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MG MGB Technical - Newbie questions

Hi All,

Im new to the board and MG ownership.
Recently bought a 78 GT, with good body, webasto, overdrive and leather seats. Managed to get it cheap, but it was obvious that the car had not had the attention it needed over the last few years. It was running OK ish but not 100% , with a flat spot around 2000 to 3000rpm. So I have started on the basics and am in the process of:

Renewing ignition components (Already has electronic ignition)

Stripped, cleaned and reset carbs. Settings were some way off.

New thermostat (Taking 15 minutes to get to temp)

General service items such as air filters etc.

New rocker and side gaskets (Leaks )

But Ive hit a couple of puzzlers, that I hope someone can clarify for me. I cant find specific info on this anywhere, but forgive me if its already been asked.

1 The car has a S/S exhaust, manifold back. This is straight through with a single back box only. HIF4 needles fitted are ADD. I have struggled to identify the originals for this but think they were AAU. Can anyone confirm the original sizes and what is recommended with this exhaust ?

The Minty needle comparison program, shows the ADD are a fraction richer across most of the range, but are the same in the 2000 to 3000 rpm range. Maybe thats my problem ? Plugs look a good colour by the way.

2 Called into Moss Manchester to get hold of the side cover gaskets. Ive read that Silicone for the rear and cork up front is popular. The rear cover I removed, has a D profile gasket silicone gasket and this is about thick and fits well, into the corresponding groove in the cover.

But the gasket held by Moss is a flat profile about 1/8 thick. While both the flat and D profile are the same outline, the flat one would not sit right, in the cover I have. When I inquired about the D profile, no one there had ever heard of it or seen such a thing!

The engine fitted is a recon. About 9 years old and with about 9000 miles on it. Im wondering if its an 1800 off another Leyland model? (Intend to try and check numbers tonight) Or do I have some modified / improved rear cover ? And more importantly, where can I get a D profile silicone gasket?

Thanks for reading this far! Bob
R owen

Bob,
I always recommend the same for new owners and that's buy and read the Driver's Handbook (which will tell you the original standard carb needles), do a staggered 36k-mile service (of the whole car) whilst regularly driving the car on reasonable length journeys, the full and proper service of the whole car will find and help to prevent problems as will regular use throughout the year, there are many dry, sunny, salt-free opportunities throughout late autumn and winter

(Ref: 0053) - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

as you've already discovered new made parts are not always the same as the originals and some are p8ss poorly made such as some ignition, electrical and many parts with rubber in

for ignition parts go to the Distribution Doctor - http://www.distributordoctor.com/

Ashley Hinton for various parts - http://stores.ebay.co.uk/ashleyhintonmgparts2002/

to get the engine properly set up you want the whole car fully and properly serviced and run then take it to Peter Burgess for an excellent value and performance rolling road tune up - http://www.mg-cars.org.uk/peterburgess/

for servicing set up then set up - tappets/(CB points)/plugs/timing/carb mixture - in that order, if you have to adjust any item in that chain then you’ll also need to check/adjust all the items that follow it in the chain

there's a John Twist video that tells you about those side cover gaskets (and loads of other info in the vids) - http://www.youtube.com/user/Universitymotorsltd#g/u

an excellent site for MGBs is regular poster Paul Hunt’s –
(Spanners) http://www.mgb-stuff.org.uk/hammertext.htm

I've got a set of non-technical, non-mechanical notes I sometimes offer to new owners, usually youngsters, but almost all find them OTT and don't follow them but they're from my 20+ years of using various classics as dailies, very expensive experience offered for free - if you want them just email me
Nigel Atkins

According to Clausager original needles were ACD on all rubber bumper MGBs. Usually it's fitting K&Ns that can require a 'richer' needle, an exhaust change on its own shouldn't make as much difference. ACD is slightly richer at the middle stations, and slightly weaker at the upper stations than ADD, but not much different. AAU is very slightly richer than ACD in the second quarter, then progressively richer from the mid point onwards. AAU becomes progressively richer after one third of the travel, compared to ADD. With ADD at the moment it strikes me you would be best off with AAU to start with.
Paul Hunt

AAU are the standard needles. Your exhaust system won't affect the fuelling requirements, unless the induction has also been improved. i.e., K& N's, worked head, better cam etc., even then AAA or ABD needles will often suffice. But choosing different needles can be a lot of expensive guesswork without a rolling road session.
ADD's were used on export Mini's.
Personally I'd go back to standard needles, set the valve/rocker clearances, set the timing then tune the carbs.
Allan Reeling

According to Clausager AAU were used on the first HIFs for the 1974 model year i.e. the last of the chrome bumpers, but were changed to ACD in 'late 74'.
Paul Hunt

Many thanks for the helpful info guys. Nigels collection of notes are great too - thanks again Nigel.

