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MG MGB Technical - electrical miss

I have a terrible miss in the ignition that just developed. With a timing light attached to individual ign wires, there is sporadic light when reving up, but good light pattern when reving down. Engine dies at idle. I have pertronix Ignitor. Did the ign timing get messed up?


Tyler, Check under the distributer cap and see if there is a misalighment between the rotor and the cap. This will show up as a burned section after the terminal lug in the cap. Pertronix units often are not perfectly alighned with point mounting holes and can throw off the timing as well as burn through the cap. Keep us posted with your progess. Ray

When accellerating there are greater cylinder pressures, which makes it harder for the spark to jump the plug gap, which means the voltage rises higher. If the cap or rotor are marginal this can cause a misfire, I've had this on two cars now. If you connect the timing light to the coil lead and the flashing is regular (albeit 4 times faster than when on any one plug lead of course) when accellerating then the cap or rotor is the problem, change both.

If the flashing from the coil lead is just as erratic then the problem is in the coil or triggering. You should always get a burnt *section* of the rotor contact, not just a burnt spot, if you have vacuum advance as the effect of vacuum advance, unlike centrifugal advance, is to vary the relationship between rotor and cap, this is known as 'phasing'. After-marget triggers like the Pertronix can seriously affect phasing but in my experience the burnt section of the rotor can often reach one end even with points, implying that under extreme conditions the rotor may be moving away from the cap when the spark occurs, although this hasn't caused misfiring on my cars. Unfortunately the only way you can diagnose problems with Pertronix, and any electronic ignition, is by substitution, which is much cheaper and easier with points.
Paul Hunt 2

thanks Paul and Ray,
I installed a new cap and rotor and had the same problem. I get a good light pattern with the timing light attached to the primary wire, which eliminates a bad Pertronix (right?). But I only get light on the secondary wires when decelerating from a high rev. Tried rotating the dizzy, but that didn't help. This one's got me scratchin' my head. Any ideas?


what kind if wires are used on this setup?
May be cheap cupper core wires will help.


It's beginning to sound like 'points' phasing being way out, this can be an issue with after-market triggers. Are you sure you have the right Pertronix? Did this only happen when the Pertronix was installed or has it been working OK before? If OK before could the gizmo on the cam shaft have slipped? Or the Pertronix itself? Did you do anything else to the distributor when this problem started? If the phasing is out then under conditions of low manifold vacuum (accelerating) the rotor is too far away from the cap contact to give a reliable spark, but under high vacuum (decelerating) it is OK. Only vacuum advance changes phasing, which is why rotors are usually burnt around a significant portion of the rotor and not just at one spot. One way that might confirm this is to disconnect vacuum advance from the distributor all together, but then you might find in this case that the engine wouldn't run at all. But if it does run, and you get the same erratic triggering from the plug leads decellerating as accellerating, it would tend to confirm that phasing is the problem. Really you need to be able to apply vacuum or vacuum advance all the time and see if that gives a consistent spark, you should be able to apply some vacuum advance by sucking on the end of the pipe (removed from the manifold, block the port) with the Mk1 mouth. However that should only be for diagnosis, advanced timing during accelerating could well cause pinking.
Paul Hunt 2

Thanks for the help. I now think something slipped out of place inside the dizzy, maybe the Pertronix magnet, but it can only be installed one way. I will next try to find the rotor position that fires No.1 plug, and check to see how far off position it might be.

Is it possible the dizzy dog gear slipped? Sounds impossible.

I'm still trying to figure out how the engine runs (at high rev) and I get no timing light signal to individual cylinders.


Tyler, is your mechanical advance operating smoothly? Its possible that the advance is binding and causing intermittent issues. Its also possible that the trigger wheel fits loose on the cam and is moving around upon acceleration and deceleration. Add a bit of RTV sealant to the inside of the trigger wheel when you install it.

I've never seen phasing as an issue with the Pertronix, but I have seen the wrong kits installed. You should see the center of the module lined up squarely with the center of the trigger wheel. If its not, then you have an issue. You should also have a gap between the module and trigger wheel approximately the thickness of a credit card. You can slot the retaining holes to make it fit better.
Jeff Schlemmer

Either slipped drive gear or dog (which I agree is nigh-on impossible except with exteme wear) would cause the timing to slip but not affect the phasing. If either the Pertronix or the magnet slipped that would affect phasing as well as timing. Note that if the distributor drive dog is not fully engaged with the drive gear the two can slip in relation to each other, this can happen if the distributor is clamped up with the drive dog 180 degrees out to the gear - it can drive the distributor but is bound to slip eventually. With everything fully engaged the 'ears' on the clamping plate should be tight onto the block and there should be no gap between the flange on the distributor body and the top of the clamping plate.
Paul Hunt 2

This thread was discussed between 13/02/2007 and 26/02/2007

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