That awful colour MGB GT

[Alf892]

Driving along then stopping suddenly will not be timing for definite.
Unlikely to be fuel if cut off was totally sudden...like no on/off bits.
Condenser is good place to start as Hooli says.....have a good look at rotor arm too

[SiC]

Something else. When I got it going the second time, I accelerated hard onto the motorway. Then got up to 70mph and climbed up the Avonmouth bridge at 60-70mph, which is quite steep. Didn't feel like it was wheezing up it.

This here:
https://maps.app.goo.gl/JAJDRZYhgtzfAQJUA

Then going down the hill I was on part throttle. Felt a slight "bump" again and then accelerator did nothing. Knocked it out of gear and revs dropped to zero.

While thinking about it, I am 99% certain the revs were reading when it lost power. Didn't think much of that as driving at the time. However that does mean that the points must be switching away to some degree.

My current thoughts are the rotor arm. Cap and centre pin looked ok. Rotor arm nothing obviously a miss as much you can tell looking down there.

[Alf892]

Get out there man......get it fixed!

[SiC]

Just nipped up to fetch the battery. Turned ignition on and the pump merrily ran away to reprime the system. Couldn't see any overflow out the bottom. Gave the carbs a tap with a mini claw hammer. Didn't make any difference.

Pulled plug number 1, it wasn't wet. Hard to tell under the security lights in the car park but it did look a bit on the lean side.

Tried turning it over but it managed about 2 rotations before the solenoid started clacking. Battery is now on charge, ready to go back to the garage tomorrow morning.

I'm sort of suspecting fuelling issue. Can't imagine stuck floats, as to happen to both carbs together?? As Phil said, the B-series should be capable of running two cylinders - albeit badly. I think I've done it before by putting my hand over a carb to stop any airflow in when tuning them.

Need to make a list of things I've changed and any observations when it failed for the garage.

[panhard65]

The black rotor arms can give trouble, I had one fail on my old Jensen there was nothing obvious to be seem but putting the original back in sorted it. They reckon the red ones are the best you can get as there are so many shit ones about now.

[SiC]

This is the exact rotor arm that is on it:

https://www.moss-europe.co.uk/rotor-arm ... 101hq.html

I think its a genuine Distributor Doctor item. Certainly had the letters DD on it.

[Hooli]

Sounds like it could be a dodgy connection somewhere in the low tension side. From memory I think the rev counter comes off the coil? Then the coil earth's via the points. I know I've had random faults due to bits in dizzys earthing when they shouldn't, would that fit with what you disturbed removing the points assist box?

[SiC]

The points assistance unit has a wire straight from the points to the box, without going through the loom. The other connections then connected onto the Coil. I also have a direct connection from the fuse box unfused side to the coil positive. When I disconnected the points assistant unit, I reconnected the old wire from the coil to the distributor. 

I've looked in the points assistant unit and there is no obvious failure to it. However I've not had a chance to power it up on the bench for a proper test. Could well be that has failed and also the loom wire to the distributor is failing. IIRC it was a bit iffy now I come to think about it.

[SiC]

Freshly charged battery on it. She started right up! Fast idle possibly a bit low but she would sustain running off throttle and a chunk of choke at 800rpm. Bleeding bloody bastard thing.

Went through all what happened with the mechanic and I've left it with them to diagnose. Couldn't think of anything obvious off the top of his head. I did suggest a vacuum leak and he said he seen rough running if the manifolds aren't sealing properly after mixture adjustment. But didn't think it would cut out suddenly.

One thing I did think of was that both sections of the motorway I cut out on were steep inclines and then low/off throttle downhill. So I suggested the vacuum advance may have got stuck at full. He reckoned it'd still run, just roughly and pinking. Also he said when he did the timing check, the vacuum advance didn't look like it was working. Common thing that he almost always sees on them. I moved the plate by hand earlier, it moved fine and smoothly.

[LynehamHerc]

Something moving around in the petrol tank?

There was a case quite a while ago where the label came off the sensor in a T2 and blocked the outlet at times.
I wouldn't expect it to die suddenly though.