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MG MGB Technical - Manifold leak

Hi all,

I've been told I have a leak in my manifold since a recent clutch replacement where the mechanic removed the engine and didn't do a great job refitting it.

I could take it back and have them fix the problem, but I'd rather tackle it myself if it's not too difficult.

I have bought a replacement set of gaskets, manifold and exhaust flange.

It seems reasonable straight forward to me. Remove the manifold, clean the surface, replace gasket with new one and bolt it all back together. Same for the exhaust flanges.

Is there anything I should be careful of (aside from letting the engine cool down of course)?

Any suggestions will be appreciated.

Cheers,
David
Sydney - Australia
D O'Brien

David. I should like to know what symptoms are causing you to believe you have a manifold leak and whether it is on the exhaust, intake, or both manifolds. What tests have been done to indicate that the problem is a gasket leak?

Process of replacing the gasket is fairly straight forwards, as you note. However, replacing the gasket will not correct a cracked manifold. Nor will it correct a stripped stud or nut. Hard to give any advise without knowing what the problem is.

Les
Les Bengtson

My regular mechanic (not the one who changed the clutch recently) noted the problem when he adjusted the carbies recently. He told me I should take it back to the other mechanic and get them to rectify the leaky manifold which he is sure is being caused by either a poorly fitted gasket or possibly they forgot the gasket.
He didn't charge me for adjusting the carbies as he wasn't able to do them properly until the manifold leak is fixed.
D O'Brien

Dave. Please forgive me for saying so, but that is a large pile of horse droppings.

Let us start at the cylinder head. At that point, there is a single gasket which fits between the cylinder head and both the intake and exhaust manifolds. It is not completely covered by either of the manifolds and, if the first shop had "forgot the gasket" it would be very obvious--you would not be able to see it.

After that, it gets more complex. I assume that you are running the standard twin SU carb set up. On the inside of the intake manifold is the gasket to the cylinder head. On the outside of the manifold are two connection points, one for each carb, and these have four gaskets associated with each carb. (This from memory as it is late, my GT is parked outside and I do not feel like clearing a space for it in the garage so that I have a lighted area to examine it. So, please allow me to work from memory, which I believe will be found correct and, if not, others will provide a more exact answer.)

On each carb connection, you have a gasket between the flange of the intake manifold, followed by the heat shield, followed by a gasket, followed by a plastic spacer, followed by a gasket, followed by the carb. Total at this point, six gaskets, one heat shield, two spacers, and two carbs.

Each carb has fitted to it, an air filter and each air filter has a gasket between the carb and the air cleaner holder.

At this point, we have a total of nine gaskets which may, singly, or in combination, leak. Thus, the second mechanic's claim of I cannot tune the carbs because someone else did not install a gasket correctly, causing a leak, becomes somewhat suspect. A competent mechanic should have been able to trouble shoot the problem down to a specific gasket, replace it, and tune the carbs for you.

Start the engine and warm it up. You need it to be able to hold an idle. You may have to adjust the idle speed upwards as you have had one mechanic playing with the carbs and failing to bring them into adjustment properly. He may have gotten them badly out of sync. But, get the engine running, get it warm, and adjust it so that you do not have to keep your foot on the pedal to keep it running.

Take an unlit propane torch, or some carb cleaner/WD-40, and find out where any air leaks might be. With the unlit torch, gently move the tip of the torch around the various junctions--cylinder head to intake manifold, intake manifold to rear of heat shield, front of heat shield to rear of spacer blocks, and front of spacer blocks to rear of the carbs. (The gasket between the air cleaner housings and the carb bodies will not have an effect on the running of the carbs unless the gaskets are installed upside down--then it will have an effect. But, they cannot be tested using the propane/carb cleaner technique.)

The carb cleaner/WD-40 is used in a similar fashion, except it is sprayed at the various junctures. In either case, where there is a leak, the propane will be drawn in making the engine run smoother or, the carb cleaner will make a temporary seal and the engine will run smoother.

Additional tests. Remove the air cleaners and note that the carbs have two holes above the main opening. The air cleaners will have similar holes as will the gaskets between the air filter housing and the carbs. Make sure that those holes line up or the system will not work properly.

