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MG MGB Technical - Sick Engine - 1967 B - Part 2

Hi all,

I posted recently on this bulletin board about my MGB and its sick engine. I had some excellent feedback from Neal, Paul and William.

The car sounded like a tractor with a huge loss of power to the point I had to get it towed home. It's been sitting idle over winter since then.

With advice from that previous post, I removed the spark plugs and ran a compression test.

This is the first time I had run a compression test (new experience for me which was relatively easy to action). The test was run with the engine cold. I ran the test twice. Once with no throttle, second with throttle. The results were the same for each test. When I did it, the battery was not fully charged so it was a little slow in turn over, but still turned over ok (battery charger said it was sitting at 60%). Is that a factor that could effect test results?

Anyway, the results were as follows for the cylinders below:

1 - 110
2 - 105
3 - 105
4 - 105

I've had the car for 35 years. In that time, no 'engine work' been undertaken on the engine block, head etc.

Further to the previous thread, some points to note:
* Tach was not jumping wildly
* Exhaust smoke was sort of grey the day it failed, but had been sooty black prior to the engines power loss.

My next step is to test/replace the ignition leads. But before doing that, I thought it would be great to tap into your collective knowledge for some feedback on the compression test and the condition of the spark plugs etc.

I'm not too mechanically minded, but always want to learn more as I encounter these issues. So your advice is gratefully received.

I have attached a PDF showing the condition of the plugs and one of the readouts from the test.

So if you could give me your thoughts on the test, I would be most thankful.

Kind regards
Mark
(1967 Mk 1 B Roadster - New Zealand)
M Duggan

Compression figures are balanced albeit low, but they will be lower with a cold engine, should be about 130 for low compression and 160 for high, what's your engine number prefix?

For the plugs 1 in particular and to some extent 2 are paler than 3 which could be a front carb issue, but then so is 4 which leaves 3 pretty black so that could have not been firing.

For the carbs turn on the ignition and check the fuel pump clicks at least a couple of times and when it stops - assuming it does so! switch off and remove the float chamber lids (easy on HS carbs) to compare fuel level. You might need to remove the overflow pipes from the lids but should need to remove the fuel delivery hoses. Alternatively it could be just that the carbs are unbalanced and have been for a long time and that's not the cause.

For 3 possibly not firing lay a spare plug on the block and with each plug lead connected to it in turn crank the engine and check you get a good fat spark each time. If the plugs have been in a while it might be a good idea to get a new set in case one is faulty. You could prove that (if you can get the engine running) by disconnecting each plug in turn and noting how much the revs drop each time. They rarely all drop the same amount, but if one makes no difference then it's not firing. Swap that plug with one of the others and if the fault moves with the plug it's the plug. If it stays with the cylinder then it's the lead or the cap, you could also swap leads (at the cap as well as the plugs) to determine which.

You can get very rough running to the point of not running at all if the condenser is failing. Modern replacements from the usual suspects are frequently rubbish and fail very quickly, the easiest check is to connect another one in parallel from the points terminal of the coil to earth and see if that makes any difference. If so the condenser in the car has failed, if not it could be good ... but they both could be bad! You can do a go/no-go test as shown here youtube.com/watch?v=3nW7laT4dAA&source_ve_path=MTc4NDI0
paulh4

BTW it helps to add to the original thread rather than starting a new one for additional information.
paulh4

It looks to me that your engine is well worn from your compression figures.
I tested mine that I rebuilt 4/5 years ago that was rebored with new pistons. Don’t know how accurate the gauge was but all were between 165/175.
Trevor Harvey

Thanks for the input guys. In particular, thanks Paul for the comprehensive reply. I will work through your suggestions. Engine prefix is '18GB RU-H'. Also noted your suggestion about the linking to previous threads. Thanks.

And yes Trevor, I suspect the engine is due for some significant work given its age. My goal at the moment is to get it running, then go from there with the engine condition. Good to hear your feedback on your engine. Really appreciate that.

M Duggan

18GB is right for the year, the 'R' of RU-H indicates supplied with overdrive and the 'H' high compression.

Have you checked the valve clearances? They can have a significant effect on compression figures as too large (or small) a gap affects when they open and close which because of the overlaps affects how much air can be drawn in and pushed out.

Another issue can be where the point of largest gap occurs. For whatever reason mine are all over the place (and I'm not the only one to have found that) which affect things if you use the 'rule of nine' to set them. I could never get repeatable results on some valves until I discovered that some of the gaps were changing at that RON point which meant that unless I put the engine in _exactly_ the same position ever time the gaps would be different. I noted the point of maximum gap for each valve and since then they don't change from one year to the next.

Incidentally, did you keep cranking on each cylinder until the needle stopped rising? That can take several revolutions. All plugs out as well.
paulh4

Headgasket's ok then- don't worry too much about the lowish readings-as long as they're even it's ok -winding slow on a cold engine they're always going to be down--so next check valve clearances if you want to, but they don't usually create an issue on a single drive-
Usual causes now then , could be ign. points closing down, dead rotor button, spark plugs/plug leads/coil lead, maybe ign.coil -
On the fuel side-the only thing that might happen that suddenly would be a sinking float but you'd have fuel running out the overflow----or rare as, a fuel metering needle has dropped out of the vac.piston blocking fuel off on a carb--i've had that and took a little while to sus it out, even had the aircleaners off and noted the pistons rising ok but missed the fact that one needle was staying down in the jet---might be worth checking that'
willy
William Revit

Hi Gents. Apologies for the slow reply... been a hectic week. To reply to your last messages...

Firstly, Paul, you are spot on with your interpretation of the engine prefixes. Most impressed with your knowledge there. In regard to the valve clearances. I havent checked as I am tinkering with an area I'm unfamiliar with. With the compression test; I turned over the car cold approx 10 times with all plugs out.

William, I was pretty relieved to see the compression test 'passed', thus getting the all clear from a head gasket issue! (fingers crossed)

As mentioned, my mechanical knowledge is not as strong as yours is, so I tackle what I can. In this case, I decided to note the points you said and have gotten the car to the mechanic I usually go to. He's onto it with a good steer as to possible solutions.

Thanks again for your great advice. All going well, next week I will be zipping around again in my cherished B.

I'll report back the results. Fingers crossed it's nothing significant for the fix!

Regards
Mark
M Duggan

Mark - Good stuff that you were keen/interested enough to have a crack at something you hadn't done before (compression test) It'll be interesting to see what your mechanic finds---hopefully something simple.
Cheers
willy
William Revit

This thread was discussed between 02/11/2025 and 07/11/2025

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