MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - rough running

looking forward to some feedback on this. I had my 71B tuned by a real mechanic (not me) in April. I drive the car at least once a week. Coming home the other day, all of a sudden it starts running rough again. There is fuel coming from the tank to the carbs and filling the bowls so I'm thinking "spark" right? The plugs and wires look fine, checked, cleaned and gapped anyways. The points had a small mark on them but I did not pull them. Now, my question. If the points are burnt, what burnt them? Could it be the coil or would the car not even start if it was the coil? There is a little slop in the throttle shafts but I find it hard to believe that this would happen all of a sudden and not gradually.
Thank you so much for your input--Keith

Keith Yarbrough(TD 1275)

check and if required reset your dwell, if its out then also check reset the ign timing, make a test condenser, clip it a good earth, connect the lead to the terminal on the coil marked - or CB, the one with the White wire with a black trace on, then give it a road test


Andy Tilney

"There is fuel coming from the tank to the carbs and filling the bowls so I'm thinking "spark" right?" Not necessarily if the fuel is not being controlled by the float chamber valve and float, causing flooding and therefore rough running
Burnt points = condensor. Check the points gap or dwell if you have a meter. Could be that points have closed, which, of course affects the timing.
Allan Reeling

I owe both you gents a pint. Dwell turned out to be 90 so I pulled the dizzy and set that correctly then retimed it. Runs great again. Thank you sirs.
Keith Yarbrough(TD 1275)

How is so called dwell checked? Is it with a meter of some sort and is this altered by adjustment of the points gap. An explanation of this would be appreciated from our more knowledgeable contributors.
Trevor Harvey

Points always get marked from arcing to some extent, it's one of the reasons why they are a service replacement item, but a failed condenser will cause them to become visibly burnt and damaged. Andy's tip is a very quick and easy way of getting you back running - make sure you carry one.

Automotive multi-meters usually have dwell, and tach, functions as well as voltage etc. Dwell is another way of measuring gap, and more conveniently as far as I'm concerned as the meter only needs clipping onto 12v, earth, and points wire terminals, you don't need to remove the cap or fiddle about with feeler gauges possibly leaving oil behind. It's also much like dynamic timing being preferable to static as it shows you what is happening when the engine is running.

In fact I set new points up using dwell on the bench as it is far easier there than groping round the rack shaft. To do that all you need is an ohmmeter, connected between the points wire and the distributor body, and rotate the distributor shaft. This can by done by lightly clamping an Allen key to the drive dog and twiddling with a finger. This does show dwell in percent rather than degrees, so has to be converted, more info here http://www.mgb-stuff.org.uk/ignitiontext.htm#adjust
paulh4

Trevor, a tach/dwell meter is a very good service/diagnostic tool and well worth buying, as Paul has rightly said points are a service item,and don't last for ever, to set the points you can use a feeler gauge, but this only works with a new set of points, if the points have done a few miles, then a feeler gauge is no good as the points face will have a pip on it making it impossible to measure accurately, so this is why checking/setting with a dwell meter is better,(Dwell angle means the number of degrees the distributor cam turns during the time the points are closed) to check the dwell. connect up the meter, observing the cars polarity, so for + earth cars, red wire from the meter to earth, black wire from meter to coil terminal marked either + or CB (the black and white wire) for - negative earth cars black wire from the meter to earth and the red wire to coil terminal marked either - or CB, set the meter to dwell and the number of cylinders the engine has, you can check the dwell with the engine running, or if it wont start by cranking it over and observing the reading,(this is best done at normal cranking speed, not with low battery voltage) EG for an MGB with a 25D distributor you should have a dwell angle of 60+ or- 3 degrees, the bigger the angel the smaller the points gap, if you need to adjust the dwell, you can do this by taking off the cap removing the rotor arm , slacken the adjuster screw inserting a stubby plain screw driver in the adjuster slot,and cranking the engine over, at the same time watching the meter and making adjustments with it is still cranking, when correct tighten the screw refit the rotor arm and cap, you can check your points cap in the time it takes to open the bonnet connect up your meter and start the engine, when you are making yourself a test condenser, make yourself a remote starter button at the same time then with these 3 bits of kit, a test condenser, a remote starter button, a tach/dwell/volt meter you can diagnose 99% of the faults why your car is faulty
Andy Tilney

