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MG MGB Technical - Misfire around 70mph

Can anyone assist with a random misfire (and backfire) that occurs when sustained running at 70mph. Once it starts the only option is to back off to around 60 - 65 mph where upon it is immediately fine. The sense it is going to start is, ironically, after period of really healthy running but is usually accompanied by a premonition/imperceptible change but is never a total surprise.
M Coleman

Fuel starvation, perhaps?

What sort of fuel pump do you have?
Dave O'Neill 2

Electronic I believe. Previous owner component!
M Coleman

Backfiring if you mean in the exhaust usually indicates missing sparks.

What's the tach (assuming it has one and not a rev counter) doing? An ignition LT issue usually shows up as fluctuation on that.

Points or electronic ignition?

If points condensers rarely go erratic but you can eliminate it by connecting a second one from the coil CB or -ve terminal (i.e. the terminal that goes to the points, not the one supplying 12v to the coil) to earth.

Could be HT, with a wider throttle the cylinder pressures increase which makes it harder for the spark to jump the plug gap. If there are any marginal components in the HT that can be enough to break it down, and backing-off 'clears' it.

Could also be fuel starvation as said although less likely with the backfiring. Assuming it is an SU-type pump you can eliminate the pump and supply to the carbs when you are in a quiet and safe area. Do what ever is necessary to get it misfiring, and while it is doing so switch off the ignition while the throttle is still open. Bring the car to a halt and listen for the fuel pump when you turn the ignition back on. If it chatters away then for whatever reason the pump wasn't keeping up with the demand and the float chambers were emptying, which could be a restriction anywhere from inside the tank through to the float valves. If the pump does nothing or maybe makes one click, and the car drives off normally, then the carb float chambers were full of fuel so not fuel starvation unless it is needles/jets onwards.
paulh4

Runs on a lumentron 123 ignition with new coil and leads. No vacuum advance option if that helps
M Coleman

OK, no condenser but the rest still stands.

I take it the original carb or manifold vacuum port is plugged?
paulh4

Yes, manifold plugged. The original points distributor was non vacuum so replaced like for like.
M Coleman

This thread was discussed on 18/10/2020

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