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MG MGB Technical - misfire

Ive a 79 mgb with an aldon ignitor and the restive coild - its starts fine but gets to 2500 rmp and then misfire. New plugs/dizzy cap and rotor (from the dist dr)

any ideas - suspect its coil but no way6 of checking without buying a new one.
Richard Perry

I've a suspicion that Aldon coils are the same as the Petronix ones. I know of a few problems with the Petronix units. Apart from putting a meter across the + & - terminals to measure resistance, either 3 or 1.5 ohms, I don't know how else to check it.
Allan Reeling

What does the tachometer do? Is this still reading steady engine speed or is it fluctuating at all? If fluctuating it's probably an ignition LT problem.

How the Ignitor is connected on a ballasted system (as a 79 would originally have been) can also cause problems. Ideally instead of the red wire going to the coil +ve, as the instructions may have said, it should be taken back to the white/brown wires at the fusebox.

Once you start getting the misfire - and where it is safe to do so - I'd immediately switch off, dip the clutch and bring the car to a halt. Then turn the ignition on and listen for the pump. If it chatters a bit then the problem is fuel starvation. If you get nothing or just one click, and the car restarts and runs normally, then it probably isn't fuel.

Which leaves HT. Probably not leads as it is engine speed related, which leaves the coil. They aren't that expensive, given what Allan says just get a new one, and a standard one i.e. not another pertronix.

You will need to determine whether the ignition is as from the factory i.e. ballasted and needing a 6v coil i.e. one that measures 1.5 ohms between the spades, or whether it has been converted to unballasted so needing a 12v coil i.e. one that measures about 3 ohms. The easiest way to do that is to remove the Ignitor wires from the coil, and what should be a black/white wire on the coil -ve, just leaving what should be two white/light-green wires on the coil +ve. Connect an earth to the coil -ve, turn on the ignition, and measure the voltage on the coil +ve. If you have ballasted wiring you should see about 6v and need the lower resistance coil. If you see 12v you have unballasted wiring and need the higher resistance coil.

Any other voltage indicates a fault, which could include the incorrect coil for your wiring, you will have to measure your existing coil (wiring disconnected). If you have a 12v coil on a ballasted system you will see about 8 volts, and be getting weak sparking.


paulh4

Hi Richard,
I had a recurrent misfire to you albeit at slightly higher revs which would rectify itself when I slowed down.After trying a different coil,leads.distrib cap,checking for fuel starvation it turned out to be one excessive gap on one plug. I was convinced I had properly checked them! Worth having a second look as it is a no cost exercise!
(I have a 123 electronic distributor).
Charles
Charles9

A good point (not punny). A large gap results in a higher peak HT voltage (not good for HT components), and the wider the throttle the higher the compression in the cylinder which makes the plug harder to spark, which pushes up the HT voltage even more, and any weak points in the HT system could well break down. Over-large gaps although mentioned in various places are not always a Good Thing.
paulh4

This thread was discussed between 18/04/2018 and 19/04/2018

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