Ignition and running problems

[Gareth91971]

I have a 1972 MOD that I have had for over 27years. In the last two years of I must admit limited use in the summer it has developed a fault when accelerating away or whilst driving and pressing the accelerator it stutters and backfires and runs like dog. This is what I have done to try and cure the problem. Rebuilt the carburettors and set them up so it appears to run well whilst starting and idling. New quality HT leads, distributor cap and rotor arm, new flame thrower coil all from Distributor Doctor. New battery and tested to the best of my abilities tested and checked the wiring and fuel feed. I fitted a Newtronic electronic ignition about 26 years ago and I am thinking now that this is probably the problem? I am thinking of fitting a Pertronix system which does away with the control box, but I know there are cheaper alternatives on the market. I welcome any thoughts on all of the above. I am based up north near Preston and unfortunately I don't, know of any Stag Guru's near me. I am sure it is a simple fix but working it out is proving elusive. Thanks Gareth.

[Goldstar]

You could try one of the £40 units (Powerspark) but they’re cheap for a reason. I think Newtronic are pretty reliable so it may not be that. Have you checked fuel flow? I had an issue similar to yours and it turned out to be the fuel pump earth was flaky and failed under load. Worth checking the wiring first and then the flow - you can do this really easily. First check the filter in the engine bay has fuel in it. If not the pump might not be pumping. If it does, check flow info this by lifting up the inertia switch, taking the feed off the carbs and then putting the ignition to position 2!back to the engine bay, put pipe end into a jar or similar and then push down on the inertia button. If fuel flows ok you’ve probably discounted the pump.

[Gareth91971]

Thanks Goldstar I'll look at the pump wiring.

[Lingen]

Gareth.
I hate to argue with Goldstar but I think it is more likely to be a weak spark. A pump problem is unlikely to cause the symptoms you describe, as there is a reservoir of fuel in the carburettor float chambers that will allow a good burst of acceleration before fuel starvation occurs. If the "Dog" behaviour occurs the moment you put your foot down, then it is almost certainly an ignition issue.
I know some people turn their noses up at inexpensive systems like Powerspark, but I have had one for the last 6 years and am very happy with it. As they were so cheap and easy to replace I bought two- just in case- and put the spare in the glove locker. It is still there. In your place, at under £40, I would fit one and see if it cures the problem.
Also, the fuel filter in the engine bay will only fill up if you mount it vertically. In a horizontal position it will only ever be half full at most.
Mike.

[Goldstar]

Gareth.
I hate to argue with Goldstar but I think it is more likely to be a weak spark. A pump problem is unlikely to cause the symptoms you describe, as there is a reservoir of fuel in the carburettor float chambers that will allow a good burst of acceleration before fuel starvation occurs. If the "Dog" behaviour occurs the moment you put your foot down, then it is almost certainly an ignition issue.
I know some people turn their noses up at inexpensive systems like Powerspark, but I have had one for the last 6 years and am very happy with it. As they were so cheap and easy to replace I bought two- just in case- and put the spare in the glove locker. It is still there. In your place, at under £40, I would fit one and see if it cures the problem.
Also, the fuel filter in the engine bay will only fill up if you mount it vertically. In a horizontal position it will only ever be half full at most.
Mike.

It’s not arguing Mike, it’s sensibly adding info to the debate, it could also be a diff HT lead even though they’ve been changed or a broken spark plug, never easy to fault find.

[Keith Kidd]

Sounds like ignition. You didn’t mention setting the static timing and checking the vacuum advance mechanism. Maybe you’ve already done that but it sounds just like my Stag when I bought it, the static timing was way out.

[richardthestag]

In my humble opinion could be either.

Symptoms could be either fuel or ignition related.

Gareth91971 is it backfiring through the exhaust or the inlet? inlet might suggest an overly weak mixture whilst exhaust suggests unburned fuel mixture is getting into the exhaust system

Hard to tell without being in front of the car but I might start with the following;

Check that both carb breathers are attached to pipework and that it is not cracked and broken. The breather should be connected via a t-piece to the RH cam cover. cracked and broken pipes will upset the carbs

Find the little stub on the front of the carb pedestal that should have a non return ball inside it. connect a short length of vacuum pipe to it and plug the other end.

