Running rough

[Tom Brearley]

I wonder if anyone can help me with an engine problem?

She developed an intermittent hesitation a few weeks ago, which would manifest itself shortly after starting, or when coming off the choke and accelerating in 3rd. Once the engine was warmed up it seemed to disappear. Sometimes it didn't do it at all. I wondered if it was running rich: the plugs seemed a dark.

I started to give the car a tune-up to see if that would help (or at least to dial-out any overlaid issues and make diagnosis easier). So I set the timing, adjusted the valves and balanced the carbs. The problem seemed to disappear but the plug colour now seemed lean so I made a number of test runs, to check the plugs and adjust the mixture. However, immediately after setting off from one such pit-stop, the engine ran so rough that it was barely driveable. I pulled over and checked that I hadn't made some gross mistake, like leaving a rag in the intake, but all seemed fine and the problem wasn't reproduceable when I revved up at the side of the road.

Set off again, and the problem re-appeared; only worse. Pulled over for the second time and again it seemed fine when stationary. A bit non-plussed, I thought I might be able to limp home, but this time she cut out altogether 50 yards down the road. My RAC membership came in useful and we got an ignominious tow home.

I've been able to re-start in my garage, but the engine is very lumpy, sometimes running on 3 cylinders. When it does that there is no spark at the dud cylinder. (I'm checking with a timing light clipped to the HT leads.) It's not always the same cylinder. The position seems to be improved when increasing the advance by turning the distributor. But the timing light gives uneven flashes and the engine is still unhappy.

I have replaced every component in the ignition system over the past few weeks, and tested the live feed to the coil and the earth straps. I've also serviced the carbs for good measure. Fuel flow and compression are good.

Is there anything else I should try? I'm fairly out of ideas.

[Vic Butler]

Have you changed the coil? Is the rotor arm one of the red ones as the cheap ones with the rivet don't last five minutes? I presume it's running on points and not electronic ignition.
It could be the coil breaking down. I had that problem on an old Mini and my current Land Rover. Changing the coil cured it. A 1973 MGB won't have ballasted ignition so you must use a 12v coil with a resistance of 3 ohms and make sure you get a decent coil. Red rotor arms are available from the Distributor Doctor and the MGOC sell them too.
Change one item at a time. If changing one doesn't cure it then change the other.

[Paul Scott]

There is a lead inside the dizzy connected between the moving base plate and the body. They do fray and cause issues.

I have a lighted switch connected between + & - of the coil. Helped me track down an LT issue. I still have it connected , but switched off.

Does the tacho drop out, when it struggles?

Regards
Paul

[Tom Brearley]

Thanks Vic, but I've already replaced the ignition components one by one: plugs, HT leads, distributor and coil. No luck.

Paul - the tacho doesn't drop out and seems to be reading correctly.

[Vic Butler]

If the tacho doesn't flick about when it happens it is most likely to be in the HT circuit.

[Allan T]

Although you say you've replaced all the ignition components, you don't say specifically if you've followed Vic's advice to use one of the Distributor Doctor rivet-free type rotor arms. If I hadn't known better, I'd have thought the big brass rivet was an obvious signifier of better/more robust construction, and preferable to the plastic-only type. Turns out the opposite is true, and your HT can find a route to ground through that rivet, leading to exactly the sort of erratic/hard to diagnose misfiring you're experiencing. Good luck!

[Tom Brearley]

Yes, I should say I'm using a Distributor Doctor red rotor. Together with a Pertronix electrical pick up, in a refurbished 25D body. My other distributor, which I'm switching in for test purposes, is a 123 fully electronic unit. The 123 gave years of trouble-free service. So I'm confident the issue doesn't lie in the dissy itself.

I was thinking about the connections between the various ignition components today and particularly how the dissy earths. It does so through the body. So I removed the clamping plate to find quite a bit of corrosion underneath and around the collar. Having cleaned up the surfaces, the engine is running better - with only the occasional miss.

That may be due to the rear carb leaking fuel through its overflow(!) which I will try and fix tomorrow. I suspect there's contamination in the flat bowl from the recent removal and reconnection of hoses.

[Vic Butler]

Let's hope you've cured it. I must admit it's an unusual fault.
After a bad experience with a 123 distributor on my Land Rover I sent a proper Land Rover distributor to the Distributor Doctor who rebuilt it and fitted Pertronix electronic ignition. I fitted the distributor, timed the engine by ear and that was done about 12 years ago and I haven't needed to touch it since apart from the occasional drop of oil.
The 123 distributor was returned to the supplier to find a fault that caused a misfire and fortunately they lost it and refunded me in full even though it was out of warranty.

[Allan T]

Neither was MG-related, but I've now had two things returned to the suppliers/makers for diagnosis and repair, 'lost' and refunded in full.

I suspect it's a pragmatic business/economic decision to not waste time looking for the fault, and to avoid acknowledging there ever was one!

[Tom Brearley]

Well, the dissy-earth seems to have been a red herring since the misfire has returned. This time there _was_ a spark at each plug, so I looked at the carbs again, thinking fouled plugs were a possibility, and found a fair bit of contamination in each float bowl. Tiny metal particles in the rear one, and larger brown bits in the front (even though they were clean a couple of days before). Suspecting that the fuel filter might be coming apart, I changed it, flushed the lines and cleaned the carbs out.

But the engine still seems unhappy. It sounds (to my inexpert ear) like one cylinder isn't pulling its weight and occasionally missing. The float bowls are clear. The plugs all have a similar grey/black colour. I'm just running the engine in the garage at present and haven't ventured onto the road. Adjusting the timing doesn't cure it. I will have a go at playing with the mixture tomorrow. But any further suggestions?

Wearing a hole in my hair with all the head-scratching.