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MG MGB Technical - Misfire and Hesitation - answers

Dear all

I thought I would write this up to help someone in the future diagnose a misfire or hesitation problem on acceleration and especially when the car is warm. It would start OK and cruise OK but when you cruised to a halt and tried to start again then it would die or splutter.

My problem was caused by the COIL BEING WIRED THE WRONG WAY ROUND! – and it will still work. Mine is a 74 MGB Chrome and therefore should have the White/Black wire connected to the -ve terminal and the White only wire to the +ve terminal (colours vary on earlier and later models). It is a 12V coil and shows a resistance of around 3.7 Ohms across the LT terminals. Car drives just great now – like a modern car.

I reckon it has been like this since I got it back on the road 3 years ago and fitted a new coil. I have spent many hours tuning the carbs, taking the dizzy to pieces, fitting new points and condenser. I was even considering more drastic action like taking the fuel tank off to see if the pick up was not corroded, or buying new carbs or buying a new dizzy. Main thanks, as ever, go to Paul Hunt (http://www.mgb-stuff.org.uk/) and Les Bengtson(http://www.custompistols.com/cars/articles/index.htm) who, via their sites, gave me just about all of the reference material to solve this problem. Also the archives where by reading a lot of posts you can piece together some great info. It was Paul’s comment that converting positive earth to negative earth means flipping the coil connections round but even so the coil will work if wrongly connected but not as well – and boy can I vouch for that.

In my research I found many possible explanations of the cause of a misfire and maybe someone will find it useful to use this non-exhaustive checklist based on my reading of other’s problems:

COIL – check the wires are connected the right way round! – If it is getting too hot to touch comfortably then this is probably the cause. Replace it.

DISTRIBUTOR
- get a new condenser and points = cheap.
- Check the ground wire from the base plate to the distributor case – they can disintegrate inside the silk cover. Test with Ohm meter should be 0.3 Ohms. This is special wire that is designed to flex with the plate don’t use just anything.
- LT wire from plastic clip on distributor case to contact spring – same check as ground wire
- Centrifugal weights – are they free to move
- Centrifugal springs – check out Les Bengtson’s site for specifications. Turn the distributor arm anti-clockwise whilst holding the drive firm – you should feel the springs in action
- Check the cap is OK , no cracks or dirt – new ones are cheap
- Vacuum advance – vernier adjuster set to half way, suck on it (beware petrol inside) does it move the base plate – with maybe a bit of help. If you can suck air then the diaphragm is caput – get a new one. A loss of vacuum in the inlet manifold could cause misfire. See below.

PLUGS and LEADS
- New plugs are cheap. NGK seem to be best.
- Are all the plugs the same colour. Look in the back of the Haynes manual for correct plug colour or diagnosis of causes.
- Test plug leads using Les Bengtson’s advice

CARBS and MANIFOLD
- No oil in the dashpots. If you can feels reistance when pushing the piston in then there is enough oil.
- Week dashpot piston springs = pistons rise too quickly = to much air on accelaration
- Balance and Mixture need to be set (after you’ve got the ignition system right)
- Air getting into the manifold other than through the venturi could cause misfire. Check: Vacuum advance and pipe; Pipe to the Brake Servo; Diaphragm in the brake servo; inlet manifold not bolted tight enough or gasket caput.
- Throttle spindles – loose in their bushes = air getting in
- Fuel pipes not of the right grade, therefore breaking down and letting fragments of rubber into the float chamber = blockage
- Floats caput or not set right
- Jets worn or have dirt in them
- Needles not the right ones or worn
- Throttle plates not centred in the venturi
- Fuel Filter blocked (incidentally – it is ok to have an empty fuel filter under all circumstances except fuel starvation, it’s just difficult to know it is OK!)

FUEL and FUEL SUPPLY
- Changing to unleaded – received wisdom is to advance ignition 2-3 degrees to compensate for lower octane ie. instead of 11 degrees at 1000rpm then use 13/14.
- fuel pump – should deliver 1 pint per minute in free flow
- Fuel pick up: either pick up blocked by rust debris or pick-up pipe holed through rust and thus pump losing suction = fuel starvation


ELECTRICAL
- Poor battery connection
- Poor engine earth strap connection
- Poor connection of brown wire to starter solenoid
- Someone even needed to earth their headlights better to fix it.

ENGINE
- Set your tappets right – mine needed 0.014 to 0.016 thou so set such that the bigger won’t budge but the smaller fits easily.


It is amazing how many possible causes there are and I am sure there are more – please add on………………….
Richard Evans

Richard, that's a very interesting post. I don't have a problem but was very aware of the fact that a coil can be connected reverse polarity and still operates. From memory it produces about 80% capacity. I would have thought that would be sufficient to produce good running but it just goes to show how marginal the whole ignition system is and the need for proper and accurate tuning even when the polarity is correct.

Thanks for that little gem.
Iain MacKintosh

Richard
As a newcomer to the mg scene plus only a very basic knowledge of tuning ect i found this article very interesting so i have saved it to my mg files for future use thankyou very much for the time spent and detail it is better than the haynes book i use happy driving
Regards
Richard H
Richard H

Great post.
There's one thing I don't get.

You say to compensate for unleaded you advance ignition? Shouldn't it be the other way around (Retarding)? I thought that since lower octane burns faster, it would require a later spark...And then, what about pinking with unleaded?

regards
Philip
Philip

Thanks for the feedback guys.

Philp, yes I stand corrected. I just checked on the MGOC site at their Unleaded Technical Bulletin and it confirms that Unleaded burns FASTER than leaded fuel and thus you should RETARD the ignition around 3 degrees. My MGB requires a factory tune of 11 degrees BTDC at 1000rpm so to retard it 3 degrees that means it should go to 8 degrees BTDC.

I'm still learning.

Les Bengtson - you posted to this thread but it has disappeared. I've sent you an email.
Richard Evans

Richard. I had noticed that also. Les
Les Bengtson

Hi Richard,

Thank you for your thread. I just checked my '72 Chrome Bumper and it appears mine too should be W/B -ve, W +ve!

I also notice you're very close to me, I'm from Barwell, only ten minutes from Bosworth!
M J Kirkham

Michael

It must be local gremlins goin' round changing them!.

Yes. 10 minutes literally, I'm in Cadeby. Drop in if you're passing. I'll send an email with location.

I am also worried that the coil has suffered some damage from getting too hot so I might buy a new one.
Richard Evans

Does anyone know what would cause hesitation on letting off gas or just starting off, i still have a zeinith carb in my 79, runs fine and not leaking a drop otherwise. thanks,Rick
RLH Rick

This thread was discussed between 29/06/2005 and 03/07/2005

MG MGB Technical index

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