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MG MGB Technical - Unreliable when engine is hot

Please help ,
My usually very reliable B has started developed a fault after about 30 minutes driving. The rev counter starts flicking when idling or in traffic then the car starts mis firing and stops. I have changed the coil, condenser and rotor arm. The car has a 12v battery and a kenlowe fan, does any one know what the problem is please ?
Thanks
Andrew
A Simpson

If the tachometer is flicking when it starts to run rough, the problem likely has something to do with the low tension circuit in the ignition. The first thing I would check is the very flexible small wire that runs from the breaker plate to ground in the distributor. This wire is insulated by fabric, and is quite thin, in order to be very flexible to withstand the repeated rotation of the breaker plate plus to not impede the rotation of the plate. It may have a break in it that fails to conduct when certain temperatures are reached inside the distributor, or if the fabric insulation is worn to the point that its causing a short somewhere that could cause your problems.
You might as well also check the condition of the points, for fouling and pitting. Just because the condenser is new does not mean it works properly. The coil and cap/rotor are not likely to be involved if the tachometer becomes erratic at the same time as the engine begins to stumble.
Erick Vesterback

Thanks Eric I will look at that but sounds like you could be onto something.
A Simpson

Andrew,
Assuming you have not done anything to the car recently (either routine or repair), I guess the area to start looking for faults will be the ignition circuit - are there any variations to the makers' circuits ? are you running transistorised ignition (relatively short-lifed and always suspect) ? have you a faulty non-standard electric fan which shorts out when the thermostat operates ? If you find nothing obvious when the car is cold, you may need a trial and error approach when it is warm but stopped; will it run with the tacho disconnected ? Do you have a faulty ignition relay? Is there current to the coil? spark at the plugs ?

You don't give us a lot to go on but if your car is completely original and the wiring is in good order, the ignition relay would be my first suspect.

HTH
Roger W

Thanks Roger the ignition is original 1970, but at a recent service the kenlowe fan was reconnected to the thermostat. So it could be faulty or the common theme seems to be I need to upgrade to electronic ignition ?
A Simpson

I'd go with what Erick has put and add just because a rotor arm is new doesn't mean it can't be faulty

you don't have to upgrade to an electronic igniter head at all - but I always do but this stills leave a possibly very worn dissy below which were, so I'm told, getting out of step two or three years from leaving the factory so you can imagine how they might be decades later

personally I like a fully electronic dissy and have the 123 myself and very happy with it but it's not cheap and not necessary for your existing problem and only really relevant if the rest of your engine and ignition is in good condition a fully and properly regularly serviced, maintained and repaired

igniter heads too have delicate wires that can be chaffed inside the dissy and outside if not installed correctly, owners that do this blame the unit rather than the installer and go back to trusted CB points

for all things about tradition dissys the Distributor Doctor is a reliable source of good parts and components - Distributor Doctor - http://www.distributordoctor.com/

I thought the Kenlowe fan used its own probe slid under the rad elbow hose, I'm not into this business of having a override switch rather than trusting the thermoswitch like you have on new cars for 30+ years
Nigel Atkins

A long shot but I had a similar problem with a Ginetta G15 that used
a similar tacho. After going through everything mentioned above without solving the problem, I removed the tacho and found a loose connection.
V Todman

"or the common theme seems to be I need to upgrade to electronic ignition?"

Absolutely no need to at all. And depending on what type you fitted and what the present problem is, you could still have the problem! Diagnose and fix this problem first, then decide an anything else afterwards.

As said the tach flicking about indicates an ignition LT problem which could be a bad connection anywhere from the ignition supply from the ignition switch, through the tach pickup (on a 72 or earlier), the coil, points, and all the wiring in between. The points plate on the distributor is continually twisting back and fore as you alter the throttle position, which is bending the points wire and the points plate earth wire back and fore as well. 25D4 distributors (with the vernier adjustment) have a cloth-covered wire from the spade on the outside of the case to the points, and a similar wire from the points plate to the distributor body. These are quite likely to be original to the distributor at least if not the car. The later 45D4 distributors still have the first wire but it is part of the condenser, so changed with that. They still have the other wire.
Paul Hunt

Andrew,
Thanks for yourlast - the consensus is that you do not necessarily have to change the ignition system although you may end up looking in detail at your dizzi.
I suggest the common thread may be heat - either something electric which gets hot such as a coil or a capacitor or perhaps something which is heated up by the coolant and the kenlowe thermal switch is the first port of call.
It's tedious hunting these problems down but immensely rewarding when you sort them. A multimeter would be a good investment anyway.
R
Roger W

multimeter can be very useful if you know about them and how to operate them but many things can be sorted with a test bulb and different fitting and length of leads

as always you will need to do step by step diagnostics to save wasting time and fitting unnecessary new parts

good luck
Nigel Atkins

Hi All
I have just got in from work and read all the the very helpful responses and just wanted to say thanks for taking the time.
Andrew
A Simpson

"something which is heated up by the coolant and the kenlowe thermal switch is the first port of call."

