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MG MGB Technical - Rough running

This is a long story, but please stick with it. Prior to a rally that took us to Italy and back I fitted new BP7ES spark plugs, rotor arm, points and condenser to my '73 MGB and set the timing up at 12 deg. before TDC at 1,000 revs., and the dwell at 50 degrees, both as recommended a few years ago by Peter Baldwin on his rolling road. The car ran very well for 4 or 5 days with no signs of a problem. During the last couple of days of the rally the driver did notice some 'rough' running at mid range, 2-3,000 rpm, but we put it down to over a 1,000 miles of hard driving over mountain passes including the Gavia, the Stelvio and the Umbrail.

On the way back on the toll roads the issue became more frequent and was particularly noticeable when climbing gradients at around 3,000 revs in overdrive 4th gear when the engine would begin to loose power and the rev counter would swing wildly between 3 and 4,000 rpm with out any change in road speed. Dropping out of overdrive would immediately restore normal running, the car would accelerate and would continue to pull well if overdrive was re-engaged. As the car has spare coil and external condensers mounted ready for use I systematically changed first the condenser and then the coil, but to no avail. We managed to get the car home, a non-stop 640 mile trip in 14 hours by which time when running round the M25 the car was clearly sick.

During the subsequent investigation I again changed the points, rotor arm, plugs and also the distributor cap and plug leads. The problem could still be induced when accelerating hard. At a friends suggestion I replaced the wire to the the coil positive terminal and the problem appeared to be cured.

We then set off for a rally in Devon, but around 30 miles from home and on the M4 the hesitant running re-appeared. We decide to continue the journey as the car was drivable and our previous experience suggested that we would be able to get there and back. All was well until the second day of the rally when the problem occurred more and more, but not catastrophically. We then had a nightmare 5 hour journey home on the M5 & M4 on a Sunday evening and again by the time we arrived home the car was running very roughly. It was a 610 mile round trip over 3 days, including 2 days of competitive rallying.

Now home I have removed the Aldon BR101, 45D4 based distributor and checked that the vacuum advance is working OK using the suck and tongue technique. Using a dial gauge I have also measured that all the lobes are opening the points the same amount, 0.013 inches, which gives the desired 50 degrees dwell for this distributor.

The question is, what next please?
Graham Gilmore

Graham,
It would help to know which version you have so we can look at a wiring diagram with you - taking the fluctuations of the tacho as the first clue - is the problem heat related ? Does it occur when the fan is running ? What work has been done recently ?
Roger Walker

Roger,

Not quite sure what you mean by version. It is a 1973 1800cc Roadster. Other than routine servicing of what is a fairly modified road rally car, 120 BHP, it has not had any modifications for sometime. It may be heat related, but the car never runs overly hot as it has both a mechanical fan and a Kenlowe. Not sure that there is much more that I can add I am afraid. It has run extremely well for sometime, certainly there were no problems that I can recall last year.
Graham Gilmore

Graham. I would check out the grounding of the distributor. It would be good to know when you have full mechanical advance. From your description, "dropping out of overdrive would cure the problem", might indicate that the mechanical advance moved (due to higher engine rpms in direct gearing). If there is some form of grounding fault, the swing of the tach and the resumption of normal running due to movement of the points plate would seem logical.

Les
Les Bengtson

Thanks Les, but I am not sure that I understand your 'grounding of the distributor' comment. Are you referring to the the connection between the points and the coil -ve terminal or the wire that connects the plate that the points are secured to and the base plate of the distributor? The distributor is grounding through the engine block.
Graham Gilmore

The fluctuating tacho does suggest an electrical issue. Picking up Les’s comments about the grounding of the distributor I assume he’s talking about the flexible cable that links the movable points plate to the distributor body, this gets flexed every time the vacuum advance operates and can fracture. The wire is a specific type of cable and should be very flexible but could still fracture over a period of time. You may think that as you have an electronic module in place of the points this cable isn't required, this isn't the case, it provides the ground for the module (or at least it does in all the electronic modules I've used which doesn't include an Aldon unit).

I'm not saying this is the cause of your issue but I think it’s worth checking. If you do find this cable‘s broken and require some replacement cable let me know, I've got a reel a trafficator repair wire which is identical.

Another possibility is that the electronic module is starting to fail. Aldon claim their units are good up to 300 Deg F, this assumes all the internal components are good, if one of them is starting to fail this could be considerably lower.

Bob
R.A Davis

Bob, I don't have electronic ignition, it is still on points which I buy from Distributor Doctor. I will closely examine that cable though. Thanks.
Graham Gilmore

Graham

That’s what comes from replying without re-reading the original entry. It didn’t help when I did a search on a BR101 distributor and couldn’t find a match, it did however take me to Aldon’s website and the nearest match to the number seemed to suggest it might be fitted with one of their electronic modules.