Managed to find more info on original needle sizes last night.

TELERIZER - ACD
SU - ACD
HAYNES - ACD

So, that does look like the favorite. The Minty comparison program shows that I'm running leaner than standard around the mid range which may explain the flat spot I mentioned. Course that's all muddied by the fact the carbs were not set right anyway and had definitely been tinkered with. Chokes were unbalanced, one float way out, Mains wildly different settings etc.

Regarding your comment about the exhaust, Paul, I believe it does affect the flow, in just the same way K+N at the front does. Intake, engine, exhaust are all part of the same air flow line. Make either inlet or exhaust freer and that will help flow, do both and its even better. The lump in the middle will be the hindrance!

As for the problem of identifying the odd cover gasket arrangement, that still remains a mystery.

Found a picture that shows a cork 'standard' profile. This profile is what Moss sell for front and back covers and both cork and silicone are approximately the same thickness and profile. You can see in the picture, the right hand silicone ones are a completely different profile - and this is what I have on mine.


R owen

no problem Bob, might be far too basic for you but they include links to help you with you current questions

the relevant Driver's Handbook will list the standard needles, I've found that both my 1974 and 1980 Haynes for my midget have mistakes and omissions and that the SU web site has errors when I've looked at it - there can be many inconsistencies when you start looking for information

parts are a bit of a minefield, different model years of Bs could have different parts, all catalogues need their information checking and cross referencing, many modern made parts are not well made or match original parts, in my experience all parts suppliers that cover a wide range of parts have some poorly made modern parts in their range

for the cover gasket find the John Twist video as he mentions which to use and which are not so good and it is about the profile, sorry I can't remember which he recommended but just look for one of his vids on the B engine

a common newbie mistake is to change the engine oil and then start fiddling with the carbs rather than as I think you said you were going to do set from the tappets through timing to carbs

on the exhaust I believe the standard B exhaust manifold is considered good to keep on and after that you can improve the rest and match with the filtering

(allowing for the variances of a worn dissy) once you have your car fully serviced a excellent value session on Peter's rolling road will sort the timing and needles for your car's set up
Nigel Atkins

Spent a few hours on car over the weekend .

New distributor
New rotor
New silicone leads
New plugs
Carbs (HIF4) cleaned, reset and refitted.
New air filters (Standard)

Checked ignition timing (electronic) with strobe. Spot on with and without vacuum.

Synced carbs by ear. (Iffy!)
Reset choke mechanism.
Set jets to 2 turns down from bridge.
Starts and runs well, with / without choke.
Colourtune shows perfect flame on both carbs. No adjustment needed on jets - though I did try a few turns back and forth to make sure the jets were actually moving.

Car running really well now, but I suspect it would rev a touch cleaner with richer needles. But will give it a few weeks and see. Now to complete the basic servicing as mentioned by Nigel. :>)

Regarding the side cover rear gasket, I still haven't identified when the difference in gaskets appeared, but the thicker ones do exist on the GT and would appear to have been fitted to the later models (78 onwards?)

MOSS CORK PART # 12A1139 FLAT PROFILE
MOSS RUBBER GUG5505GM FLAT PROFILE

SUSSEX CLASSICS 12A1175 D PROFILE RUBBER
Though the website doesn't say what years??

Hope this helps anyone else stuck over numbers!
R owen

Bob,
well done on getting it running well

if I'd know you were after new HT lead set I'd have put you on to some excellent made British leads at a reasonable price - for others (and no I'm not on commission just a very satisfied customer) only available by phone order via their website as they don't want to upset their customer who they make leads for for them to put their name on - 'performanceleads' - http://www.performanceleads.co.uk/

as regards the cab needles, after the whole car has been serviced, I'd strongly suggest you leave that to an excellent value rolling road tune up by Peter Burgess - http://www.peter-burgess.com/index.html

with regard the gaskets and part numbers generally do not take the parts suppliers part numbers as gospel and that the parts they sell for those part numbers are to original spec

I don't know if the part numbers are original or not but I do know that getting the correct original part to your model isn't always getting the best part for the job as changing parts wasn't always to improve on what came before

have a look for the John Twist video and he recommends the best gasket to use, sorry I can't remember
Nigel Atkins

This thread was discussed between 18/06/2014 and 23/06/2014

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