Correction that last line should read tach/dwell/volts, and ohms meter, not tach/dwell/volts meter,
Andy Tilney

And a test-lamp ...
paulh4

Andy, can you post some info regarding the "Test condenser" I am not very good with electrics on cars, I suppose others might/will find it usefull.
john wright

John, firstly have a look at the picture, these are very simply to make, you need a condenser, 1 or 2 small crocodile clips a length of cable, a piggyback terminal and a very small nut and bolt, length of shrink rap,
1. bolt 1 of the crocodile clips to the condenser securing lug.
2.cut the terminal of the condenser wire, solder the extension cable to the excising wire, cover joint with shrink rap.
3.on the end of the extension cable either fix a piggyback terminal, or the other small crocodile clip. that's it made. To use, clip the condenser to a good earth, then clip the cable to the coil, negative earth cars, terminal marked - or CB (the one with the black and white cable, positive earth cars coil terminal marked + or CB, you can also connect the cable to the terminal on the side of the distributor if you cant access the coil terminals, that's it job done, this is nothing clever or new, I made my first one of these nearly 50 years ago.



Andy Tilney

Thanks for that Andy. I spent most of my working life as a truck mechanic and a brake technician. I will make one ASAP.
john wright

Thanks for the info re. explanation of dwell.
I assume if electronic ign. is fitted and have no points in the dizzy such as petronix etc. This does not apply.
Trevor Harvey

Trevor, Technically the Dwell is the time when the coil is "charging"i.e., time between discharges so does apply up to a point
Allan Reeling

Trevor

Dwell does apply to most electronic ignition systems, it’s important to understand the significance of dwell or you can have problems. Coils need a finite time to “charge” their magnetic field, this is the dwell time or the time when the points or electronic equivalent closed. Current flows through the coil building up a magnetic field, when the points open the current flow stops and the magnetic field collapses, this induces a voltage in the secondary winding which is used to generate the spark.

If the dwell time is to short, which can be caused by the points gap being too wide, the magnetic field doesn’t charge to its maximum, the result is a reduced HT voltage and a weaker spark.

If the dwell time is too long, the magnetic field will reach its maximum but as the current is still flowing through the coil, the excess just generates heat which can cause the coil to overheat.

The ideal situation would be a long dwell time at high engine speeds but a short dwell time at low engine speeds, this is because the coil takes a fixed time to “charge”, typically about 3mS, and is the same irrespective of engine speed.

Points are a compromise, for a 4 cylinder B the angle is between 51 and 60 degrees depending on model and year, ideally it would be shorter and low engine speed and longer at high speeds. Electronic systems can overcome this limitation by using variable dwell, in practise this means setting the “on” time to about 5mS. Not all electronic systems do this and at least one has a fixed dwell angle of 70 degrees.

The exception to all this is CDI ignition systems. The CD stands for capacitive discharge, these systems were popular in the early days of electronic systems and are still used in high revving engines. They work by charging a capacitor to a high voltage, several hundred volts, and then dumping the charge through the coil when the spark is required. This can produce an extremely high voltage in a very short period. These are not generally suitable for our cars due to the relatively low engine speeds and the short duration of the spark.

Bob
R.A Davis

Bob and Andy,
From my calculations the dwell time on a 4 cylinder engine at 1000 rpm and 60 deg dwell angle is 20 mS (3.3 mS at 6000 rpm) and as Bob says only 3mS is required to charge the capacitor, then is the important reason in setting the dwell angle correctly is to have 60 degrees where points are open to allow the HT spark to form and fully dissipate at the spark plug electrode? What time does this need?
Where do you find the correct dwell angles for various points distributors i.e Lucas DM2, Bosch as they don't seem to be given in the tables ie
http://www.mgaguru.com/mgtech/ignition/pdf/lucas_distributor_specs.pdf
I assume all 4 cylinder distributors are all around 60 deg with only distributor cam profiles causing small changes?

