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MG MGB Technical - Condensor?

I put in a 40 Kv Lucas Sports Coil, and now my points are burning out faster. Will a bigger condensor prevent this?
John Berger

No, ALL condensors are rated @ 1/4 mu or micro farad.Check if a resistor is needed with this coil.
Many coils are wired for 12v during crank, and then thru a resistor during run position of the ignition switch.
Len Fanelli

Len-
Not quite so. Advance Distributors is marketing a higher output condenser (capacitor) (Blue Streak Part # LU-206). This condenser (capacitor) has a higher output value of .22uF, a value notably 21% higher than the .18uF output value of the Original Equipment Lucas condenser (capacitor).
Steve S.

According to Rick Astley the original condenser is .2uF, not .18, but they have 20% tolerance anyway, i.e. .16 to .24, which gives a 50% chance of being higher capacity than a .22uF anyway, not that a slightly higher capacity will change sparking much. You can do a crude go/no go test of the condenser by disconnecting it, then flicking the points open by hand while you variously reconnect and disconnect again.

If you are getting significant burning with the new coil compared with the old then unless the original condenser has failed co-incidentally it is more likely to be the wrong type of coil as Len says.

Chrome bumper MGBs used a 12v coil powered directly, rubber bumper cars and all V8s used a 6v coil powered through a ballast resistance for normal running, which is bypassed when cranking to supply a boost voltage to the coil. Original 12v coils measure about 3 ohms, original 6v coils about 1.5 ohms. 12v aftermarket 'sport' coils measure about 2.4 ohms, and 6v versions about 1.2 ohms. If your coil measures much less than 2.4 ohms between the primary terminals then it needs an external ballast resistance of about the same as what the coil does actually measure.

But if you have a chrome bumper car you aren't going to be able to bypass this ballast without changing the solenoid, or using electrickery to replicate its function.
Paul Hunt

Its a 40 Kv 12 volt Lucas Sports coil which is supposed to be compatible with my chrome bumper ignition system. It measures 2.4 ohms, just as Paul says it should. Why wouldn't a bigger capacity condensor help mt points life?
John Berger

Well, that's the point, it won't, especially only being fractionally higher capacitance than the original and with the tolerances overlapping by the huge amount they do.

So you do have the correct coil for your wiring. A 2.4 ohm coil *will* spark more than a 3 ohm, but only marginally so, and it shouldn't reduce effective points life to less than the 10k change intervals, sport coils of that resistance are very common.

Have you tested the condenser by checking the spark with it connected and without? Is is an old condenser? Coils feed back around 200v into the condenser as the points open, it's more than likely that a higher output coil will feed back a higher voltage, which may just have been enough to tip a marginal condenser over the edge. The easiest thing would be to just change it anyway and be sure, but unfortunately many currently (pun not intended) components available today are nowhere near the quality of the originals and premature failure is very common.
Paul Hunt

John,
I have one of the Lucas Sport Coils on my '57 MGA and have had no problems with burning of the points with standard Lucas components. I don't even know if this
would even cause this problem, but could the connection polarity at the coil be reversed. I think it will still run like that but can cause other problems. I could be way off base though.

Good Luck,
Ralph
Ralph

They do say to reverse the coil connections if you reverse the polarity because, apparently, plugs fire better if the current flows one way rather than the other. However wasted spark system tend to fire two plugs one way and two the other, so maybe not. Another theory says that if the plugs have been running with current flowing one way *for some time*, then reversing the current on those, i.e. old plugs, makes them harder to fire. Whatever, the difference between connecting it one way and connecting it the other is minimal and won't cause rapid points burning if connected the wrong way.
Paul Hunt

Paul - "They do say to reverse the coil connections if you reverse the polarity because, apparently, plugs fire better if the current flows one way rather than the other."

You have to go back to the days of valves (vacuum tubes for the colonists reading this). If the electrode emitting the spark is made negative, it will act as a cathode, enhancing the current flow. A coil hooked up backward will supposedly reduct the sparking efficiency some 10 - 15 percent. For older engines this could make a difference, albeit a very small one. As you stated, reversing the coil connections would not make any difference in the operation of the condenser or cause the points to burn more rapidly. I would look for an improper dwell angle or points gap as a possible reason for the points burning excessively. Cheers - Dave
David DuBois

This thread was discussed between 17/01/2009 and 21/01/2009

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