MGB Register Forum

Hi to all,
I own a 72 bgt which has developed an irregular misfire which gets worse at higher revs. What makes it particularly interesting is that by removing the vacuum advance pipe and plugging at both ends will remove the misfire completely. But I don’t want to run without the vacuum advance, and obviously something isn’t right.
The car is pretty much standard 1800, twin SU’s etc though I have an electronic module in
Place of points with absolutely no problems. I’ve checked the vacuum advance which appears to function perfectly, rotating the distributor base plate. I’ve tried
adjusting the timing but the misfire persists. Carburettors are pretty well set up.
So runs fine with no vacuum, but misfires with, even though everything is doing what it should.. or so it appears !?
The distributor electrodes show some possible arcing though this could be where I’ve been trying timing adjustments.
I can’t seem to solve this one, can anybody help

Thanks
Andy

My approach would be to substitute all of the electronic gubbins with good old fashioned points/ condenser etc., connect up the vacuum advance and see how it runs.
Original equipment in good condition is so much easier to deal with and quite adequate for most folk.

Ian F

With the vacuum advance disconnected then the base plate isn't moving so I reckon the fault is related. It could be pulling a wire which then makes intermittent contact.
This nearly happened to me. When I removed the magic eye from the distributor I found that the wires inside the grommet in the distributor body had lost their insulation. This was because when I clipped the wires in the distributor I hadn't allowed enough free play to allow for the movement of the base plate when the vacuum advance was operating.
I suggest you check the LT wires in the distributor. If they are frayed and the ignition system is Lumenition give them a call and they'll supply a replacement magic eye.

Vic - just a quick question - don't the weights move the base plate as the distributor speed increases?

Ian F

Yes, it does but if disconnecting the vacuum advance solves the problem then it has to be linked to the base plate moving. It's still worth checking the LT wires in the distributor. I don't know but the vacuum advance could move the base plate more than the centrifugal. It's certainly worth checking the wiring first as it's the easiest option.
I wonder if the engine was run at high enough revs to trigger the centrifugal advance when the vacuum advance was disconnected.

I wondered about going back to points. I think it’s got to be worth a try so I’ll get into it next week. I’ve had another look at the LT wires and all appears fine.
My understanding is that the Centrifugal advance kicks in with more revs and the vacuum drops out as the throttle is opened. This all appears to be ok.
Changing to points should hopefully give me some answers...thanks

The vacuum advance only operates on a light cruising throttle whereas the centrifugal kicks in at higher revs. However rubber bumper B's have the vacuum advance take off on the inlet manifold which gives advance on tickover where it's certainly not needed.

Another thought. Is there an earth wire in the distributor? Not all electronic ignition systems need one but it's worth checking if there is one.

Vic Butler wrote:
> Yes, it does but if disconnecting the vacuum advance solves the problem
> then it has to be linked to the base plate moving. It's still worth
> checking the LT wires in the distributor. I don't know but the vacuum
> advance could move the base plate more than the centrifugal. It's certainly
> worth checking the wiring first as it's the easiest option.
> I wonder if the engine was run at high enough revs to trigger the
> centrifugal advance when the vacuum advance was disconnected.

Thanks Vic

Ian F

Andy,
On a different tack,have you checked all the plug gaps are correct? An oversized gap on one plug will cause a missfire intermittently at high revs/speed.. I know because it happened to me last year. I was sure i had checked & double checked them,& swopped all the leadx,tried a new distributor cap & rotor arm , but still the missfire occurred. ( I have a 123 distributor but otherwise the car is as came out the factory engine wise ). A neighbour checked the plug gaps & found one to be excessive. Once corrected, the engine has not missfired since!
Cheers, Charles