Thanks, I'll try them.

Just been out for a test drive with #2 mixture screw out 3/4 turn to match the others. Ran beautifully. Stopped to let fan kick in and when I set off it was spluttering, coughing and sneezing through the carbs. 1/2 mile down the road, temps dropped a bit and back to normal.

All plugs not sooty / brown tips.

I'm going to try the hotter plugs and a new coil.

This is a really annoying problem as it makes me not want to risk using it and getting stuck in traffic.

[661]

I'm on NGK BP7ES plugs

I'd try bp6es, the 7 can foul if you idle for longer periods.

I'd tend to agree that this is worth a try

[Andy8421]

From the symptoms described, I would suspect ignition components. Unless your engine overheats to the point it starts boiling and loosing coolant, I don't think that in of itself is the cause of your problems.

Some years back, there was bad batch of distributor caps in circulation that would fail when hot, and as mentioned above, dodgy rotor arms have been suspect for a number of years. After-market electronic ignitions have been known to show similar heat related intermittent failure. Don't assume new components are fault free when you swap them out.

If you can manage to do it safely, when the symptoms appear next, can you pull over and see if you are getting a decent spark at a plug cap? If you are, then it is unlikely to be ignition, if not, then you can trace back through the system to find the fault.

I had a similar problem that I couldn't pin down. I kept swapping out ignition components, eventually I traced it to a faulty distributor cap. I had swapped it for a new cap a number of times in desperation from different suppliers, only to finally realise all the usual suspects had sourced their caps from the same faulty batch. I don't trust the usual suppliers any more for ignition components and buy from a specialist (Distributor Doctor) who I have found to supply reliable components.

[Graham B]

Hi Mark,
Noticed you mention the sputtering happened when hot and the fan was on.
I have had an ignition switch which caused misfiring after a short run. Just to eliminate this, run a temporary wire and switch from the battery or starter solenoid direct to the ignition circuit.
Easiest way is to disconnect 12v feed at coil or ballast resistor (if fitted) and connect temporary 12v feed here. The standard tacho will not be working. You may also have to link the 12v to the electronic ignition if fitted.
In my case the car ran fine for 5 miles and then gradually misfired to the point it would not pull in top gear! Left to stand for a while and it ran ok but repeated the sequence. A new ignition switch solved the problem.
Ignition switches do age and can cause low or intermittent voltage to the ignition.
At least another avenue is eliminated.
Graham

[2cams70]

If you are using a coil designed for a ballasted system (i.e designed to run on 8-10V) in a car that doesn't have a ballast resistance installed (i.e supplies 12V to the coil in the 'run" position) it will overheat.

If you are replacing the coil make sure you install the correct type for the supply feed set up you have in your car.

[mbell]

Yes worth checking the coil. The coil or coil + ballast should be ~3ohms. Also insure the wiring for ballast is correct and the ballast isn't being bypassed when running.

If you have a 1.5 ohm coil and no ballast then you'll be getting significantly higher current flowing into the coil. On a 4 cylinder there not enough sparks to use the power entering the coil, so that power ends up as heat in the coil. Causing it to get too hot. It will also increase the current flowing through the electronic ignition unit, likely increasing its temp.

[Mrmci]

Thanks so much all, it's so great to have this community.

So.....

I did think back and my misfire is apparent after being stuck in traffic and therefore extended periods of idling. I've searched the forum for plug fouling problems. I let my car idle for a good while and checked the plugs and they were properly sooty. I've leaned off the idle mixture and fitted NGK bp6es plugs, I've been out to try to get it to misfire and it's behaved beautifully. It still has a little "cough" when you prod the throttle after a long period of tickover but no misfire once up and running.

I was convinced I had a fuel vaporisation problem so thank you so much for the second, third,....., opinions.

Mark

[Mrmci]

Yes, checked timing all good.
Plug colours are now good (I think)
Weber 31s iirc

[mbell]

Glad to hear it. Thanks for reporting back for future readers to know.