On a related point I’ve been considering an Aldon Amethyst, I’ve tried a number of different electronic units including my own design all of which work (until they fail). I like the fit and forget design of the electronic units but I do carry a set of points for insurance.

Bob
R.A Davis

Graham, The Tach fluctuating does indicate an ignition problem. The tacho gets it's "pulses" from the coil negative. What coil have you got, 6v ballasted or 12v? It's possible you have a coil issue.
Allan Reeling

A 73 would originally have had a non-ballasted ignition system i.e. a 12v coil. I've heard of people converting ballasted (RB only) to non-ballasted, but not the other way round. However 1973 was the first year the tach was fed with pulses from the coil negative instead of being in series with the ignition switch and coil SW or +ve, but that doesn't change how the tach reacts to problems in the ignition circuit.

The erratic tach does indicate an ignition LT problem as has been mentioned before. Any intermittent break in that circuit, i.e. from the brown through the ignition switch, white, coil, white/black, points, and points plate earthing wire to the distributor body can cause it. I'm not familiar with the internals of the Aldon (101BR2 incidentally, not BR101) but 45D4 distributors have a wire coming off the condenser and points that passes through the distributor body to join to the white/black. Like the points plate earth wire that wire can also fracture internally due to the continual twisting back and fore of the points plate every time you move the throttle. On my 73 it was caused by the rivetted spades on the original coil working loose - a known issue, later coils have threaded studs and nuts.

An intermittently shorting condenser can also cause it, although usually once they go they stay gone. Other shorts on the ignition LT circuit will also cause it, and the switching out of OD restoring things to normal is perhaps another clue. If anything in the OD circuit shorts to earth it will pull the ignition supply voltage down. It's not fused from the factory, and whilst a full short would normally be expected to burn wiring, a partial short may not, but still affect the ignition supply voltage. OTOH that may be a red-herring, as the rev change switching OD out may well be enough to 'cure' it. It's intermittent, which is why switching OD back in again may not make it happen again immediately.

My advice is to drive it with a voltmeter in the cabin where you can see it, connected to the white at the OD manual switch. If that still shows 12-14v when the problem next occurs then you can discount the OD wiring at least. After that I would do the same with a dwell meter - which can be connected to the white/black on the back of the tach instead of wiring it through to the coil -ve, and then an ammeter in series with the white to the coil. In all cases what you are looking for is a change in reading from what you have previously noted as 'normal', as the misfire comes on.

If none of those show anything, then it's going to be HT or fuel, or possibly condenser.
Paul Hunt

Thank you for your ideas Allan & Paul. The car does not have a ballast resistor and I have switched coils, I have two mounted in the engine bay along with two condensers, but with no improvement. I am now in the process of replacing various parts of the ignition wiring, which does include a FIA spec Kill switch which may need bypassing. The problem is the testing, as there are no fixed conditions which induce the problem. I let you know what I find out.
Graham Gilmore

Building on Paul's last post, I experienced a similar problem with my '67 B some years ago. It turned out to be the OD power wire rubbing on the crossmenber causing an occasional short circuit. RAY
rjm RAY

I replaced the black cloth wire inside the distributor and mounted a new condenser inside the distributor. Took the car out door a 15 mile test run, driving it as hard as local conditions allowed which included some dual carriageway. Was unable to induce the problem, but it is too early to be conclusive. One thing that I did notice is that the tacho seemed to be much steadier all the way through the range and certainly at constant running speed.
Graham Gilmore

A further update is that we took the car on a 150 mile round trip on Sunday and it behaved perfectly, no sign of the previous misfire/hesitation. I am keeping my fingers crossed that the short black cloth wire inside the distributor was the culprit. As it was electrically sound my conclusion is that somehow its position was hindering the movement of the points plate when the vacuum advance/retard was trying to operate.
Graham Gilmore

Good to hear it seems to be sorted, but I doubt that was the cause.

As one of the symptoms was a fluctuating tach that points to electrical interference with the ignition LT circuit - either a break in it or a short to earth, rather than the points plate simply being prevented from moving. The only thing that is likely to do is dampen part-throttle acceleration a little, or worsen cruising MPG.
Paul Hunt

I appreciate your point Paul. As I also changed to an internal condenser at the same time, unfortunate I know to do two things at once, it could have been the previous lead that goes from the pillar out of the dizzy to the harness that was at fault. That had not been changed for some time as I had external condensers mounted by the coil.
Graham Gilmore

This thread was discussed between 29/09/2015 and 07/10/2015

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