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MG MGB Technical - Ballasted Ignition Confusion
|I have a late model MGB GT with an ignition system that has been modified by previous owners. There is a ballast resistor fitted and I am getting a voltage reading at the coil of 7 volts with ignition on. The battery is fully charged and showing 12.5 volts. This leads me to the conclusion that the ballast resistor has not been bypassed. However, the resistance of the coil is reading as 3.1 ohms, which would be normal for an ignition system without a ballast resistor. There is also a Lumenition optronic system fitted.
The car runs really well but failed the NCT (Irish MOT equivalent) on CO (just above limit) and HC (3 times the limit).
Would you expect the car to run with this configuration or have I made a mistake with the measurements?
Also, what spec coil should I use if I want to replace it.
Thanks in advance
Stephen Elster (new to the forum)
|Standard fit on later MGB's was a ballast resistor in the form of a resistor wire, together with a 6volt coil of 1.5 ohm resistance. This wire originates at the fuse box white and terminates at the coil as 2 white and green wires. The other one comes from the starter solenoid. In between is pink and white. Are you saying a separate ballast is fitted?
Check what you have got, PO's sometimes bypass the resistor wire and fit a 12 v, 3 ohm coil. So if you have a separate ballast resistor fitted who knows what combination you have!!!
Failing the emissions could be down to timing, mixture, air filters etc..
Thanks for the response. I can't see a separate ballast resistor so I assume I have the pink resistor wire. I certainly have the white at one end and the green/white wire at the coil. This is giving 7 volts at the coil but I seem to have a 12 v, 3 ohm coil.
I wanted to replace the coil because all other ht components are new but I dont know what type of coil to use.
|If you are measuring 7v at the coil +ve then that does indicate there is a ballast in circuit. However for you to see that voltage the points (or other trigger) must be closed. If they are open then you should see 12v. There are other faults that could cause the voltage to read low, so you need to remove the wires from the coil and measure the voltage on the wires, and you should then see 12v. If you don't then there is a problem in the wiring somewhere.
Running a 12v coil on a ballasted system results in a weak spark, which may be contributing to the high emissions. However that is normally dealt with at the carbs, it may be that they are set too rich. Ordinarily when making adjustments at the carbs only ever do them on both carbs by the same amount in the same direction, or you will completely mess up the balance. It may be that they need setting-up from scratch anyway. Try weakening the mixture screws (anti-clockwise) in 1/8th turn increments, and listening to the idle. Position them for the highest idle, but just as the idle starts to drop on the weakening side. You should then use the lifting pins to fine tune, but this requires practice particularly on HIFs to get the momentary rise then fall back as each piston is lifted by about 1/32".
As to your question, it depends if you want to keep the ballasted system (which does have advantages) or not.
If you do, then you need a 1.5 ohm coil. Manufacturers and suppliers get confused with coils and how they label or describe them, so the only way to be certain what you have is to measure the resistance.
If you don't then simply bypass the ballast resistance with a wire direct from the ignition supply to the coil +ve i.e. the terminal that the white/light-greens are on. These can be left on if the coil has a spare terminal, but if not and you take them off then the ends must be insulated and taped up out of the wire so they can't short to earth.
It depends if your car is 77/78 or later as to how the ignition was wired. The earlier version has two in-line fuses under the fusebox, the later three. The earlier version has the coil wired from the fusebox white/brown, the later direct from the white on the ignition relay. It doesn't really matter which you use, but can cause confusion if diagnosing another problem later on.
|Allan and Paul|
Thanks for your informative comments. I will double check my voltage and resistance readings before doing anything then I will try the extra wire to put 12 v into the coil. Job for the weekend and will post outcomes.
|Work on this was held up by a starter motor problem but that is sorted now so back to the emissions problem. After carefully checking all voltage and resistnce readings again I ascertained that I did indeed have a standard 12 v coil in a ballasted ignition setup.
