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MG MGA - Vacuum line question

I'd appreciate advice from the BBS relating to the vacuum line. I was checking the ignition timing on my '61 Coupe the other day and I must have unknowingly caused the copper vacuum line to shear where it comes out of the vacuum advance cannister when I was adjusting/turning the distributor. The car was driving very well last Saturday until about 15 miles into a journey going at around 50mph when it started to hesitate quite a bit and I had to pull into a layby to check the engine out. I couldn't find anything glaringly out of order and when I travelled on for the next 5-10 miles all seemed well again. On the journey back it was again all okay for around 15-20 miles and the hesitation/jerkiness came back again - it seemed as if the car wasn't running on all 4 cylinders. Managed home okay and found lying on the garage floor the fallen off vacuum line. Could it be that the missing vacuum line is the cause of this issue? NB, I've relatively new coil, leads, dizzy cap & plugs.

A second question is related. I attach a photo of the small brass olive fixed to the copper vacuum line that sheared off. Is this simply soldered onto the end of the line?

Thanks. I'll get the vacuum line reinstated asap and it will be interesting to see if I still have the issue.

Tim



Tim Prime

Tim

Bottom line is that you only had the mechanical (bob weights) advance available to you. Once those are fully flung out that's it. So at higher rpms your timing would start to appear retarded with the engine running flat and perhaps hesitating as you describe. i.e. the burn in the combustion chambers would be taking place too late and only partly combusted fuel/air then being exhausted.

Steve
Steve Gyles

Yes, it is likely a result of the vacuum line as Steve explained.

In my recommendation the end fitting is soldered on to the end.

Depending on the age, I'd recommend just replacing the whole thing. If its old and been vibrating you never know where there is metal fatigue that will lead to another leak.

FWIW, I just got my car back from a reputable MG mechanic, he told me that unless you are going for a concourse, don't used the clip that goes under the head bolt. You just can't get a good torque.
Tysen McCarthy

Thanks Steve & Tysen. That is really helpful. I'll get the vacuum line reinstated and try and give it a good run soon to see whether that has sorted the issue I was experiencing. Cheers. Tim
Tim Prime

Tim. I disagree with the other posts.

The purpose of the vacuum advance is to increase the timing advance at a constant rate of speed--i.e. when "cruising". This slightly increased ignition advance allows for slightly better fuel economy, something that is desirable on a touring vehicle. Race vehicles, because fuel economy takes second plate to reliability and performance, most commonly use a straight mechanical advance system with no vacuum advance used. Thus, the result of having a non-functioning vacuum advance is slightly reduced fuel economy and, perhaps, a slight air leak into the intake manifold depending on the nature of the vacuum advance system failure.

The failure of the vacuum advance system would not cause the problems you describe, at least in and of itself. It might have a very, very small impact on what is happening, but should not be the root cause of the type of problems you describe.

The first question is what distributor you are running. Since you mention that the vacuum advance line is attached with a small nut, I assume you are running a vacuum advance can off a Lucas DM4 or early 25D4 distributor, but what is the basic distributor you are running? Are you running the original points type system or has it been changed over to a "pointless" system. Has the distributor ever been removed, properly checked out for functioning, and been properly lubricated on a regular basis.

The problem you describe is most commonly found to indicate some form of spark triggering system failure or current transfer failure. This can range from a bad coil to a distributor grounding problem. And, between the coil, we have the points (or replacement system) and coil as part of the low tension circuit. On the high tension part of the circuit, a rotor that is starting to fail, bad distributor cap, and failing spark plug wires all have to be considered, along with such odd ball things as a small amount of water being splashed on a spark plug when driving through a puddle and the spark travelling down the outside of the plug rather than internally.

Yes, you had a problem and found a broken vacuum advance system. Fix the vacuum advance system, then, let us try to figure out what the real cause of the problem is.

Les
Les Bengtson

Tim

Les beat me to it. Clearly fix the vacuum pipe but they aren’t the cause of your symptoms. I had a similar problem earlier this week (in the Isle of Harris) and I fixed it by replacing the plug lead on no 4 cylinder. This is the second HT lead problem this summer but luckily I always carry a spare set. Like Les it sounds like an ignition problem so if your distributor is ok and it sounds like running on 3 cylinder it probably is which points to a HT leads or a plug.

By the way my best investment ever was going to pertronix inside the dissi, and their Flame Thrower coil. Over the my 5 decades of running the A ignition issues have been sources of most problems. When I was doing 30k miles a year in early 70s if I had an ignition problem I always replaced all the cheap ignition bits, i.e. points, condenser, plugs and leads. I still carry a spare distributor with leads and coil.

Cheers

Paul
Paul Dean

Thanks very much Les & Paul. The vacuum line is now back in place. I'm running a 25D distributor with a Powerspark electronic ignition instead of the original points. I've just recently replaced the rotor arm, the distributor cap, the coil, the leads and the plugs and so these should be okay. I will however check that all the plug leads & coil lead are securely in place and will take the car for a the good run over the next week or so to see if there is any repeat of the issue. I'll report back. Tim
Tim Prime

Hi Tim

Beware of assuming new means its OK. I had major problem that resulted in us being rescued from Glencoe. I assumed that the problem couldn't be in the new Moss distributor as it was only 2 week old and done just over a 100 miles, wrong! In the end it was the condenser in the new distributor and following discussion on the list I found this is not a rare occurrence. Some of the components are of appalling quality and as I found when looked inside the failed condenser. As i understand it condensers and rotor arms are particularly suspect and in the price list Moss now list a standard (cheap) option and a high quality one. I would never touch anything except the high quality ones. With my Pertronix system I now longer have a condenser and I think my rotor arm is original MG.

Your problem could well be the condenser as with my problem it would run ok for a few miles and then stop. When it cooled down it worked again for a few miles. I am not sure what the symptoms are for a failed rotor arm as I have never had this problem.

If you have more problems we could meet as I have lots of spares in this area including a complete spare distributor with leads attached.
Paul Dean

This thread was discussed between 21/06/2022 and 26/06/2022

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