Puzzling misfire

[Murph]

My car has recently developed a misfire and I've been having a lot of trouble tracking it down. From what we've determined so far, it's a problem of not getting enough voltage to the plugs, as putting a new set of plugs fixes the problem for about a week, then it returns. Over the period of a day or so it will miss here and there, and then it all goes to sh*t.....missing at all revs/loads to the point the car is undriveable. There's a lot of carbon on all the plugs when it's like this, so it's not just one cylinder with the problem.

Btw, pretty much everything in the ignition system was new as of 6 months ago.

So far I've tried

•two different distributors

•new points

•two coils

•two sets of lead

•new HT lead

•two sets of plugs

•new condenser

•carbs and valve clearance were all set about a week ago so no probs there

•two dizzy caps

•two rotors

•played with timing

None of these things have made a damn difference.

Voltage at the coil is good, and the problem isn't the plugs or leads, so it has to be coil->HT lead->rotor->dizzy cap. HT lead is new so we can rule that out. The coils, rotors and dizz caps I've used weren't new, so it might be them. Next on my list is to try a third coil, then if that doesn't work, then buy a new dizzy cap and rotor.

Is there something here I've missed? This really has me stumped.

[Alan Pugh]

With the amount of chasing you've done on the electrical side of things, my next place to start looking would be why are the plugs carboning up.

Too rich?

Loss of compression?

With what you've changed already I would start looking further afield.

[Murph]

All the plugs are carboning up. I also know for sure that one week ago, that both the carbs were running a good mixture.

I'm still sure it's electrical, as if I put new plugs in, it will run fine.

[Zedrally]

My bet would be either as Alan suggested, mixture or valve seals.

If clean or new plugs work well, then the electrical system isn't the problem. Look further a field as to why they are fouling?

Good luck..

MOM

[Murph]

If clean or new plugs work well, then the electrical system isn't the problem. Look further a field as to why they are fouling?

Good luck..

MOM

New plugs work well....but cleaning the week old plugs doesn't. So it's not the fouling itself that's doing it. They're only getting a light coat of carbon that comes off quite easily.

The explanation from my mechanic was that a brand new plug will have a lower resistance, hence getting a decent spark. Then over the next few days the resistance of the plug will increase. It pretty much fits the pattern of what's going on.

Mixture is known to be good. Valve stem seals are on their way out, but I see no reason why they'd be any different now to what they were 2 weeks ago. Besides, what's the likelyhood that 6 of them would totally die at the same time?

If my logic/reasoning on any of this is wrong....then please point it out! Any ideas are quite welcome at this stage.

[Zedrally]

What brand plug are you using?

MOM

[Murph]

NGK BP5ES is what it has run since I've owned it. When I first replaced the plugs thinking it was just old foulded plugs, I bought the BP6ES (cooler) as that's what the book recommended. It ran well for a few days then started missing again. The thing is, I cleaned the plugs, but it still ran like $^!#. You'd think if the problem was fouling of plugs, then it'd run ok until it had fouled them agian. So I went out and bought the BP5ESs, and it ran great.....for a week...

Again, cleaning them makes no difference.

So my mechanics theory of there being a week link in the chain, and the low resistance of a brand new spark plug fit perfectly.

[Zedrally]

Well.... armed with this info, it could be a break down in the coil.

I gather you haven't actually tried a new one?

MOM

[Murph]

No not at this stage.

This avo if I have time I'll be trying the coil out of the parts car I just sold, which is not an original. The one currently in my car is a 6 month old bosch GT40.

[my_mad_z]

Have you checked the ballast resistor? Not sure if that's causing the problem, it's sounds as though you've tried just about everything electrical.

I have a similar problem with the L28 in my 240z that I'm building at the moment; when I start it, it runs like crap, misfiring pretty bad.... I decided to disconnected the battery while the motor was idling (very roughly) and the revs picked up and the motor ran really smooth....I then reconnected the battery lead and the revs dropped but the motor ran smooth.... Not sure why this is happening, could be that the battery (not standard) I've put in the Z is way too big or the voltage regulator isn't doing it's job.

Not sure if this will be the cause of the problem, but are the tappets correctly adjusted?

If you're willing to make the change, get your hands on an electronic dizzy; much better than the points system.

[Murph]

Ballast resistor is fine. Voltage to the coil is normal. Still misfires if you give hte coil the full 12V.

My mates old man was around when I was having go at it an hr or so ago, and he's an old school mechanic. He spent a good hour and a half playing with just about everything (I'd rather he hadn't messed with EVERYTHING), but it's still doing it. Nothing made a difference. Mind you it's running even worse now have he played with fuel mixtures.

Tried another coil btw, no difference.

One thing he did was to put a piece of plastic under the rotor, so that if it has a crack, it wouldn't jump down to the dizzy. No difference.

I think I'm just going to have to tow it over to my mechanic and leave it with him.

Oh...will be going electronic as soon as I get my tax money.

[speedyblue]

How much fuel have you gone through since it started playing up? Bit of a long shot, but maybe you got a dud batch and it's causing problems.

Good luck.