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MG MGB Technical - Intermittent dropping revs and now no starting

I have a UK home market 1972 MGB.

Converted to accuspark electronic ignition (a whole new distributor rather than just the module) by a previous owner some years ago. Standard everything else. Has been running great until now.

I've found that intermittently my revs drop whilst driving along (even when the engine is relatively cold) or whilst at idle. Also get a bit of power loss. The size of the drop varies, and sometimes at idle, it drops sufficiently low to illuminate the ignition light. No backfiring, misfiring or lumpy running when this happens. Fuel pump was replaced after the previous one died about 6 months ago and has been working fine.

video showing an example of the revs dropping whilst at idle. When I apply the throttle, the rev counter moves up smoothly.

https://youtube.com/shorts/O1PSIDtJJvM


I got home and left the car idling whilst I wobbled the wires from the distributor to the coil and alternator, associated connectors and all HT/LT leads and the vacuum advance hose to try and replicate the fault without success. Idle was rock solid with no wandering about. Ignition coil feels warm as expected after a drive on mild evening, but not excessively hot.

I then switched the ignition off to open my garage door, then try to restart the car - ignition light comes on and engine turns over, but it fails to start, so had to turn the engine over in gear to get it up the slope. It got dark so I abandoned the task.

I'm going to investigate further today, what should I be looking out for now I've got 2 issues?

1) The coil- connections to it and the coil itself?
2) Any connections on the LT side
3) Rotor arm/distributor cap - but I think if these were bad, I'd get misfiring rather than revs dropping
3) Connections to the starter motor
4) check the fuel pump (it was new 6 months ago)

thanks
Nat

Do the easy things first. Check for spark at one of the plugs and then check for fuel in the carb bowls. If the motor is turning over all you need is fuel, air and a spark. Good luck.

Jud
1972 US MGB
J K Chapin

I think I can hear what sounds like a single misfire in time with the needle dropping, and that is classic symptoms of an ignition LT fault i.e. the circuit from the ignition supply through the coil, trigger/points to the distributor and engine earth.

If the drop is long enough for the ignition warning light to illuminate that indicates the supply through the ignition switch itself is OK.

I can't see the details printed on the the face, does it read RVI-something or RVC-something? In the former case the ignition supply to the coil is routed through the tach pickup so that is a possible point for a failed connection, in the latter it's direct off the white at the fusebox with a second wire from the coil -ve back to the tach to drive it. If that latter wire fails tghe tach stop but doesn't affect the engine. Unless it earths!

It won't be fuel for the symptoms in the video as each interruption is far too brief. Also unlikely to be fuel now as when switching off a running engine the carbs remain full of fuel for an immediate restart (and even from cold hours later) ... unless the pump packed up a few minutes before you switched off and the carbs emptied just as you did so. Very unlikely. Not plugs or HT which would give a misfire without the tach drop for single misfires at any rate.

Maybe your waggling triggered a complete failure of the connection which is why you can't start at all now, but again that would be a coincidence that it didn't cut out and die before you switched off.

Electronic ignition is tricky to diagnose as there are different types that do different things at different times. Best thing is to connect an earth to the coil negative, turn on the ignition and measure the voltage on the coil +ve. It should be 12v, if not then there is a break back towards the ignition supply, see the comment on tach type.

If you have 12v on the +ve then tap the earth on and off the coil -ve, and you should get significant sparking. If not the coil (or your earth) is open-circuit. You _might_ get a spark at a plug when doing this - remove the coil lead from the distributor, connect it to a spare plug laying against an earthed surface - but without a condenser in circuit (as there would be with points) the park will be weak and may not jump the plug gap. If you happen to have a spare condenser then connect that between the coil -ve and a different earth, then tap your first earth on and off the coil -ve. That should give a much smaller spark at your tapped earth and a good spark at the plug.

If that's the case then your electronic ignition has failed. Depending on how long it has been on it could be that one of more of the wires feeding the coil has fractured internally because the 'points' plate is continually twisting back and fore as you accelerate and decelerate. For that reason factory wires are deliberately made to be very flexible and last a long time, not so sure about after-market electronic ignition, and there have been reports of failures.

If the electronic module is entirely under the cap also check the earth wire from the points plate to the distributor body is present and sound, originally that was also a very flexible cloth-covered 'tinsel' conductor wire.
paulh4

Thanks both.

Paul, my Tach is an RVC.

I've had a partially successful day.

Have found the cause of the non-starting - a dodgy rotor arm, now swapped out with a spare. Some coincidence that it decided to die whilst I was wobbling all the wires to trouble shoot something else!

Double checked fuel - all pumping ok & no water present.

Coil resistance across the terminals is fluctuating a bit (with nothing else connected to it).

But... now, I've sort of solved my rpm issue in that now my tachometer is not working at all, so there are no rpms to see a drop!

I may have dislodged something - and my wiring is not exactly 100% stock as the PO installed a toggle cut off switch which seems to connect off the negative from the coil, go through the switch up to the tach on the black wire (I think ground).

