Overheating coil?

[Philwesty]

Thanks for all your help folks.. here's an update. tests made when cold:
Voltage out at regulator reading a steady 14.2v
Existing Viper dry coil 3.5ohms
Old Lucas sport coil 3.5ohms
New today Lucas sport coil 3.5ohms
Starts first turn. I have a electronic dizzy cartridge. Red rotor
I notice i dont seem to have an ignition light (not sure i ever did tbh) and no relay for same.. the little tin can one.. This has never been on it since the rebuild, and as i say, the misfiring when hot thing is recent. it was fine before
Will fit the new coil and road test when i get some time and report further!

[Hugo]

Do those electronic distributors have condensers? If so I'd suspect that. If not, then it probably isn't! Don't suppose you have the original distributor? I hate electronic stuff cos I never know how to test it. I like things you can fix on the side of the road. Like Morris Minors. And E types come to that. None of this new-fangled stuff!

[Philwesty]

nope - no condenser... i had the original diz converted to electronic so its identical from the outside. forgotten who did it tho! the old ROM isnt what it ought to be..

[Philwesty]

Fitted the new Lucas sports coil and now i have a misfire all the time - right from cold!
Losing the will to live.. not thinking straight. only beer can save me now..

[Hugo]

Well at least that's progress of a sort - if in the wrong direction. Clutching at straws here - could the go-faster gold coil be pumping out more voltage & that is leaking out of the HT leads? I would run it in a dark garage & see if there's a light show under the bonnet. I'd be very surprised if there is, but won't take a minute to check.

[JagWaugh]

Fitted the new Lucas sports coil and now i have a misfire all the time - right from cold!
Losing the will to live.. not thinking straight. only beer can save me now..

Bad HT lead between coil and dizzy.

Bad rotor.

Bad Cap.

[cactusman]

The coil resistances sound about right. It seems very unlikely you have three dud coils so I would be suspecting an HT breakdown somewhere especially as it happens once everything is hot. There have been reports of fake red rotor arms that fail like the riveted crap black ones. Might be urban myth though. As already suggested get the car to mis behave in the dark and look for blue sparks where they should not be. Check the dizzy cap for cracks. If you have the repro champion plug caps with resistors check them when hot. Resistance should be about 5000 to 10000 ohms.

[adam]

I am having very similar problems to you with my 3.8 .Only started a few weeks ago with a slight miss,then to terrible misfire,I have a123 dizzy and have changed rotor arm cap leads plug caps and plugs,original coil was a lucas DLB105 sports coil changed to an NGK1079 still same problem would run ok somtimes then misfire returns,changed coil again to a new DLB105 still the same.I have cleaned out fuel tank sump, fuel filter carb filters and float bowls.I have fitted new floats in the carbs,new float needles and checked float heights.I have cleaned suction cups and pistons.The car has a dynamo and dosent appear to be over charging on the ammeter. Not sure what to do next any advice would be most welcome.

Adam

[mark10337]

Seems you haven't looked at any of the electrical elements. I would check as per Julian's advice above.
Plugs, leads, rotor arm, dizzy cap, earth strap.
Measure the voltages, resistance and connectivity at all the links along the chain.

[adam]

I have changed plugs plug caps leads rotor arm and dizzy cap and coil.

Adam

[Hugo]

But the latest coil change has at least made a difference, which suggests the fault is on the ign side. I would like to get away from that electronic distributor & see what happens. Another thing worth trying perhaps is to try it with each plug lead removed in turn - first at idle then on the road. It should of course produce a uniform drop in performance across all six. In the unlikely even that it doesn't, we're back where we are now. You've tried a few coils I believe? I presume you still have them? Try switching them over & see if it makes any difference - or rather to see if the latest quirk is consistent (whereby the go-faster coil actually made things worse and the previous one only causes a misfire when hot). And rotor arms - it takes seconds and costs pennies to switch rotor arms (again!). In the good old days rotor arms were never a problem, but now the Chinese have got their hands on production, the little buggers have counfounded many a mechanic. The rotor arms, not the Chinese, that is.

[Heuer]

Philwesty

The standard Lucas coil produces about 17kv whilst the Sports coil produces 25kv (both measured on an oscilloscope) which is enough to jump a 1" gap let alone a spark plug. Fitting the Sports coil obviously exacerbated your problem because of the increased voltage so I would venture to suggest the problem lies on the HT side of the ignition and the ignition leads is where I would look. What manufacture/type are they?
e.g. copper, silicone, suppressed

[adam]

I think its ignition based, on the 123 the cap and rotor are bosch items and used genuine parts.I think it could be the dizzy itself but dont know how to test it and havent got a spare.

Adam

[mgcjag]

Hi Adam.....one thing that gets overlooked is the actual connections in the low tension circuit...the spade connectors on the coil, dizzy and back through the Cct.....renew them.....but as a temporary measure start the car then run a direct feed to the coil, this will bypass all the ingition Cct and any possible bad connections...you can take the car for a run with it..ps only recently someone had a faulty 123 that had to be replaced

[adam]

I used silicon resistor leads as recommended by 123 and the original lucas DLB105 gave no problems for 9 years.
Adam

[Hugo]

Again clutching at straws, but do you have resistor leads AND resistor plug caps? Could that be causing the spark to leak out somewhere?

[adam]

Non resistor caps and plugs.

Adam

[Heuer]

Get rid of the 123!

[adam]

I will leave it for a couple of days and start working through it again.

[cactusman]

FWIW I would be looking at ignition and not fuel or carb set up. The original ignition set up, while crude, was easy to diagnose and fix. The 123 system is not. If all else fails go back to a standard set up and if the problem goes away you have your answer. I have read mixed reports on the 123 system as fitted to other cars. Some swear by it and some do not. I have an accuspark points replacement in mine and it has performed perfectly. Also, if you have silicone leads with carbon inners they don't work well with the old style acorns or screw on caps that were intended for copper cables. Years back I fitted them to my mgb and it developed an intermittent misfire that was eventually traced to a poor contact with a screw in plug cap....but two RAC men and me could not find it....only the old salt at the local garage got to the bottom of it.....sometimes original is actually best....