Andy, for your test condenser does it matter much what spec the condenser is? MGA workshop manual say 0.2 microfarad with DM2 distributor.
Regards
Mike

Crankshaft speed 6000 rpm
Camshaft speed 3000 rpm
Distributor speed 3000 rpm
Time for 1 revolution (360 deg) 0.020 sec
20 ms
Dwell angle 60 deg
Dwell time 3.3 ms
Mike Ellsmore

Mike no time to explain today my son is getting Married this morning, and I haven't finished polishing the MG
Andy Tilney

Good luck Andy. I'll wait. Mike
Mike Ellsmore

"The ideal situation would be a long dwell time at high engine speeds but a short dwell time at low engine speeds"

Not strictly accurate as the dwell i.e. 'on' time only needs to be the same duration at any rpm. The problem with points and some electronic systems is that in order to get a sufficiently long 'on' time at peak revs, it is way too long at idle, which creates wasted heat.

So 'variable dwell' ignition systems were developed, although strictly speaking that description is confusing if not incorrect as it is the relative lengths of the 'on' time to the 'off' time that is varying.

The 'on' period has has a fixed length throughout the rev range (although it reduces slightly at peak revs so as to get a long enough 'off' period to generate a full spark) and it is the 'off' period that varies, being very long at idle and short at peak revs. This means that the heating effect in the coil is greatly reduced at anything except peak revs, and even then it is only equivalent to 'fixed dwell' systems like points and early electronic systems. That's why the factory were able to fit low-resistance coils with the later 45DM4 electronic systems - 0.8 ohms in conjunction with a ballast resistance, which significantly boosts the power of the spark. If you replace one of these coils with a so-called 'sport' coil you will actually reduce spark energy.

The Lucas Fault Diagnosis Manual states "the standard ignition system will quite adequately meet the requirements of a six-cylinder engine up to about 8000 rpm". Given that a 6-cylinder engine is generating 50% more HT sparks than a 4-cylinder at the same rpm, that means that points will meet the requirements of a 4-cylinder engine at 12,000 rpm. If you don't believe that, a V8 is generating twice as many as a 4-cylinder, and will all too easily rev into the red at 5200rpm, which would equate to 10400rpm on a 4-cylinder.
paulh4

Paul,
"The 'on' period has a fixed length throughout the rev range". I take it you don't mean a fixed "time", but a duration relative to rotational speed?
Presumably the "on and off" times can be better controlled when each cylinder has it's own coil pack?
Al
Allan Reeling

Paul

“"The ideal situation would be a long dwell time at high engine speeds but a short dwell time at low engine speeds"

Not strictly accurate as the dwell i.e. 'on' time only needs to be the same duration at any rpm”

This is completely accurate as I go on to say in the next paragraph “in practise this means setting the “on” time to about 5mS” and you also state “ 'on' time only needs to be the same duration at any rpm”

Having a fixed “on” period, generally described as variable dwell, is effectively the same as “a long dwell time at high engine speeds but a short dwell time at low engine speeds"

Can you explain which part is “Not strictly accurate”?

“confusing if not incorrect as it is the relative lengths of the 'on' time to the 'off' time that is varying”

Dwell is measured in degrees of rotation of the distributor, so if the “on” time is fixed (a fixed time) the number of degrees of rotation will vary depending on the engine (and therefore distributor) speed. It’s difficult to think of a term, unless you call it a fixed on time system, or a variable off time system, that would be more accurate and less confusing.

Bob
R.A Davis

"Technically the Dwell is the time when the coil is "charging"i.e., time between discharges "

and

"The ideal situation would be a long dwell time at high engine speeds but a short dwell time at low engine speeds,"

Can't both be correct, which is why it's confusing as I said.

'Time' to me is as in hours, minutes, seconds etc. Which is why I said it's not strictly correct as in a variable dwell system the 'on' period is relatively constant throughout the rev range, it's the 'off' time that varies being long at idle and short at peak revs.

Dwell is displayed on a meter as degrees, but is actually measured as a proportion or percentage of the 'on' time to the 'off' time. You can see this on an ohmmeter across points with no other wiring connected, and is how I set my points gap or dwell on the bench. Dwell in degrees gives very different readings for 4-cylinder, 6-cylinder and 8-cylinder engines, but the same percentage for all. Some meters can display either. A 'fixed' dwell system displays more or less the same reading through the rev range, whereas a 'variable' dwell system displays a low dwell at idle and a rising dwell through the rev range. But that rising dwell represents a relatively constant 'on' time in milli-seconds.
paulh4

Paul

I now understand your confusion, I shouldn't use the term time in connection with dwell I should use angle. The reason for this is whenever I make any measurements I use an oscilloscope which as I'm sure you’re aware display results in voltage versus time, I then calculate the dwell angle from the times.

