356 Ignition problem?

[mikrokosmos]

My 356C has developed an intermittent ignition problem. The car usually starts fine, usually runs OK, although sometimes with a ‘soft’ misfire at idle. It can then suddenly cut out at any point – maybe after 1 minute, or half an hour – or not at all! Often, it will restart immediately, but sometimes it can take several minutes.
The problem can happen at any engine speed.
I replaced the coil and condenser. This apparently fixed the problem for about 2 months, although the slight misfire was still there. Now it’s back – worse than before.
I’ve checked the supply (12v) to the coil – and when the engine cuts out, the supply is still fine. So it has to be after the coil I think. I’ve checked the HT lead & the earth return from the distributor to the coil – all seem fine. I’ve fitted new points and plugs.
The mechanic who serviced the car last week had the problem happen – and he found no spark. He thought it had to be the supply to the coil, but that is definitely OK.
Could the coil or condenser have started to fail again? Or is there something that could make them fail? I don’t want to keep replacing parts if the problem keeps returning… the only common parts (assuming the new coil & condenser are OK) are the HT lead and the distributor. I know, don’t assume anything… Please help!?

[vwretrosteve]

I wrote a reply then realised I hadnt read your post correctly -

- assuming your checks have covered the ususal suspects - cracks in coil nr terminals, points wire /condensor wires ok ...
If you are 100% certain it isnt a power to coil related issue (bad connection, broken wire ) you are either unlucky or your distributor may be the problem.

I would seek the combined wisdom of PRS, as they are certain to have experience of the symptoms etc . -

then i would junk the lot for electronic ignition

[roy mawbey]

Just a few points you might check:

1). Is the copper strap earth lead from the gearbox casing clean and not corroded? I presume you have checked the copper earth strap from the battery negative to the body. It might give an intermittant problem.

2). Presume the misfire is from the exhaust and not from the carbs. I presume petrol is being fed correctly so that you are sure its ignition based.

3). The carbon brush in the distributor cap is okay? Is it well worn or the spring very weak? also is there any tracking marks on the distributor cap which might show a crack? Did you check the rotor is that okay? When you move the rotor clockwise to the stop does it spring back okay?

4). Are you sure the contact breaker points insulation at the connection for the coil wire is perfect? Also the ignition lead from the coil to distributor is this in good condition. Have you checked the resistance with a multi-meter when you twist it around?

For the engine to die is odd, you might expect a sparking plug or lead to go down but that would still leave other cylinders operating. It does seem all contact to the points is lost. The condenser stops the points from burning and if its the correct one, the points should be un-pitted.

Finally, ignition switch, all okay there??

I am no expert, maybe someone else can give you further leads on this.

Hope you find the problem let us know! Best of luck

Roy

[Mike Smith]

You covered it Roy - Check Everything

Can we offer you a job ?

[pauldvern]

I had a problem a bit like yours when I first got my car on the road. It turned out there was a square sort of washer on the bolt that held the points to the outer body of the distributor, this would slowly rotate after a few miles and short the points to earth inside the distributor.


Paul

[mikrokosmos]

Thank you everyone, for your ideas & comments. I'm having another go at it today - I'll let you know!

[mikrokosmos]

Well, at the end of the day it was the earth return from the points to the coil. I had checked this several times with a meter, and it always measured fine, but eventually I figured it couldn't really be anything else... mainly because I had changed everything else. Why did the easiest, cheapest solution have to be the fix? Why didn't I just change it first? Anyway, after I'd cleaned up the (new) plugs, which were completely fouled by now, it started. Still misfiring a bit, and sometimes blowing blue smoke... and the thrust bearing complains loudly when cold... it all seems to be going horribly wrong. I wonder if the constant misfire/intermittent spark has caused some damage? There will have been a lot of un-burnt fuel swilling around, but I don't know if this could cause problems. Time, I suppose, to visit an EXPERT.