The stock SU intake manifold has two plugs, one at each end, which can come loose and cause an air leak. Check these with your torch/spray. While doing so, check any other holes/fittings on the upper side of the intake manifold.

Check that all of the nuts are tight that hold the intake/exhaust manifold to the cylinder head. If these are not tight, the system will leak. You would also expect to find the exhaust manifold leaking if the studs are bad, nuts are bad, nuts are not properly tightened, or if the washers (one large, bridging the flanges between the intake/exhaust manifolds) and their lock washers are missing. This can also happen if a stud(s) have been removed and not fully reinserted into the cylinder head. Then, there may not be sufficient threaded length available to allow the system to be tightened together.

You seem to lack confidence in the mechanic who replaced your clutch. I lack confidence in your second mechanic because he should have been able to trouble shoot the problem down and replace the offending gasket with no problem. May be a good time to do your own testing and make some decisions based on what you find out?

Les
Les Bengtson

I 100% agree with not going back to the incompetant biz that screwed up. This is not a difficult job, unless they broke a stud off flush with the head. If the did that and then just gave me the car back no way would they get their hands back on it. The 6 studs where the 2 parts of the cast manifold meet can be aproblem, but you do not need to split it there to do this job, or the clutch, so you should be OK there.
Stan Best

In defense of my second mechanic, he didn't look into it closely because he was trying to help me get something done for free under warranty from the first mechanic. He did this because I had just been screwed over a warranty issue with the fuel pump he had just fitted for me. In short, my pump was less than 12 months old but it wasn't convenient to organise a warranty repair so I simply bought a new one. The mechanic only did the labour.

He didn't say the gasket could be missing, that was my suggestion. All he said was the manifold was leaking and I should take it back to the place that did my clutch so I can get at least one thing fixed under warranty.

Meanwhile, I did have a look at it yesterday and found the front screw which was the only one I could get my socket to (couldn't find my spanner) was loose. I tightened that up and now the engine is idling a whole lot better.

I'll do what you suggest with the carbie cleaner to test where any leaks are, but I think I might have solved most of the problem anyway. I guess if they left one screw loose then there's a good chance another is loose too. Now where did I leave that spanner?

Either way I'll still let the second mechanic take a look at everything next week. He has the advantage of a nice hoist where he can walk underneath the car. I only have a set of wheel ramps and axle stands. He can also do a much better job at tuning the SUs than I can. I'm not too bad it at after practicing plenty of times, but he really is an expert with the SUs (it's the reason I chose him in the first place). It's just a pity he wasn't available when I needed the clutch done and I probably wouldn't be having these problems now.

Thanks for all the advice btw. It clears up a lot of things for me.
D O'Brien

David. It begins to make more sense now. But, you are wise to do your own initial diagnosis. That way, you can describe to the mechanic what you did and what you found. May save him some time. The bolts at the front and rear and the nuts on studs in the central portion do need to be checked for tightness after the engine has been run up and taken for a drive. The gasket can compress somewhat, causing an air leak to the intake and exhaust leaks around the flanges of the exhaust manifold. These can, just after start up and before things become too hot, be felt by running the fingers near the flanges.

Les
Les Bengtson

I dont understand what the whole comotion is about. Its very straight forward to change gaskets. So change them... If you want put some hi-tack gasket sealer on as well, make sure studs are seated good and have good threads, along with bolts (brass) and change them out. Its very cheap and shouldnt take so long. Just when undoing the SU's as It sounds like this would be your first time, note how the linkage goes together, or better yet take a picture and then go for it. Then if you still have problems come back on here. Im sure Les is just trying to save you some agrivation from changing the gaskets and it not fixing everything. But that is reality... Especially with an older car.
James

Another possibility. Is your exhaust manifold standard or aftermarket steel? Sometimes the flange thickness doesn't match the inlet manifold flange. When fitting this is often "made up" by putting small packers under the washers. (Sometimes even pieces of welding rod!) The nuts could be done up tight but not putting pressure on one of the manifolds.
Allan Reeling

This thread was discussed between 08/08/2007 and 09/08/2007

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