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MG MGB Technical - Missing engine when warmed up1970

1970 MGB. New distributor, coil, plugs, wires and carbs. Car cranks and runs fine for about an hour at highway speed, then develops a miss. Tach bounces and the longer you, drive the worse it gets. If car sits and cools down, it runs fine again for a while and symtons return. Any ideas????
G.L. Mills

Coil or Condenser (even if they are New it doesn't mean they work) would be my starting point..
K Harris

The condensors available now are often very poor quality. I read an article recentley where some were dissected and the use of an external professional grade one was recomended. You could buy a 02uF non polarised with a rating of 100v or so from an electronics vendor, remove the one in the dizzi and wire the new one from SW to ground at the coil. This may clear the problem and work forever.
Stan Best

"The condensors available now are often very poor quality"

Thats putting it nicely, might be worth while putting a Electronic Ignition in the Car.
K Harris

Tach bouncing means there is an intermittent break in the ignition LT circuit i.e. from the ignition switch through the tach pickup, coil, points, distributor earth wire from the points plate to the distributor body. If the ignition warning light is flickering as well then it is the supply from the ignition switch that is failing.

Condenser is unlikely to be the cause unless it is intermittently going short-circuit, but they usually go open-circuit and usually once gone stay gone. Good ones *are* available (like rotors and caps which are similarly iffy these days) from ignition specialists rather than the usual MG parts suppliers.

It seems to be heat related, which points to the coil more than anything. Early coils had riveted spades and these work loose (BT, DT), later coils have studs and nuts. Once it starts happening you can diagnose further with a voltmeter on the white wire of the coil. If that is still steady at 12-14v when the misfire occurs and the tach is jumping then the path back through the tach and ignition switch is OK. Put it on the other side of the coil i.e. the wire to the distributor (which should normally be giving a steady reading of around 6v) and if that is jumping around then the coil is the problem. If that is still steady then it's the points or the distributor earth wire.
PaulH Solihull

I had a very similar problem at the weekend. After about 40 minutes driving the car ('68 Roadster) began mis-firing getting progressively worse until I had to pull over and call the AA (recovery). This had happened before with this car, on it's last long run-out. Previously when it cooled down it was fine again.

The AA man (as always a nice bloke, and previously an MG owner) diagnosed points and gave them a clean. Set off again with him in pursuit and the problem returned within a mile or so. This time he rummaged around in the back of his van and produced a new set of points and condenser. Fitted by torchlight the new parts did the trick. My drive home was another 45 minutes or so and she ran as sweet as a nut!

I didn't notice any bouncing on the rev counter, but to be honest I was more concerned with finding a safe place to pull over!
Neil_M

I meant to write that the title of this thread reminded me of the Quantas problem sheet where the pilot reported (on a 4-engined turbo-prop plane) "No. 3 engine missing". The mechanics response was "Engine found on right wing after short search".

http://www.webmaster-talk.com/general-discussions/34216-qantas-airline-pilots-versus-maintenance-engineers.html
PaulH Solihull

Stan,It goes on the CB/ Black&White wire, - for neg earth cars,+ for poss earth cars, not SW/ White wire or +for neg earth or - for poss earth cars,A.T
andy tilney

Eh? Jury-rigging an external condenser it *always* goes on the wire going to the distributor, regardless of polarity or coil.
PaulH Solihull

Some replacement points have goofy plastic that gets soft and/or sticky when hot. It causes the points to stick open or to close random/slowly when hot, due to sticking on the pivot post. Some kinds of "lubricant" will cause the same, especially if it is old. This gives all the symptoms of bad LT circuit, plus random timing. All returns to normal when it cools off.

And sometimes the plastic gets soft enough that the riveted joint between the point blade, spring, and plastic gets loose - this is a permanent fault, so after it is bad enough you can find it. One of the very few times I had to walk home from a British car was an Austin America with newish aftermarket points that did this. They looked exactly like Lucas ones but were not - I burnt my finger on the loose/high resistance rivet joint in diagnosis; fortunately I was only 1.5 miles from home, but it been acting as described for a week or so. Before it is loose enough to quit, the heat anneals the spring starting at the joint, so you get the symptoms coming in gradually.

FRM
FR Millmore

This thread was discussed between 12/10/2010 and 16/10/2010

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