This would have no effect on the ignition problem i.e. where the tach starts flicking around and the misfire starts.

A problem with the Kenlow switch would either result in the fan running all the time or not at all, the latter causing the temp gauge to rise significantly, which should still not cause the ignition problem.
Paul Hunt

Andrew,

You post just after the hot weather we've been having. Are you still having the same problem now its cooled down. If not your issue could have been caused by vapour lock in the fuel system. Have a read of:

http://www.mg-cars.net/mga-bbs/more-on-vapour-lock-2011062721514121762.htm

I R Dingwall

The flickering tacho points to the primary circuit, for which Paul H's list summarises the potential culprits. While other causes suggested above (rotor, vapour, etc) can produce similar symptoms in the engine, they don't also make the tacho misbehave.
If the Kenlowe is run via the ignition switch then it could be the straw that overloads a marginal ignition switch or fuse box connection.

Best way to progress would be to
1. Fit a new set of points. You haven't renewed them yet and old points wrongly gapped are the most common cause of primary circuit failure. If this makes no difference, confirm or eliminate the potential causes which are external to the distributor, and then if need be investigate inside the distributor:

2. Drive until the problem occurs
3. Check whether the fuse box terminals to which the white and brown wires are connected are unusually hot. If so this would indicate a poor connection.
4. Hot wire the coil, direct from the fuse box, then test for any difference. (Note: this will bypass the tacho so it won't work during the test)
5. If no change, reconnect the input to the coil and move the hot wire to link coil to distributor.
6. If still no difference, check the fine stranded leads inside the distributor as noted above.

Paul Walbran

"3. Check whether the fuse box terminals to which the white and brown wires are connected are unusually hot. If so this would indicate a poor connection."

The ignition doesn't go through the fusebox on any MGB, nor involve the brown wire at the fusebox. On 1973 and later cars the ignition supply for the coil comes from the ignition switch on the white wire and does go to the fusebox before daisy-chaining onto the coil, but it still doesn't use the spades as the two wires are in the same spade connector. You could pull these wires off the fusebox and it won't affect the ignition. On 72 and earlier cars with a tacho as in this case it doesn't go near the fusebox at all, being from the ignition switch to the tacho to the coil.

If it's the ignition switch or the 12v supply to the switch on the brown wire then it will be causing the ignition warning light to come on as well when the tach drops. A UK 1970 has four white wires coming off the ignition switch - one to the OD manual switch, one to the ignition warning light, one to the fuel pump and the fusebox for the green circuit, and one via the tach to the coil. So if the warning light isn't coming on when the tach drops then it could be this last wire that is making a poor connection, other wise the problem will be further towards the coil and points. You will need a test-lamp or volt-meter attached to the coil +ve to see which direction the problem lies. Hook this up so you can se it while you are driving, and the instant the problem occurs you can glance at it and see whether the fault is back towards the tach and switch or forward through the coil and distributor. the problem with stopping after the problem occurs, and doing tests and swapping connections after you have raised the bonnet, is that the fault may well clear anyway and the results of your tests or wire swapping will be inconclusive.

Although vapour lock has been talked about for many years I've never understood how it can be a problem in our cars, and in any case it wouldn't cause the tach to flicker. For a start these cars have been running in desert states for 50 years without problems like this - unless there is a contributory fault on the car in question.

Secondly the design of the system is that the pump will keep pushing fuel through the system until the float rises to stop it. If you have vapour in the system such that the float drops, which may well cause a weak mixture, then the pump will keep going pumping fresh fuel through, and the vapour out of the vent/overflow, until the float does rise and restore the fuel level. The one possible cause is if the heat shield is defective on cars with HS carbs, and it's the fuel in the jet pipe that is vapourising. But even then strictly speaking the bubbles should flow up through the float chamber or jet to lower the fuel level and open the float valve.

I've heard the fuel boiling in the float chambers of my V8 after a run on a hot day, which will empty them pretty quickly, so the pump chatters away at the next switch-on, but I've never had running or starting problems in the hottest weather, which it certainly hasn't been in the last couple of weeks. However twice in the last few weeks I've switched it off, then not been able to start it again until it's cooled down a bit, the last time just a couple of minutes with the bonnet up while I was checking the spark was enough. It was quite warm then, but not the previous time when shortly afterwards it started raining. It's possible this is something to do with ethanol as I do run basic supermarket 95 in the V8, investigations continue.
Paul Hunt

Paul someone on here told me that BP Ultimate is zero ethanol and Tesco clearly show their Momentum99 is a maximum of 5% so perhaps you could use a tank full or two of either to test your ethanol theory

I know you don't need the extra octane but it wont do any harm (will it?) plus the added package of cleaning additives in each help

personally as you probably know I think there's too much worry about ethanol for our classics and overseas posters put how they've been using much higher levels for many years without problems

yes I do realise petrols are different in different counties but I personally have had no problems that I know of from using the petrols available with various levels of ethanol and AFAIK the enforcement of 10% has yet to be introduced in the UK or England at least
Nigel Atkins

Yes, different fuels will be part of the diagnosis. If it happens again I'll try Momentum 99 (as Tesco pay me to buy their fuel) then Shell V-Power nitro. A waste, but there we are.

