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MG MG Y Type - XPAG misfire at higher revs: faulty coil

My engine started to misfire at higher revs and it got progressively worse, so the search was on for the cause.

I first suspected the rotor and/or play in the distributor shaft, because the rotor marks in the contacts of the distributor cap. I took the distributor apart to fit some extra shims to lessen the axial play. The radial play wasn't especially bad. In the process I managed to break the top fiber washer; can you buy them loose somewhere? I now fitted a copper washer but that may be too soft?
In the rotor the spring was missing so a new red rotor was fitted and I was amazed about how much tighter/better it fitted.
Alas this didn't solve the misfire.

Then I tackled the distributor cap, which I replaced with a NOS Lucas side entry example that I bought two weeks earlier. For good measure I cut to length and fitted new copper core HT-cables, with NOS bakelite sparkplug caps (with the text 'made in Germany' in small print on them, so the search is on for other British made alternatives).
Alas this didn't solve the misfire.

Changing the Champion plugs for NGK of course didn't solve the problem either, it just added one because on a test run one managed to loosen itself (or just maybe I forgot to tighten it?).

Adding a spare condenser in parallel had me crossing the original(!) condenser of the suspect list....

In desperation I targeted the coil. It had the correct 3 Ohms resistance on the primary circuit (measured between the two connectors) and a suspect 4,500 Ohms on the secondary circuit (measured between one of the connections and the HT outlet).
An old Lucas coil from the parts bin, measured 6,000 Ohms, but that had sliding connectors instead of the original screwed connectors. So I made up some round-to sliding-connector-adaptors to rig up the spare coil for a test.
This time my efforts were successful and I revved the engine enthusiastically and may or may not have yelled YEEHAA!.

The noise thus generated caused SWMBO to storm in through the side door of the garage, which I accidentally left open for ventilation purposes after an earlier test. She not so kindly wiped the broad smile of my face when she drew my attention to the clock; it was 22.30 hours and I live in child friendly neighbourhood where the houses are built relatively close together..... I'd better not show myself outside this weekend. ;-)
Willem van der Veer

Was you old coil getting warm too Willem?

Conventional wisdom says Coils do not fail - I beg to differ, I have had several fail over the years and the tell tail sign is they get warm to very hot in use!


Paul Barrow

I don't know if the coil was getting warm, I never touched it with a running engine.

Is there a source for old style coils with the screwed-in HT lead holder?
Willem van der Veer

NTG, Moss Europe, Moss Motors USA all sell them Willem. Just make sure when you are ordering you ask for MG TD compatible old style screw in type.

Paul Barrow

I second Paul's remarks about coils failing. A couple of years ago, my YB was misfiring and generally performing poorly. I thought maybe the coil, so I fitted a new one that I purchased from the Octagon CC. It lasted approximately 15 miles, finally giving up as I pulled into my driveway. I put the old one back on and the car started immediately and it is still going strong. The OCC replaced the faulty one with another one, but I have not had the courage to fit it; I carry it as a spare, with some doubts!

I eventually found that the problem was a hairline crack in the distributor cap. I replaced the cap with a new one and I had no more problem.

M Long


You may have simply been unlucky - it happens. I would say switch them over and give it a go. If it is to fail again quickly it will, you can then put the old back in and get going again but on the other hand if the new one holds up then no worries. A coil will either quickly fail ... or give years of service then fail.

Paul Barrow

Anybody able to post a pic of an original Lucas coil as fitted to the TD? If Paul Barrow is correct they are the same as the Y Type. Be intersted in the visual difference between it & the sports coil. Cheers
Peter TD 5801
P Hehir

Well, I thought I'd update my findings on the quest for a good running engine at higher revs.

At first I thought I'd solved it with a better coil, but this was only a result that covered up the cause. It turned out that (probably) the voltage of my system wasn't high enough.

My father said he had struggled with his MGB with the same faults and that it had been solved by the fitting of a new alternator. With this in mind I cleaned the contacts in the regulator and now it is finally running like it should do; I had an indicated 75 MPH on the speedo today!

I think the old coil couldn't get a proper spark with 10(?)V, the newer one managed a bit better.

With the engine running 2000 RPM I still only get 12V, where it should be over 13V ideally, so some extra care for the regulator is in order.
Willem van der Veer

This thread was discussed between 22/01/2014 and 29/03/2014

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