Low volts on ballast wire & misfire at speed

[chrismc]

Bizarrely I have been struggling with the same confusion with my v8 conversion. Ticked over fine but miss fired badly when taking its first trip to the MOT.

The car is running the Range Rover electronic ignition with 12v coil but with the original Magnum loom. It seemed to run better on full choke but was miles too rich. This lead me down the path of carburettor strip downs, fuel pump testing etc...

After no fault was found I moved on to the spark and found it was irregular. weak and yellowish.

I trawled a few forums for 'Weak spark' and it dawned on me the original loom had a ballast resistance wire which was incompatible with the 12V Coil and Ignition Amplifier.

It was getting late on Sunday night so I jumped a 12V supply to the Coil from the back of the Alternator and immediately got a big fat blue spark and a revving engine without a misfire.

This weekend I will wire it up properly and see how it runs

The next day I logged on to this forum and read this thread.

Paul and I seem to be living in a parallel universe

This is what forums are for.

[Paul Dawson]

As mentioned in the restoration thread, I've fitted the Accuspark electronic modules, fitted a new 12c 3ohm coil and bypassed the ballast resistor. I've also fitted a new rotor and dizzy cap. I'm waiting for new HT leads. The timing has been checked. The misfire is still there, exactly the same.

It's more of a hesitation to be honest - a touch of choke and it's fine. If the new HT leads don't sort it (which I don't think they will) then it's time to look at the carb.

[1972nail]

Paul, is it a 'hard, stutter' type of misfire or as soft 'flash spot' type. A hard misfire is most likely to be sparks while a soft one is fuel.

[Paul Dawson]

I'd say it felt more like the hard / stutter type of thing, but it's had new plugs, ignition system, coil, rotor, dizzy cap and a change of dizzy - the only thing left are the HT leads. But after changing so much of the ignition system it is exactly as before, so unless the HT leads do the trick I'm thinking a look at the carb is in order.

If you pull out the choke slightly you can floor it no problem and it'll rev hard and free, so there's no fuel supply issue. The carb is a Zenith 301Z which aren't brilliant. It may be dirty, it may be set up badly, it may be leaking through the spindle (I hear they're prone to it).

Changing the fuel pump connection for the accelerator from summer to winter (ie a bit richer) made it slightly better. So I'm pretty sure it's a carb issue.

[ADRIAN]

Check ....... and double check, and look very very closely at THE DIZZY CAP.

Every OHV distributor cap sold in the UK , wherever it comes from or whoever makes it, they are ALL faulty from new. The inside ring ridge, which clips over the metal top is too big. This means that the cap does not fit snuggly onto the distributor. It is either located slight on the " P### " , (rocking) or if it is forced onto the metal body the cap splits. It will be a tiny hairline split, usually at the clip area. This minute split will cause an annoying misfire when engine gets warm.

The cure is to sit down with a tiny file, and remove plastic around the underside ring until the cap fits the distributor properly. It is better for the cap to be slightly loose than very tight.
Only the original Delco Remy distributor caps were fine. These fit correctly as you would expect. But you cannot find these now. Any other cap is trouble.
Go look at your caps............

[Paul Dawson]

Thanks - that explained why I noticed that the new cap is a poor fit which struck me as odd. I thought I'd got a dud. I'd set about it before fitting and made sure it wasn't forced into position or sitting crooked. I've checked it and it's not cracked, neither was the old one which is actually a much better fit.

I've since cleaned out the carb and that's made it about 80% better, so I suspect it's a fuel issue.