I have now done the following
1. Adjusted valve clearances. They were fairly close anyway
2. Fitted the correct coil for the system I have. Measured coil resistance is 1.6 ohms.
3. Adjusted the static timing to 10 degrees BTDC. I dont have a timing light so cant do dynamic timing.
4. Set carb balance using a dial type synchrometer
5. Set the mixture using a colortune and the pin lifting test.
The car is running really well now but I dont have a way to measure emissions directly. There is a slight but persistent misfire at idle and I would like to resolve that before doing the test again.
Any ideas on what to look at next?
|Plug condition? Air filters clean?
You do really need to do a dynamic timing test, to check the operation of the vacuum advance system as much as anything. However you can do a simple go/no-go as follows:
Remove the distributor vacuum pipe from the inlet manifold and suck on it as hard as you can, and you shouldn't be able to draw any air through. Next with the distributor cap off as you suck hard and release on the pipe you should see the points plate twist clockwise as you suck and return as you release. It does take quite a bit of suck to move it.
If you can draw air through either the pipe or the vacuum capsule is punctured, which will be upsetting the mixture and the carb balance.
There can be other problems affecting carb balance, if you have the correct operation to the lifting pins, and have a dial caliper, then remove the carb piston covers and pistons and compare how far down into the jet guide each jet is. Ideally they should be the same. I've seen 0.085" for 4-cylinder HIFs recommended, but I've also seen 0.060" for HSs but mine ran really well (and correct according to the lifting-pins) at 0.045", so it's more about them being balanced than to a specific depth.
Another possible issue is different fuel levels in the float chambers, but that is much more difficult to check on the HIF than HS, and shouldn't be affecting emissions if you get the correct lifting-pin results on both carbs.
1977 and later models used vacuum advance as an emissions reduction system, by applying near maximum vacuum advance at idle, and to do this the vacuum pipe is connected to the inlet manifold instead of one of the carbs. The engines are more efficient with more advance at idle (even though too much can make starting more difficult hence the lever on the steering wheels of vintage car with START (retarded) and RUN (advanced) positions), which increases the idle speed, so the idle screws can be turned down so reducing fuel consumption and hence emissions at idle.
So for a 77 or later another go/no-go test is with the engine idling, remove the vacuum pipe from the inlet manifold and block the port, and you should find the idle speed is lower than with it connected, possibly stalling. Ideally adjust both idle screws by the same amount in the same direction so that it will idle without vacuum connected, then there should be a definite change in idle speed as you connect and disconnect. If there is no change then you probably don't have the required timing change at idle to reduce emissions.
With a timing light you should clearly see this change, and dynamic timing must be done with the vacuum disconnected and the manifold port plugged.
At the end of the day, it may be necessary to weaken the mixtures (by the same amount on each carb) for the test, and enrich them afterwards, if you find it runs less than ideally on the weaker mixture. But only enrich by enough to run well, if that ends up slightly weak on the lifting pins you should be OK.
Thanks for another comprehensive answer. I will try the vacuum advance tests as described and I guess I should splash out on a tming light.
Plugs, HT leads, distributor cap and rotor arm are all new. Air filters about 4 months old. Reconditioned carbs fitted a few weeks ago.
Something that is confusing me is that the engine number 18V582F H5803 seems to be from an older car yet the distributor and carbs are correct for my car, which is an LE model first registered in 1981. This makes it difficult to work out what specs to use for timing.
|A 582 would originally have been for 71 to 73. The distributor was quite different for a 1980 model - a 582 reached 20 degrees additional advance at 2000 rpm whereas for a 1981 it needed 5000 rpm, and it is quite possible that the earlier engine has higher emissions as it was never designed to be tested, they have differently sized valves, for example.