Using this method allows me to get a accurate measurement (apart from terminology) across the entire rev range.

Having said the above, a quick search would suggest I'm not alone in using time in connection with dwell, albeit these articles are mostly about modern ignition systems

http://www.dtec.net.au/Ignition%20Coil%20Dwell%20Calibration.htm

http://www.useasydocs.com/theory/setdwell.htm

http://spdispark.com/pages/frequently-asked-questions-dwell.html

Bob
R.A Davis

Indeed it's a common misuse in those writing articles about it, who really ought to know better and use the correct description.

The first link doesn't work for me but the second raises an interesting point about the effect of inductance on how long a coil takes to be fully charged. Originally coils consisted of nothing more than many turns of wire around an iron core to achieve a nominal 3 ohm primary, which creates an inductance, one of the properties if which is that it takes time for a connected current to rise to a steady value.

When ballasted systems were brought in to boost the spark while cranking, the coil primary was reduced to 1.5 ohms, and the ballast provided the other 1.5 ohms so the overall coil current stayed the same. Not only does this reduce the heat in the coil significantly, but it also reduces the inductance, which reduces the time needed to fully charge the coil, and so could produce a satisfactory spark at higher rpms than a 12v coil could.

At some point someone had the brilliant idea of putting the ballast in the can with the coil, to be used on 12v unballasted systems. It ends up with the same heating effect as the original 12v coil, but still benefits from the reduced inductance, reduced coil charge times, and higher rpms.

Because variable dwell systems only charge the coil for a very short time at all rpms, the coil resistance can be lowered still further (0.8 ohms in the case of the MGB), which reduces inductance and coil charge times still further, as well as delivering a higher-energy spark from the increased current. It also means the coil heat is reduced still further, despite the higher current, because of the very short charging pulses.

The problem is, it creates even more confusion for users, suppliers and even manufacturers when they start talking about a 'ballasted coil'.
paulh4

Mike now our household is back to normal and my hangover has gone, I will try to answer your questions,
1. interesting about, does it matter which condenser you can use for a test condenser, to be honest I have never really thought about that, what ever car I have been working on I would just go to the stores give them the car details, and fitted what was issued, the condenser used on the midget range (except 1500cc)A 1600cc, B 1800cc, the B 3500cc, and on the C 3000cc, are all the same lucas part number with the exception of the B 18000cc fitted with a 45d distributor,so I would say for our cars a lucas GSC2111
will do them all.
2. For the dwell angle, you need to check the vehicles workshop manual, as all 4 cylinder cars are not the same, (B 1800 has 2 different setting depending on what distributor is fitted).
3.with reference to dwell and coil saturation, if you read the posts by Paul and Bob you will have more information on this subject than you will ever need, and you will be able to bore people to death in the pub about it for months (only joking) A.T
Andy Tilney

43/45D4 is different as the condenser comes with the quick connector to the points spring and the wire that leads out of the distributor body, as in the attached.

FWIW the Parts Catalogue gives GSC 101 for the 25D4 distributor and GSC 108 for the V8. I don't know what the difference is or was, one source says Intermotor 33710 is equivalent to both those, plus GSC 111, 2011 and 2111. Mind you, how much credence you can give to that is debatable, they also say it is suitable for MGBs from 62 to 81, which is patently incorrect as the distributor changed to the 45D4 with its different condenser on rubber bumper cars, so buyer beware.

Also whatever thry came out of the factory with, many will have been replaced by now, not necessarily with the identical item, so you have to go by the distributor when buying replacement condensers, points and caps, not the year of the car.



paulh4

Now I think everything, everyone needs to know about dwell and dwell angle, coil saturation, etc has been covered in this thread, I would just like to add, there is nothing wrong with points ignition and if correctly maintained it is very good, and I would not have a aftermarket bolt on electronic ignition kit on my car even if it was given for free,
Andy Tilney

Nor me - until decent quality points and condenser become impossible to get. Then I'll buy two ... and try both on the car before relying on one of them as a spare!
paulh4

Paul and Andy,
A ship sailed of the end of the flat earth, the other day. LOL
Allan Reeling

This thread was discussed between 30/07/2017 and 10/08/2017

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.