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MG MGA - Carb Inlet Manifold

I had a good run in the car yesterday and all was going well. I stopped at a set of traffic lights but when I pulled away there was a "pop" noise and the engine died!
It turns out that one of the core plugs in the inlet manifold had blown out. I've since replaced it and the car is again running OK but I don't understand why the plug would have come out!
I was under the impression that the inlet manifold was under negative pressure and this would have kept the plug attached.
Any ideas why this would have happened? Is there something I should look for?

P Dodds


Check compression figures or do leak down test to check for burnt / poorly seating valves.

Also check ignition timing, plug leads for breakdown between them and distributor cap for tracking.
Chris at Octarine Services

One backfire is enough to blow it out if the plug was not properly tight. Be sure the new plug is properly set, about half flat, but just a dent in the middle.

Next question is why it would backfire. Check valve clearance adjustment, ignition timing, properly adjust fuel mixture, and do a compression test.


Checked the valve clearance and all OK @ 15 cold.
Warmed the engine and timing normal.

The compression readings are:


Is the low reading on No3 telling me something?

As Chris would know the B head has only been on a couple of years, as he shipped it to me from the UK ex Peter Burgess.


P Dodds


Those readings should be more even
- can you do a wet test as well?

Chris at Octarine Services

Hi Chris

Re did the compression test today.

No1 155 180

No2 160 180

No3 145 200

No4 165 180

BIG jump for No3!!

P Dodds

Look like rings then! 8-(
Chris at Octarine Services

You might also check the health of the cam at the rockers. It'll take a dial gauge with a magnetic base and some patience and won't require any disassembly.

But, the wet test is pretty telling. I'd suspect rings, the cam might become a 'might as well job'.

I was hoping to do just valves and rings in my TD last winter and found 3 rounded-off lobes and 3 cracks in the crank.

Just like Forest's box of chocolates.....


For info how do you do a wet test?

Paul Dean

You squirt a shot of engine oil down into the spark plug holes before you do the compression test.

The oil temporarily improves the seailing of any worn piston rings.
So if the compression readings are then higher, you can be fairly sure that rings are worn.

Also, make sure that your carbs throttles are wide open when you do the test or you will get low readings.

Colyn Firth

Is this an engine out job and would it be good practice to replace all the pistons rings or just No3?

How difficult is this? Thank you all for your advice.

P Dodds

Depends - you do need to pull the head and measure the bores below any ridge - if they are worn by more than 7 thou then you need a rebore which means full engine out.

If they are worn less than 7 thou then you can drop the sump, release the big ends and pull the pistons to replace the rings, the bores will need a hone to break the glaze and allow new rings to bed in but this can be done with the block in situ.
Chris at Octarine Services

Oh the joys of owning MG's!

Thanks for all your advice, I'll do some probing.

P Dodds

Why would lowish compression on one cyl cause a backfire?
I can see it if it were valves at fault, but not ring seal.
Anyway, 142 psi isn't too bad.
Art Pearse


It wouldn't - taking the compression figures was a way to eliminate the valve seating (that would cause a backfire) having found the low pressures dry versus wet rules out the valves but highlights the ring problem - the cause of the backfire is as yet unknown!

142 psi is very bad when the wet readings are 180 / 200.
Chris at Octarine Services

Thanks Chris

Would I just pull No3 piston or all four? The others seem OK so why would you?

P Dodds

Once you have gone to the work of pulling one piston then it makes sense to pull the other three - rings come as a set for four pistons usually.

The other readings are not that great anyway - I would expect nearer 180 psi from good rings if the wet figures are 180/200.
Chris at Octarine Services


How much oil did you use to do the wet test? If you put in too much, you effectively shrink the combustion chamber size, and artificially boost the compression figures.

C R Huff


It wasn't an exact measured amount but roughly about the same volume in each cylinder. Just a squirt from a syringe of oil.

P Dodds

This thread was discussed between 26/04/2017 and 05/05/2017

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