As far as the distributor goes without spending a long time on a rolling-road (or with programable ignition) you are not going to get better than the Haynes recommendation which is set the distributor so that when accelerating from 30 to 50 in top at full throttle just the faintest trace of pinking can be heard. The original specs were quite conservative, and many engines could run perfectly safely with more advanced timing, which does reduce emissions. What fuel do you use? 95 octane will probably result in higher emissions than 97-99 as it will also need the timing to be retarded.
|I've just realised there is a much simpler way of dealing with this - don't get the emissions tested!
In a thread elsewhere on the MOT changes in May next year the question of emissions came up in relation to V8 conversions. I remembered that this side of the water the test that is applied depends on the age of the engine or the age of the vehicle, whichever is the older, so a 71 to 73 engine in a 75 or later car only has the visual smoke test, and NI is the same.
But then I realised you may be in Ireland and not Northern Ireland, and it's a completely different picture. The manual says "EXHAUST CO/HC/LAMBDA
(This item does not apply to vehicles first registered before 1st January 1980)" so on the face of it less stringent than ours as the date here is 1st August 1975, but yours not being registered until 1981 gets caught. No mention of date of manufacture being used, like there is here for the free road tax. Was the date of manufacture prior to 1st Jan 1980? The last car was built in Oct 80, but the first 1980 model was built in July 79. 1980 models are distinguished by having an SAX prefix to the chassis number. Any use?
|PS: Ireland specifically states that tests are based on the date of the vehicle and not the engine.|
|Lots to think about!
I am in the Republic of Ireland, not Northern Ireland so the rule that applies is based on date of first reg. There is a possibility of an exemption based on year of manufacture if adequate documentation can be presented but my vin number is GVGEJ1AG522527 and I think that means the car was built in 1980 and must pass the test to be road legal.
I have checked the vaccum advance as described by Paul and all is working correctly there. I also tried the carb cleaner trick to search for leaks but couldnt find any.
I have now adjusted the timing using the just short of pinking method and rebalanced the carbs. I havent had a chance to do a test drive yet but the engine sounds smoother. There is still a misfire though and that will probably push the HC over limit.
Next step is to get a professional emissions test done and if that still shows problems I will have to hand over to a professional mechanic.
PS can only get 95 ron petrol here
|My mistake, Clausager mentions SAX but only to say it was never used on an MGB! GVG indicates June 79 to October 80 built, and 522527 is very late as the last number was 523002, which closes that route.
With that engine you may struggle, and having the correct HIF carbs (as you indicate) may just leave replacement of the engine with one of the correct age? But if you have access to pukka emissions testing equipment try weakening the mixture to see what effect that has on HC and CO. If you weaken it to the point of misfiring then the HC may well rise, but going back to your initial post, if CO is only just above but HC is three times the limit i.e. in the region of 3000, I'd say here is a definite defect there somewhere that should be capable of being fixed. My 75 V8 has to pass the same limits, and is normally around 200HC when it has passed. Usually when it has failed CO it's been about 500HC, although on one occasion it was 7.48CO and 2365HC, but weakening brought it down to 3.0 and 652. On one occasion when I overdid the weakening it passed at 0.31CO and 31HC.
If you have a misfire, then that is probably the cause. A timing light should tell you if it is missing sparks (watch for missing flashes), and if not that a Colortune may show up something. However high HC does sound more like missing sparks and hence unburnt fuel going into the exhaust. You have electronic ignition, I'd try retrofitting points and condenser and see what happens.
|I have been busy with non car things but did some work on it today. I replaced the electronic ignition with points and condenser, cleaned up the distributor, set the points gap and static timing, then had another go at the carbs using balancer and colortune. The car is running fine but the misfire at tickover is still there! I am going to get the emissions tested again but I would really like to solve the misfire.
Should i do a compression test? Test the fuel pump? Think about replacing the timing chain?
Will update re tests and grateful for all suggestions.
|As long as the pump clicks a few times when you first turn on the ignition (if it has been off for a while) and ticks occasionally thereafter the misfire is not going to be from the pump, the engine will run for a minute or more with no pump just on the fuel in the float chambers.