When you set up the carbs did you pay careful attention to the float height, did you mess with the jet? there is a whole page on getting the carbs set up and correct but if the float height is too low then you will get a weak mixture, similarly a non functioning float chamber vent will prevent the float chamber from filling correctly. Any air leak between the jet bridge and the inlet manifold will also cause a weak mixture (this includes throttle shaft, temp compensator, fine tune screw etc) as will a poor fuel feed. A local friendly garage should be able to tell you the CO output from the exhaust. 3.5-4.5% co is good. They will run at 2.5% but prefer a little more richness to the mixture. be nice to know what fuel pressure your pump is running at. A gauge on a hosetail wedged into the fuel line near the carbs will tell you that. 3psi ish is nice.

Luminition you say? 3 way plug is notorious, disconnect it, clean it and make sure that it is a good connection. Did you ask distributor doctor if the flamethrower coil was compatible with your luminition setup

Check ignition timing is set correctly

Check that the vacuum advance unit on the distributor is still sealed - i.e. you cannot blow through the vacuum pipe that connects it to the carbs. if you can then it will not be doing anything at all and will need replacement

Check that the rotor arm can rotate backwards and then spring back into place - this is a check on the top section of the distributor shaft and the bobweights (mechanical advance) if it doesn't rotate then you have no mechanical ignition advance and the distributor probably would benefit a rebuild.

Not 100% familiar with luminition but typically two plates inside the distributor, one is fixed to the distributor body and the other should rotate slightly with the vacuum advance, there should be an earth connection between the two.

The supply to your flamethrower coil may not need to be ballasted - depends on which coil you have! The ballast resistor on your 1972 car should be the white ceramic block. 1973 ish the ballast "eureka" wire came in, pink / white, they can cause issues! been there, done that and have the t-shirt. Take a fresh 12v feed to the coil +ive terminal. does the problem go away? You can run a wire from the battery +ive terminal to the coil +ive terminal but be aware that the engine may not stop when you switch off the ignition (depending on where the feed for your lumition comes from) but the ignition coil will be taking a voltage and can easily over heat so this is a quick test. DO NOT LEAVE THE BATTERY WIRE CONNECTED TO THE IGNITION COIL WHEN THE ENGINE IS NOT RUNNING

What spark plugs are you using, do they all work? new doesn't mean that they will

HT Leads, I always without fail pull back the rubber boot, wd40 aids this massively, fit the connection and then slide the boot back down over the cap / plug.

Oh and you do have the leads in the correct order don't you, wouldn't be the first and certainly not the last to do this.


There is loads more I am sure...

[Gareth91971]

Thanks for all your suggestions I have plenty to go at. I am on Newtronic ignition Richard which I think was originally called Piranha , not Lumenition. The ballast resistor was white.
Thanks again I will try and keep you posted on my progress.
Gareth

[Gareth91971]

Sorry forgot to say it backfires through the exhaust.

[richardthestag]

Ah Newtronic, comes with it's own baseplate, I had one on my stag for yonks and eventually the sliding optical pickup (attached to the vacuum advance) wore a grove and became sticky in operation.

Are you still running the ballast resistor? and does the ignition coil need it?

[Gareth91971]

Hi Richard
I am not running the ballast resistor as I was tood the flame thrower coil that I got from the Distributor Doctor didn't need it anymore.

[DJT]

Hi Richard
I am not running the ballast resistor as I was tood the flame thrower coil that I got from the Distributor Doctor didn't need it anymore.

Have you got the resin filled version, or the oil filled one? He supplied me with a resin filled one, which didn't last long. When I visited his workshop he swapped it for an oil filled one, which is still going fine with no ballast resistor. He did reveal that he has more warranty returns with the resin filled ones.