The engine in my 420G suddenly cuts out while I’m cruising on the highway at any speed between 60-75mph. It doesn’t do it at slower pace though. The tachometer flicks to 0rpm at the same time. It doesn’t tend to play up until about an hour into a journey. It’s got to be an electrical issue but I’m not sure where to start checking. Any suggestions or recommendations will be much appreciated.
Check if the coil is very hot when the engine cuts out the other thing I would check is the condenser. As you surmise with the tach dropping to zero its a loss of sparks from somewhere.
The coil is relatively new and the dizzy is also a newish 123 electronic type. The engine cuts out and almost instantaneously picks up again. I’m thinking it could be something in the ignition circuit or a dodgy auto inhibit switch or relay. I’m scratching my head over it!!
Not sure if the rev’s dropping to 0 is a pointer to a electrical problem , if it cut’s out at 60mph , the engine must still be turning right , so would think the generator would still generate current for the rev counter ?
That’s if one is fitted on the back of the cam !
What kind of HT leads do you have , I have read that copper ones can interfere with electronic ignition .
DOH! I should have known better, I was looking at the wiring diagram for the ‘S’ and the tach is a closed circuit 
Iirc the tachometer and the ignition circuit are in series on the 420g (i.e. if you disconnect the tach then the ignition gets no power).
That the fault only occurs after about an hour makes it a bother to troubleshoot. I would proceed as follows:
Hotwire the ignition circuit (fused wire with a switch, straight from the batt,) and drive for 2 hours.
If the problem still occurs, it’s your coil, or the 123 distributor.
If the problem goes away, it’s your ignition switch, neutral interlock, tachometer.
And one further thing you might consider: Assuming you have a stock Lucas distributor. The small black wire inside the distributor leading from the terminal on the side of the distributor to the points has to flex to accommodate the contact breaker plate movement for vacuum and centrifugal advance. Over time this wire can go open circuit depending on how the advance mechanism is positioned or moving. If this wire is very old it may be worth changing it. Its intermittent failure can be hard to diagnose. And be sure to run the latest type red rotor as many of the aftermarket ones with a rivet had a tendency to short out suddenly causing the engine to stop. In my experience, not a intermittent failure but a sudden complete shut down of sending current to the spark plug wires…
I agree with the previous 2 answers.
- The tacho is in series with ignition…any failure in the tacho or its wiring means NO spark
(hotwire your coil )
-
If the above does not rectify the issue, inspect your points…a nuisance on a 420G
but take them out and examine them, the plastic can wear or melt & short them…check cap & rotor too -
failed Ignition barrel wiring…you will also lose power to lights, instruments etc
-
dodgy wiring in Ignition secondary
Your symptoms are indicative of 1)…even though it would be a unusual fault
A clogged fuel filter can give symptoms similar to those described, it shows up driving under load sufficiently long to empty reservoirs and load exceeding what clogged fuel line can deliver. But this does not take an hour to manifest, just getting up to higher highway speeds or a longer hill can show this trouble. Typically it will show up as a slight stumble at first and get progressively worse as the clog grows, complete cutout for even a second can occur. Bad enough and the car cannot go up a steep hill.
Thanks for all your suggestions. I’ll try the hot wire trick and see if that narrows the problem down.
I bet dollars to donuts that the key tumbler is faulty and that the keys jiggle around at speed and kill the ignition circuit.
Sometimes one person has a heavy set of keys and the other driver uses only a single key and never experiences any problems with a car. This was mentioned YEARS ago on this forum.
Let us know what happens.
Gerard
I have 2 switches wired in my engine bay…direct from battery to;
starter, momentary, turns over motor,
the other is on/off, and wired to power the ignition circuit if on, car will start & run
I have de-weighted all my key sets.
Changing out an ignition barrel on old rare cars is to be avoided, as they can be very hard to obtain