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MG MGB Technical - Running very rich on cylinders 2&3

Hi Everyone.

We recently acquired a late 1980 'B for M'Lady. It's very nice chrome bumper conversion in UK spec.

As with all new acquisitions there are one or two things to sort, I'm finding a wee bit baffled is this: It's running very rich and fouling the plugs (BP6ES) on cylinders 2&3. 1&4 are a toward the lean side, but not problematic judging by the plug colour.

Given that Cylinders 2&3 are fed by different carbs but a common exhaust port, so I'm struggling to see it being the fuelling esp., when the other cylinder on each carb is not rich at all.

Any ideas on what could be causing this?

Many thanks,

Finn
F Gheury

The inlet manifold gasket has to be a suspect - with the engine running, a trickle of liquid over the manifold where it joins the cylinder head might be revealing - it does no harm to replace the gasket anyway. Focusing then on the things which are inexpensivly sorted, I would question the quality and timing of the spark. She is an old girl and you probably don't know how she has been treated - new rotor, dizzi cap, leads and plug will assure future reliability anyway - sometimes a new dizzi is a worthwhile investment too - check the timing and the advance.

Its a practical second car and will give your partner years of fun as long as the electrics are in good condition. Good luck sorting this little hiccup.
Roger Walker

As above, but try spraying carb cleaner or propane from a gas torch (unlit!) round the ports. If the engine note changes then there is a leak.

Ordinarily I agree with changing ignition components as it could be weak spark on 2 and 3, although that would give poor running as well.

However there is a lot of rubbish out there these days especially rotors and caps, if you do replace them get them from Distributor Doctor.

paulh4

Manifold gaskets and air leaks won't make 2-3 run rich
Sounds like the start of a headgasket going to me
Compression test would be my first check and go from there----
William Revit

It would if the mixture had been enriched in 'compensation'.
paulh4

Finn,
my first thought was HGF having suffered similar with my HGF last year.

I would certainly agree with checking the free and easy to do things first and of course (as I always do recommend) doing a staggered full 36k-mile service check at least on a classic new to you.

I'm not a fan of taking the results of a compression test in isolation but added to the thread title it would be a good idea, if the readings done correctly give an unacceptable difference with 2 and three compared to the other two then that could very well be HGF.

Nigel Atkins

Gentlemen.

Thank you for your comments.

The car has only seen 3000 odd miles in the last 5 years so there are going to, and have been a few teething problems but I do think it will make a good car for her.

I'll obviously start with the easier jobs. I noticed the vacuum hose was disconnected and plugged with a self tapping screw so, I have ordered another. This of course will probably reveal a leaking advance unit on the distributor. The manifold gasket is ordered and then I'll look into the ignition components. The ignition components appear recent including leads and lucas coil, but may well be of dubious quality.

William, Could you please explain a bit more about how the head Gasket could be the issue? I'll dig out my compression tester and check it, but I'll undertake the easier jobs and some diagnosis before I embark on a head gasket replacement!

Thanks again.
F Gheury

Finn,
you probably didn't see my previous post as you were typing yours as I posted mine (this often happens to me).

I was about to add to my post - Finn, hi and welcome.

To give my completely non-technical answer to your question, gasket leaking between two and three now makes them joined in effect.
Nigel Atkins

There are many, many, piss-poor parts sold by all of the suppliers sometimes some or one supplier will sell better versions of a part so you need to shop around but always best to ask here first for experience. But do bear in mind things do change, sometimes so many complaints that even the big suppliers have to offer a better alternative but it may still not be the best you could get.

Paul Hunt's excellent mgb-stuff website is stuffed with lots of great information. - http://www.mgb-stuff.org.uk/hammertext.htm

HT leads generally are OK but the cheap Accuspark ones used to be (still are?) poor.

'Lucas' coil is made by Elta and seem to be fine, see in the Archives here for my previous post with links and info. The Archives here are a great source of info but again bear in mind things might have changed, certainly with parts.

Rotor arms have been a particular problem, hence Paul's recommendation to buy such stuff from the Distributor Doctor and for such as CB points (shudder), condenser, dissy cap and other bits. - http://www.distributordoctor.com/

My advice to you is to have a (paper printed) copy of the relevant Driver's Handbook (the one that would have come with the car when new, it's not the workshop manual) and to read it and refer to it as required as it has so much info.

As put do a staggered 36k-mile service, or at least check, in between using the car and getting to know it.

Always have the battery in good condition and well charged with all the major cables, earths and connection clean, secure and protected. You can sort and find starting and electrical issues better and faster then, plus know you can drive the car anytime.

https://www.brooklandsbooks.co.uk/product/mg-mgb-tourer-gt-drivers-handbook-2/ (I am not recommending buying from Amazon.)

Example of the contents of the good book from my personal copy for the midget.






Nigel Atkins

Had something similar a few years back. Hard cold starting, rough idle. The intake manifold nuts had loosened just enough to create a vacuum leak. Tightened the nuts about a 1/4 turn each and the problem corrected.

Hopefully this will fix your issue.

Cheers

Gary
79 MGB
Gary Hansen

Nigel,
I Can't agree more on the quality of parts. I run an S3 Landrover and some of the parts are just hilariously poor in quality.