If the switch is open, the engine can start and if it's closed, the engine won't start.
(originally when my tach was working, this PO wire could be removed from the coil which would stop the tach, but still allow the engine to run- if the switch is open - if its closed, the engine won't start...)

I will have to trace everything and do a diagram.

I'm left scratching my head....

I will follow through in checking the ignition as you suggest and will report back!
Nat

The switch is almost certainly an anti-theft device, by earthing the coil -ve i.e. the trigger wire up from the coil to the tach it shorts out the points so the engine will not start as you have found :o) Incidentally originally this wire would have been white with a black tracer. However the tach and wiring changed from RVI to RVC in 72 so it's possible a later tach has been fitted hence the black wire you have, the change was at chassis number 294251.

If that connection to the tach has gone open-circuit then the engine will run but the tach not work. The tach also needs 12v and earth of course on two other wires.

Does your coil have rivetted spades or threaded studs and nuts? Originally they were rivets and over time could work loose, I had that happen causing intermittent cutting-out many years ago.

If it has studs and nuts and you get this fluctuation with croc-clips on the meter leads on clean spades then check the meter on some other component like the heater fan. If that is stable then it looks like the coil, obviously if it still fluctuates it must be the meter.

BTW can you estimate from the fluctuation what resistance you are getting? Originally it would have been a 3 ohm coil, it could be 2.5 ohms if changed to a Sport.



paulh4

Thanks Paul,

My chassis no. predates that switch over number - its an early April 1972 car, & I think you're right, the later RVC tach was fitted.

That explains my confusion in seeing so many black wires and not a white with black trace one behind the dash - I couldn't match it up with any of the advance autowire diagrams.

I've traced the wires and done the attached sketch of my setup, which I think matches what you thought about the switch being an anti-theft feature.

In poking around, I found out that the wire from the coil to the post at the back of the tach had come off (& had been obscured by the green spade connector), so that was the cause of the dead tach!

My coil has riveted spades - didn't have a chance to do much else today with my multimeter - these darker evenings are most inconvenient!



Nat

Getting there, then ... at least back to the original problem I trust!

Waggling _gently_ are the spades firmly attached or moving slightly? I will say that with mine cutting-out was both momentary (as yours seems to be) as well occasionally slightly longer, although I don't recall the engine stopping completely. But I was aware of the spades issue at the time so changed the coil straight away.

More than 20 years later on the 'new' coil with the studs and nuts I had a repeat of the symptoms, that time it was due to original spade connectors on the wiring being a bit slack on the spades. Pinched them up slightly with a pair of pliers and again no further issues.

paulh4

Ok, so there is some play in the negative set of spades on the coil - it moves slightly with some gentle waggling! +ve spades are firm.

Also, measuring the resistance across the spades, I nominally get 4.3 ohms, but then as I hold the probes against the spades, the reading occasionally moves about and can goes up to 180/200ish to infinity.

It's the coil isn't it!
Nat

I'd say yes :o)

When you get a replacement check the resistance between the spades before fitting, suppliers (and even manufacturers sometimes) get very confused between the CB types which should be 3 ohms (or 2.5 for a Sport) and the RB types which are 1.5 ohms.
paulh4

Thanks for all the help!

Coil has been ordered, lets see what happens!
Nat

Okay, so new coil obtained and checked. A steady 3 ohms exactly.

Installed, and so far, the tach needle doesn't seem to do the intermittent drop. (maybe a slight wobble)

https://youtube.com/shorts/JflpTHzwq2I

I'll have to work out where that rattling noise at idle is coming from - maybe I haven't done up all the fasteners on my battery cover properly!
Nat

When you think how old and low tech the MGB (etc.) fuel and ignition systems are compared to modern computer-controlled injection systems are it's amazing ours run as well as they do!

I'd say yours is very stable. With my early 73 model if I tried to idle it that low the engine would be trying to escape, it needs closer to 1k to idle smoothly from a physical point of view.
paulh4

I agree with Paul. I've never got a smooth idle on the 1800 engine below 900 - 1000 rpm. Why would you want it any lower? It's stressing the engine mountings and in my experience stopping at a junction with the clutch down it's likely to stall. Much kinder on the engine to let it idle freely rather than be imminently stalling.
Mike Howlett

Thanks Paul, sometimes low tech is the best way!

The idle speed is as when I got the car and hasn't really changed in the 12 months or so - it seems quite happy to idle at that speed most of the time once warmed up, though sometimes it prefers higher revs when stuck in traffic for ages.

Compared to my previous B, yes, the speed at which it idles seems a bit lower.

I get a decent oil pressure at idle though - somewhere between 40 and 50 psi.(63 ish when at higher RPMs away from idle)

Would it be worthwhile increasing the idle a touch maybe to 800?
Nat

SU HS carbs have a habit of slowly bogging down in a long idle, something the HIF doesn't seem to suffer from as it has a special passage in the throat to prevent excess fuel pooling. It's why the tuning instructions say to raise the revs for a few seconds every couple of minutes to 'clear its throat'.
paulh4

This thread was discussed between 28/09/2025 and 13/10/2025

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