Until last year producers could put *up to* 5% ethanol in without labeling it as such at the pump. Now they can put up to 10% without labeling. No enforcement as such for any particular product, but larger suppliers will have to show that a certain percentage is supplied from renewable sources. Fortunately it seems that they can meet their obligations purely from the sales of bio-diesel.

Also at the end of 2013 the requirement for super unleaded to remain at or below 5% (E5 protection grade) was due to end, and hence could also rise to 10%. There was a proposal to extend this to the end of 2016, some sources indicate this did happen, but I've not seen an 'official' statement of that.
Paul Hunt

>>but I've not seen an 'official' statement of that<<
I have but whether it would be enough for a sceptic I don't know

if you really interested I'll go looking for it, whether it would satisfy you is another matter
Nigel Atkins

I am wondering if the term "vapour lock" is correct for what we are describing here. I have heard of a "vapour lock" in the heater core where air got into the system and caused the heater core to be mostly full of air (or vapour?) except for a small amount in the bottom through which the coolant would flow thereby not providing much warmth to the interior of the car. This supposedly happened in a Chrysler in the early nineties that made for a pretty cold trip home from university at Christmas time.

The way I think of it, if the fuel in the float bowls begins to boil, the fuel pump would push fresh fuel into the float chamber, in turn pushing the fuel vapour through the needle valve and into the carb throats, which would weaken the mixture significantly once the vapour mixes with air. So, I can see how this would cause a car to stumble or even stall. However, I don't see it as vapour being locked anywhere...

Also, the tach needle jumping around suggests something to do with the low tension circuit. Andrew, are you certain that the tach needle is jumping around when this happens?
Erick Vesterback

"whether it would satisfy you is another matter"

A document on a government web site would. AA/RAC perhaps, progressively less after that.
Paul Hunt

they might not document things that are not to be applied, did they ever mention when this austerity rate of 20% VAT would go back to 17.5%? (remember 25% VAT on luxury goods?)

IIRC it was a junior? minister answering a question on a TV program

I'd really have to stretch my memory of where to find it, obviously it is now historical but other than that it'd be interesting to see a cynic pull it apart
Nigel Atkins

sorry try as I might I can't find it, I know it's there because I copied it into a post I put up with a hyperlink to the vid from which I put printed text in my post of the relevant quote

most sites have 2013 as being in the future so obviously not updated

I did find on the AA site -
"... the standard additionally requires that any fuel sold to the new specification must be clearly labelled on the pump as ‘unleaded petrol 95 E10" - http://www.theaa.com/motoring_advice/news/biofuels.html

so perhaps I could turn the tables and ask you to tell me where E10 is currently being sold to the general public in the UK?
Nigel Atkins

My question was whether the requirement for petrol companies supplying 97 and higher octane to keep ethanol levels to 5% or less, i.e. E5 protection grade has been extended from 2013 to 2016, not whether E10 is on sale in the UK or not.

But the real question is, what is E10? My understanding - and stated in that AA article - is that it is fuel containing 10% ethanol, and does have to be labelled as such at the punmp. How about fuel containing 9% or less? Prior to 2013 any fuel containing 5% ethanol had to be clearly indicated as such at the pumps. Now the limit is 10%, the implication being that anything between 5% and 9% doesn't have to be labelled. Or does it?
Paul Hunt

well as you're paid to use Tesco petrol (how'd that come about) why not just look at the pumps next time you fill up

I don't know but Tesco are still claiming maximum of 5% on their website, if you think they are wrong or lying then it must be challenged as it shows it's not just the Government/EU/DVLA that have it in for us - http://www.tescopfs.com/our-fuel/specifications

I'll try to remember to look next time I fill up at Tescos but just mastering paying at the pump takes all my mental effort

I'm now trying just running on 95 and perhaps putting in a fuel system cleaner every 4k-miles to see how that compares to running on 99 cleaners
Nigel Atkins

http://www.tesco.com/fuelsave/
Paul Hunt

ah, yes thanks, I didn't know I'd got to use up April's by the end of May

my wife forgot to tell me about this Fuel Saver so until she was with me at the pump I didn't use them in case they had to build up over time
Nigel Atkins

Try putting your old condensor back into the dizzy and see what happens. Just because it's a new condensor doessn't mean it's good. BTDT

Cheers

Gary

79 MGB
gary hansen

Someone changed a condenser before it was faulty? Foolish boy! New ones from the usual parts houses are crap, get them from one of the distributor specialists, like rotors.
Paul Hunt

following on from Paul's post

if you're really sure you need a new condensor (they seem to last decades) then get a new one from a reliable source like the Distributor Doctor - http://www.distributordoctor.com/
Nigel Atkins

This thread was discussed between 21/05/2014 and 02/06/2014

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