Do a compression test by all means, but if the misfire is irregular any problems with that are not likely to be the main cause.
Worn timing chain/gears/auto-adjuster can cause timing jitter, viewable on a timing light, but I can't see them being so worn it would cause a misfire without them being very noisy as well.
You really need to borrow/get a timing light. Not only to make sure the timing is set up correctly, but also as a diagnostic to look for missing flashes. Until you do that, or take it to a garage that can do that for you (and some may not understand when you say you just want to watch the flashes), you have no idea what is happening ignition-wise.
|It's worth a try to run the car and removing and replacing the plug leads one a time. Taking off a lead should show a marked drop in rpm. If you have a faulty plug, lead or a badly seating valve, removing that lead will show little if any drop in rpm. When you use your colourtune, put it in all cylinders and notice the colour after raving the engine and closing the throttle. A distinctly yellow flame will indicate oil being pulled down the valve stem, with can cause a misfire at idle when manifold vacuum is at it's highest. This might also show on the plugs or plug, after idling, as "sooty" deposits.|
|Does anyone else have problem editing a post? Although I do it within the time frame the edit just doesn't appear!! |
What I tried to add was that it is worth looking at the condition of the jets. Worn jets make it difficult, even impossible, to get a good idle mixture.
|" ... raving the engine ..." - love it!
I've also recommended disabling the leads one at a time, albeit putting an earthed probe into each of the cap contacts with the rubber pulled back rather than disconnection. When disconnected the HT will rise two or three times higher than when connected, which can cause the rotor or cap to break down. Earthing prevents this, and as far as I'm aware has no down-sides of its own.
Whichever, someone somewhere in the past has said that on an MGB one cylinder is quite likely to show no drop even with no specific faults, simply because of the crude technology and machining tolerances resulting in significant differences between cylinders. But for me, whilst agreeing that you are quite likely to see differences, if you find one that does nothing that would not be 'normal'.
In SJs case one would be looking to see if the intermittent misfire had been replaced by a regular misfire, or whether there was now a regular misfire in addition to the intermittent. In the first case you may well have discovered the misfiring cylinder. In the latter then not, or it could be multiple cylinders misfiring at different times.
Test edit. Worked.
|"Raving" another example of a computer knowing better!!! I suppose I should read wot I have wrote!! Good idea about earthing out Paul, IF you can actually get at the metal connector. I should have said to earth out the lead to the block, this also gives an indication of spark strength.|
|It has been a while since I last posted but I wanted to record the good news. I eventualy found a mechanic with a pucka gas tester and he got the emissions down below the limits. However he had to retard the ignition timing by about 5 degrees to bring HC within limits. The car ran terribly like this but it passed the test. I have now reset the timing using my newly acquired timing strobe and it is running really well again. I used 10 degrees btdc at 1000 rpm with vacuum advance disconnected which is the recommended setting for my distributor.
Profound thanks to everyone that made suggestions on this forum and sorry this was such a long running saga.
|Good to hear after your saga. As I mentioned earlier I have to tweak mine before the MOT, then restore it afterwards.
I hope you don't have roadside emissions testing, there was here for a while but it raised so many false positives in winter with engines not fully up to temp it 'fell by the wayside'. However a number of Local Authorities have been designated Air Quality Management Areas, which means they can be given authority to test vehicles at the roadside and issue Fixed Penalties to drivers whose vehicles fail. There is talk of it being widened, with automated drive-by checking as in some USA states.
|I haven't heard of roadside emissions testing here but anything is possible. Most checkpoints around here are looking for cars with no tax or driving on agricultural diesel. I have heard that the testing rules are going to change over the next few years such that cars over 40 years old will be exempt. However I think testing will still be possible on a voluntary basis and it is a good way to get a basic safety check carried out so I wil probably continue to get tests carried out. Thanks again for the assistance and encouragement.
This thread was discussed between 13/11/2017 and 06/02/2018
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