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MG MGB Technical - Tachometer Swings

Hi! I've checked the archives and haven't found any exactly similiar Tach problem.. Have 1971 MGB with electronic ignition (Petronix),

1, When I turn the ignition on, the tach hits 7000 and then falls back to proper setting, runs fine until engine heats up to normal temperature and has been driven about 1 hr, then the tach begins to climb up and eventually pins. It then begins to back off to normal.
2,This past weekend, car began to miss and hesitate when accelerating. When the car missed, the Tach would immediately pin to 7000 then fall back to normal.
3, I changed points, wires, plugs and distributor cap. Ground strap looks tight and clean. Noticed that wire inside the distributor looked frayed and I insulated it with electrical tape. Test drove car, felt better than it has in a while... nice acceleration & seemed to relieve hesitation.BUT when I turn engine on, still pins to 7000.
4, Oh, additionally, when starting the engine up black gasy soot comes out of exhaust which clears up when driven. (Car is show car and is only driven a couple times a month for couple of hours.)

Are these symptoms related?

What can I do to resolve these problems?

What's up?

Thanks! Bart
BartTS

Oh, spark plugs came out with black dry soot on them.
BartTS

Reading through your posting I have a few comments to make which whilst does not clear up your problem but I hope will go someway towards answering your queries.

I see that you state that you have electronic ignition, however later in your first posting, you said that you have changed the points. I maybe wrong because I don’t know the particular make of ignition, but I thought all electronic ignitions did away with points.
There are a few things that you have not checked; 1 is the timing of the engine; 2 is the set up of the carbs; 3 have a look at the air filters too. All of these items would also affect the engine and lay the ‘black soot’ you have described on the spark plugs.

Moving onto the rev counter swinging round I would suggest that you replace the wire with the damaged covering. It is the only way to ensure it is working properly. That may stop your rev counter playing up but in case it is not, I will ask this question:
All rev counters, which measure the ignition pulse swing when you first start the car up. How long does the needle pin itself to 7000rpm?

Cecil Kimber

Cecil,

My mistake, when describing what I did, I, out of habit, said "points and plugs"... too many years of non-electronic ignitions! Indeed, I do not have points, so did not change them. The wire in question appears to be soldered in the distributor, has a cloth covering and is a flat braided wire. I do not see a listing for this piece. Suggestion?

Timing and Carbs appear to be set up fine. Had them done by the local "British Wizard"... haven't replaced Air Filters in a while. Will do that today.

The rev counter seems to pin to 7000 only as long as the starter is running. When running the vehicle, and it pins to 7000, it stays there for 20 minutes or so.

Thank you.
BartTS

Bart..
Ok.. Now I am getting into territory that I do not know a lot about! So please be gentle with me!

When the needle sticks have you tapped the glass? If so does it come off the 7000 mark?. I had a similar problem once and done this. In the end the problem went away! Normally tachometers are reliable and you may need to talk to someone who knows more about them. Don’t forget to replace that wire.

You say that the timing ‘appears’ fine, but cars can go out of tune and if you have not had it re-checked I would strongly recommend that you do. Don’t dismiss anything until it has been rechecked is a rule I exercise quite often.
Cecil Kimber

pertronix isn't really electronic ignition, just an solid-state replacement for the points. The braided wire gives a good electrical connection between the points plate and the distributor body, the two of which are moved in relation to each other by vacuum advance. Without this ground wire you can get a missfire as the points plate moves i.e. when opening the throttle.

Disconnect the Pertronix from the ignition coil (two wires?) and ground the coil -ve. If the tach still pegs when you turn on the ignition the problem is in the tach. If not, and it is still present when you reconnect the Pertronix, then that is to blame.
Paul Hunt

This thread was discussed on 31/03/2002

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