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MG MGB Technical - Sick Engine - 1967 B - Advice please.

Hi all,

I have a 1967 Mk1 MBG Roadster. I've had it for many many years and I love it.

The car has been in hibernation for a couple of years. I had it serviced and was all good to get it up and running again. Wahoo... summer and out in my B... perfect! Or maybe not.

Before I go on, please note that I am not as mechanically minded as you good people here. I will try and do what I can, but my engine knowledge as a whole isn't the strongest. Now with that been said, here's my issue.

So my B was up and running for a few months. All seemed ok.

I was driving and enjoying the fact that my wee B was out and about... when I started to notice a lack of power. It degraded over the next few kilometers. To the point I was nursing it home.

Unfortunately, it started sounding like a tractor. Very rough. Engine power diminished rapidly at one point. Oh no, I wasn't going anywhere. It was still running. But sounded awful. I coasted to the side of the road. I had to get the car towed for the last few kilometers to get it home.

I did note that the exhaust was usually a sooty black, now there was no such colour. Hmmm... does this mean something?

So, this is where I am asking for some advice. My gut tells me this could be something significant. Before I engage a mechanic to take a look (I would need to get it towed again)... May I ask your thoughts on what the issue may be and what I need to do to diagnose it properly?

Any help, tips, or advice gratefully received.
Many thank.

Regards
Mark (in New Zealand)
M Duggan

'Tractor-ish' could be running on three or less, a developing problem with plug, lead or similar. Was the tach jumping up and down or steady?

A sooty black exhaust sounds like it was running rich, that could have fouled plugs. You say it's not like that now but what colour is it? Fuel type can vary the colour. Had you refuelled just before this? Contaminated fuel from filling stations is not unknown but there would be other reports in the locality.

The first thing I'd do is take out the plugs and compare the colours with each other and a reference. The attached is from my 73 running well on 99 RON unleaded, black round the body but mid-brown on the outer electrode, paler on the insulator. My exhaust is dark grey.


paulh4

Hi Mark,
It does sound like it is only running on 3 cylinders. I had a similar issue myself on my '67 bgt and it turned out to be a broken down ignition lead.
If you can start it up, one at a time, remove the lead from each of the spark plugs and see if the engine dies - the one that makes no difference will be the culprit - make sure you use insulated pliers or grips to avoid a shock.

If it won't start up it might be worth putting a new set of ignition leads on anyway especially if they have been on a long time.

As Paul says check the plugs to see if they are sooted up as it sounds like it had been running too rich.

Good luck!

Neal
N Powell

Hi Paul and Neal.

Thanks so much for your replies. In answer to your questions... From memory, the tach wasn't going crazy.

The fuel running was is unleaded high octane (as as always run).

I'm currently away from home, and will be getting home in a couple of days. I will check the spark plugs then. So I can't comment on the condition of the plugs yet. Having said that, this is great advice as to what to check next.

The exhaust colour went from a sooty black to no colour. It wasn't blowing lots of smoke or 'blue' smoke.

Thanks also Neal for the pointers on the ignition leads. I've not replaced them in the car since I've had it. So this may be a good action to take regardless.

Oh... and good call on the insulated pliers. I don't have a pair (yet). But I remember when I first got the car... I did get a nasty shock when I touched the spark plugs when running. So I don't want a repeat of that. Ouch!

Thanks again for the advice guys. I now have an action list to follow next weekend.

Kind regards
Mark

M Duggan

Mark-
Plugs out and first thing, do a compression test--The symptoms you describe sound a bit blown headgaskety to me.
William Revit

Hi William, I did think of that being a possibility (and hoping it's not that). So I'll definitely take your comment onboard. Fingers crossed!
M Duggan

This thread was discussed between 26/10/2025 and 29/10/2025

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