I have the Haynes manual, but will get a copy of the linked handbook.

I'm actually very impressed with how well the car starts, even in 0deg. I wish my Gilbern would start like that!

Thanks Gary. I hope it's that simple.
F Gheury

Nigel

Your contents page has a GAN4 supplement, but the cover has a GAN5 Midget.
Dave O'Neill 2

Finn,
as your good lady is the driver she should read it and refer to it too, good bedtime reading. And you have a glovebox to keep a copy in.

As you'll know the information will relate to a new car and 40 years ago but it gives good starting points at least for any settings.

From what I've heard the MG parts might not be as bad as LR parts. You'd probably be able to get even more parts for a B than a Landie. The days of cross referencing parts are over as suppliers often sell substitute parts without saying.

If you use the MG part numbers you can often find suppliers with the same part for another marque. Midget owners often use Mini suppliers as their prices are lower for the exactly the same part or they offer alternative better parts not offered by the usual suppliers (mainly for Spridgets I'd guess though). They do many common items too, their Delphi brake fluid for example is at a very good price.

Just thought, perhaps your Gilbern has an A or B engine so you know all this.

Haynes will have errors and omissions of course, as will any other database or source of info and the good book being a 1976 published will have some varincies perhaps to a 1980 car but nothing really significant I'd have thought.

3,000 miles in 5 years would be a lot for many classics (including some Bs) but nowhere near enough to trust to for really. Safety items first of course likes brakes including tyres regardless of how much tread is on them but you'd be used to that. One thing you might not know about but will need to is the "Tom's (donkey) dick" to prevent damp legs and worse.

Be interesting to know what you find with the compression tester, Willy like many on here (excluding me) is very technically competent and experienced (I'm unfortunately very experienced with having car faults and faulty people in the motor trade).

Good luck.
Nigel Atkins

Dave,
it's not an original 'good book' from 1973 but a reprint publication which I bought 13 years ago for the car.

It's a GAN5 book with a four page supplement covering GAN4, two pages of which are GAN4 negative and positive wiring diagrams.

It covers 12V 778F, 586F, 588F and CE engines, dynamo and alternator and even hazards and wiring diagrams 69/70 to 73/74.

"Publication Part No. AKD 7596 (6th Edition) (Includes Supplement for GAN 4 Series car)".

I commend it to the house.

Nigel Atkins

Nigel, my '72 MGB leaks like a sieve in rain. I've searched for the source and come up no joy.

Please elaborate on, or give a citation to, Tom's (donkey) dick.

Thanks.

Jud


J K Chapin

Jud,
have you got a white or light-tan roof instead of black?

Perhaps this thing is called something different in your area?

It's like long term owners that don't have a copy of the 'good book' they aren't always fully aware of the car, but this isn't in the 'good book.

If you don't know I'll post a photo and details (if not already done by someone else).

P.S. Spridget hoods are very watertight. :)

Nigel Atkins

Ok Jud I've found the photos.

It goes from the bottom of the fresh air chamber to under the car, see photo from Gaydon cutaway exhibit (via Paul's site).

See second photo, it has a collector /dump valve to get rid of the drained water but the whole pipe can get block with debris which can allow the water to get inside the car in the soundproofing and carpet on the transmission tunnel which gives the driver and/or passenger a damp or wet leg.

http://www.mgb-stuff.org.uk/cutaway/images/rheater.jpg







Nigel Atkins

Finn
Back to your issue
How did you go with the compression test----
It's always a good idea to compression test first, at least then you're not wasting your time trying to tune a troubled engine
Next would be valve clearances ---basics first--

To answer Paul's comment
"It would if the mixture had been enriched in 'compensation'."
The mixture of 2-3 can't be adjusted separate to 1-4
The front carb feeds 1-2, the rear 3-4
If say, the front carb was running rich then it would be 1-2 running dark not 2-3
If the front inlet port(for example) was leaking at the gasket and lean at idle effecting 1-2 and compensated for by richening the mixture at idle on that carb to get correct idle mixture ,then the plugs on them two cylinders (1-2)would possibly show darker as the mixture has been richened throughout the whole range----Impossible to adjust 2-3 as a pair against 1-4

willy


William Revit

'Tom's Knob' (which is how I first heard it) is the heater air-box drain tube. If you look at one you should get the idea! If that gets blocked the air-box can fill up and water run into the cabin from there.

But "leaks like a sieve in rain" needs further explanation.
paulh4

I'm not going to hijack anymore of Finn's thread and I apologize for starting this diversion. My leak is entirely under the dash and I think entirely from the heater box due to a non-functioning donkey kong. I'll check it and if further discussion is needed will start a fresh thread.

Thanks for the info and thanks, Finn, for allowing me this brief tangent.

Jud
J K Chapin

Look in the intake and if there is standing water the outlet is blocked.

If not then pour some water in and it should run out under the bell-housing.

If it's blocked then careful prodding with a thin dowel should clear it, the attached should help you locate the drain.

Don't go mad or if the bottom of the air-box is corroded you can break off the spigot then water gets into a box-section and from there probably into the car.


paulh4

This thread was discussed between 06/02/2021 and 09/02/2021

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