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MG MGB Technical - Rough running - condenser ?

My 73 BGT was running well when I put it away last night, but this morning it sounded "unhappy" when I started it, although it started just as easily as usual. Almost as though it was running on three cylinders, but not exactly like that. As I pulled away the engine began rattling horribly under load - like really bad pinking. I checked for loose plug leads, pulled the plugs and checked the gaps and all was fine. So I've left it at home today and will check the whole HT system at the weekend. I'm wondering what could have caused this to happen out of the blue, and I'm wondering if it could be a condenser failure - does anyone know from experience what the symptoms of that are ?
Miles Banister

Don't know what your weather is like but the right combination of temp and humidity can result in condensation inside the dizzy cap. Condenser failure may have several modes, an open condenser will cause excessive burning of the points, it will also tend to cause a miss as engine rpm is increased while the idle may be fine. An intermittant short will cause all kinds of miss fire. Several years back our A would act like you turned the key off for a second at highway speed and then return to normal, drove me nuts, ended up being the condensor.
John H

Miles, that sounds like the problems I've run into a lot of recently with the Lucas condensers. Runs and idles fine until you put a load on it, then each cylinder acts like it has about 20% power after that. An even, consistent misifre! Points with the gap closed up will operate very similar, so it may just need a point adjustment.
Jeff Schlemmer

John, I know just what you mean about condensation, a Renault 12 I had nearly 30 years ago suffered so badly that I was almost glad when some clown totalled it.

Jeff, How interesting - last autumn I had a tidy up in the garage and came across an unused Lucas condensor and points set. Since I'd been running on the same repro parts for a couple of years I thought I'd give the B a treat and fit the genuine article. Perhaps I should have left it alone :-) I have a new repro on the shelf so I'll give it go.

Thanks.
Miles Banister

There could be another cause for it.. If you ever get anti freeze on your dist cap you might as well scrap it, The antifreeze is absorbed into the cap and then the cap attracts moisture overnight. Starts and runs rough until it warms up and dries the moisture. same thing is repeated again and again untill the spark burns a permanent track.
I had a 74 BGT that did exactly this.
Sandy
Sandy Sanders

Condenser failure usually results in retarded timing, so less likely to pink. I'd also expect cold starting to be affected more than running, but then I've never had one fail in 40 years. Doesn't seem to be condensation weather in the UK right now, more like cold dry Easterlies even though we are getting April showers. You can get an indication of condenser performance from two sources - one is to remove the distributor cap and flick the points open by hand (ignition on of course). When the condenser is bad you gets lots of arcing at the points, although you have to know what a good condenser looks like to be able to recognise it. The other is to disconnect the coil lead from the car and connect it to a plug laying on the block while you flick the points. A good condenser should give a blu spark with a sharp 'crack', a failed condenser a yellow spark with more of a 'fizz'. But if you do this while gradually pulling the HT lead out of the coil a good condenser should cause the spark to jump at least 1/4" and as much as 1/2", whereas a filed condenser will bareky jump a plug gap. Don't lift the plug off the block while doing this last test or you will soon find the HT will rather go via you than the air-gap.
Paul Hunt 2

That's good advice (as always), thanks Paul. However, I think I've ruled out the condenser now. This morning I pulled the distributor and checked that the advance springs were intact, then fitted new points and condenser - no improvement. I checked all the HT leads and found one that shorted if it was disconnected so I replaced that - no improvement. Finally it occurred to me that although removing HT leads in turn made no difference at idle it might do so at higher speeds. When I tried that I found that removing #3 actually gave an improvement! So out came the compression tester and when I checked #3 and #4 there was hardly any reading and a ghastly wheezing sound. So I think my problem could be a blown a head gasket. That might also fit in with something else that puzzled me. When tightening the distributor clamp (with the engine running) after resetting the timing I removed the dipstick to improve access and I noticed splashes of oil coming from its tube. I don't recall ever seeing that before, but possibly the leak is also causing pressure in the crankcase. Oh well, I suppose it's time to remove the head and take a look.
Miles Banister

Hot news from the workshop - the head is off, and the gasket has blown between #3 and #4. So in theory it's now just a matter of cleaning up and reassembling with a new gasket - but I'm puzzled as to why it blew and without any hint beforehand.
Miles Banister

Miles, One possibility is that your head studs are now 25 years old and have stretch and contracted an unknown number of times. I would recommed that they be replaced while the head is off. Also check the head and block surface to be certain that they are not warped. Ray
RAY

This thread was discussed between 17/04/2008 and 19/04/2008

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