Annoying misfire from cold

[RIDDLER]

I run a standard 1978 260Z except that it has 240Z carbs fitted.
It has always started easily on choke, and I generally find that even on a cold morning, by the time I have driven off my housing estate I can knock the choke off completely and it runs very sweetly even before the temp gauge shows it has fully warmed up.
But in the last week or so a problem has developed.
It starts on full choke, but runs very rough with the rev counter needle bouncing around. I can drive away but it feels like it is running on 5 cylinders.
After I knock the choke off the problem continues, with it misfiring until I get it past 3000revs when it then pulls.
Even when the temp needle reads normal, so it is fully warmed up, the lumpy running continues for another mile or so - then, mysteriously, it clears and the car runs beautifully.
I thought it might be dampness around the plugs, which is drying out as the car warms, so I sprayed the plugs and leads with a bit of WD40. The following morning it ran ok! So thinking I had solved it, I then purchased some conductive grease to do the job properly and put that round the plugs and into the leads. It continued to run well all that day . . . but next morning the problem had returned!
Taking it to let Mark Warburton have a look later this week, but wondered if anybody had any helpful thoughts on this?

 

[Rob Gaskin]

I had the exact same symptoms develop on my old 240Z. In fact one day I had no spark at all! The car was generally 100% reliable.

I never really got to the bottom of it but I found if I disconnected my OMEX Rev Limiter the fault disappeared. That won't help you of course. OMEX were not convinced their unit was at fault they said they either work or they don't, nothing in-between.

I had a Luminition unit so again different than you.

My guess would be Coil. They can sometimes be heat sensitive if faulty.

Get Mark to balance your throttles while you are there.

 

[jonbills]

I think the behaviour of the rev counter is probably significant - i.e. confirms the problem is ignition rather than fuelling.
I'd look at making sure the all the plugs, leads, distributor & cap, coil are clean and dry.

 

[Rob Gaskin]

I think the behaviour of the rev counter is probably significant - i.e. confirms the problem is ignition rather than fuelling.
I'd look at making sure the all the plugs, leads, distributor & cap, coil are clean and dry.

Yeh, my rev-counter bounced around too. It was def. ign.

The reference to the throttle balancing was something else between RIddler and me.

 

[andrew muir]

Hi Rob,
This is most likely ignition issue, I sort of doubt it is the coil, as these usually breakdown at high revs and would affect all plugs, I would start car from cold and if running on 5 find the faulty piston, exhaust header will be cooler etc or remove plug caps one by one carefully checking to see if engine gets worse or stays the same, if it stays the same, you have found the faulty piston etc, remove and check if plug is fouled?
If so replace plug if still faulty could be lead (you can try swapping it over and see if fault moves, also check distributor cap etc. If swaping leads if the fault follows you have found your culprit (lead). If none of the above works would suspect ignition unit or coil. you have pertronix don't you?
Worst case check compression on each of the pistons, if one is different looks like mechanical fault.

I bet its a plug though!!

During my restoration, my car started to run like a dog because plugs would get all fouled etc, these cars like to have a good long run now and again

 

[RIDDLER]

Thanks for all the replies - but as someone who isn't very mechanically-minded, can I ask the following question: If it was an issue with a fouled plug etc, why would it correct itself (as it always does) after the car has been running for about 15 minutes? Surely it would just run rough all the time? I will try removing the leads one by one as suggested. I have checked out the distributor cap and that appears to be clean and dry.

 

[jonbills]

I think you've got a constant partial discharge somewhere other than at the sparkplug, so the spark at the plug is weak. When the engine is warmed up, the weak spark is enough to ignite. When cold, it isn't.

I read somewhere that sometimes if you look at the engine in the dark you can see where the discharge is - maybe worth a look.

 

[andy smith]

Ignition

I had the same problem for about 2 weeks, a pain when I use it so much. I looked everywhere but the obvious. I replaced plug leads, electronic ignition, disty cap, coil. It would go for a few days when I did this and return. In desperation I took it to a rolling road in Cheltenham Mechsport. He sorted straight away. Where the leads went into the disty cap, over time one would work away from a decent contact, and start to miss, but still feel like it was pushed in. He cleaned the silicone out of the cap, pushed then in hard, and cable tied the rubber boot individually to the cap. That was 6 months ago, and it runs 100%.
Worth a try.

 

[mattbibbey]

Have you tried swapping the coil for a known good one?

 

[RIDDLER]

Thanks for all that advice - really appreciated. I took the car to Mark Warburton earlier this week. He has reset the points, adjusted the timing and reset the carbs - and the last two mornings it has started and run beautifully. So I still don't know exactly what it was - but at the moment I am happy!

 

[Jimbo]

Thanks for all that advice - really appreciated. I took the car to Mark Warburton earlier this week. He has reset the points, adjusted the timing and reset the carbs - and the last two mornings it has started and run beautifully. So I still don't know exactly what it was - but at the moment I am happy!

i had a similar issue and is the reason i have changed to luminition ignition.
when my car played up i would reset the points gap and it would work beautifully for a couple of days then start playing up again.
i have a feeling the condenser or something could be dead because on my car its causing the contact pads on the points to burn and get pitted causing them to stick together killing my spark.

i still haven't finished fitting the lumintion kit into the car so cant tell you its fixed the issue yet
but given my cars symptoms are the same as yours im 99% certain you have a points related issue.

 

[RIDDLER]

Thanks Jimbo - although my original post on this was January 2014 and the problem has long since been solved. In the end the chief culprit turned out to be a faulty voltage regulator. I swapped to an alternator which had a built-in regulator and had no further problems - though I have now had lumenition electronic ignition fitted as well! Car is now sweet as a nut.