Intermittent misfire

[Ivor Hawkins]

Bugger, I missed that!

Serves me right for speed reading the thread, but it still sounds electrical to me...

[12jslater]

It's got me very confused, car runs absolutely fine down the road, pulls very well, hold speed fine, and it doesn't get any worse when hot, never stalls either, starts on first pull of the starter with Little to no choke.

Very strange, the spark is a nice fat blue one, rotor arm and distributor cap clean, all plug gaps are equal and correct too.

[John Mason]

Hi 12jslater, Have you considered that you may have an intermittent short on the low tension wiring. Try disconnecting the wiring to the coil both sides and insulating them for safety. Then make temporary wires one from coil to dizzy and one from the battery to the coil. You will need a switch in this wire in order to switch off and on the ignition. If you car then runs satisfactory then you have proved you have a short circuit.

John Mason.

[Howard Wright]

Hi 12jslater

Exhaust pops can sometimes come from air leaks right down the exhaust.  I had this on my old special and it seemed to disappear once I’d made sure the manifold to exhaust and down pipe to silencer was sealed.

Horrible isn’t it when you seem to have done everything but still have the problem?

Good luck 

Cheers

Howard

[Colin Wilks]

You say the valve gaps are correct, but have you eliminated the possibility of a sticky valve as Tiger suggested? I think this could produce the intermittent symptoms you describe.

If this is the problem, I clean my guides with one of these in the electric drill. They work really well, but I can't remember where I got them! Maybe someone on here will recognise them.

[12jslater]

I don't think valves are sticking as they were cleaned within an inch of their life with petrol not too long ago to remove deposits.

It seems everything is in order, I've ran new wires to the distributor too, even though there was no where they could have been shorting out, same old, definitely not the spark plug leads as that chap with the Morris came over today to have a nose at it and we tried his distributor cap and some silicone spark plug leads I bought for my ford Popular before I solid it and never fitted, and still the same so it's definitely not a ht lead shorting somewhere either

["Slack Alice" Simon]

I cannot be sure, from the descriptions, so:

You have tried different things with the distributor.

Do you still have the original base that carries the points?

Is it of the type that has the "hidden" earth through a screw into the metal casing?

If so, the earth lead you described doesn't over-ride the earth route, you need a wire directly from the points to a good earth.

[John Mason]

12jslater, I do not wish to the appear to be rude but you say things like, I don't think valves are sticking as they were cleaned within an inch of their life with petrol not too long ago to remove deposits. When tracking a fault like you have got go back to basics and check everything from scratch and don’t assume anything even when you think it’s ok. Once checked and found its ok you can then eliminate that possible cause and move on to the next. That way you will find what is causing your problem. Don’t ask me how I know.

John Mason.

[Colin Wilks]

As Simon said, "If so, the earth lead you described doesn't over-ride the earth route, you need a wire directly from the points to a good earth."

I had thought  you had completely replaced the distributor, but if not this is worth pursuing. I spent hours tracking a fault on my land rover to a new old stock distributor earth lead that looked OK and showed continuity, but was breaking down under load.

Anyway, you have a real head scratcher. Good luck!

[AustinWood]

We had a misfire which turned out to be Nos 3 & 4 exhaust valves sticking. Initially only happened when the engine got hot after a long haul in 2nd gear but got gradually worse until it always occurred over a certain engine speed. Valve stems & guides cleaned & tappet clearance increased slightly. Been OK now for several years.