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MG MGB GT V8 Factory Originals Technical - Factory V8 Carb question

Since taking my 1973 V8 off the road for a front end rebuild I've been plagued with a sometimes bad misfire, I have replaced all ignition components, checked and rechecked points gap and set ignition timing dynamically, Once I discovered which carb
fed which cylinders I first adjusted mixture by plug colour(no 7 especially sooted up) and the thought what the hell and I have just bought a gunsons colourtune, sure enough pass. side carb(feeds 1,4,6,7)was way rich, adjusted to correct colour(stays throughout rev range)
drivers side carb (2,3,5,8)only slightly rich , once correct colour achieved again stays correct through all revs.Now drives fine, feels powerful on acceleration ... but at very light throttle loads I can feel engine surging and occassionaly missing, foot down no problem
any ideas?
N.T Griffiths

Gather you've replaced the plugs along the way. V8's eat plugs. Approx 10K miles in my experience then they start to misfire. Notice it most in traffic when heat starts to build up, engine will eventually cut altogether.

Does your vacumn advance work correctly - I gather the line to the distributor is not punctured - ?

How about plug leads?

By the sounds of your description could well be a vacumn leak under high vacumn conditions. When my exhaust manifolds start to leak I get an occasional misfire under light throttle loads especially on a gradient. May be a leak anywhere else in the carbs/intake manifold area. Check the gaskets are not perished and everythings nice and tight.

Incidentally, on the V8 Lucas distributor, there is an external dwell adjuster. That and a Digital Automotive Multimeter makes adjusting the points gap a breeze.

Food for thought at least. :)

74 V8
Neil Cotty

Funny you should say that about plugs, mine don't even last 10k. But my symptom is hard cold starting and very poor running till warmed up then it's fine, rapidly going to refusal to cold start until the plugs are changed then it starts instantly. Funny thing is that the condition of the removed plugs is always perfect - good colour and no physical wear. Had this again last week but in parallel with a knackered (again) battery but this time instead of changing the plugs I put my timing light on plug and coil leads and had no spark. Eventually found that it was the distributor cap which showed a little slight black 'crustiness' on a couple of the webs near the centre contact but no other obvious tracking inside or out. I'm wondering now whether the cap has always been iffy, and when the plugs were a bit worn plus any number of other circumstances the cap was breaking down, but not otherwise. We shall see.

This raised the previously unknown (to me) situation that if an HT lead is shorting to ground *after* where the light is connected then an inductive 12v timing light won't flash but the older style neon in-line type will, but proves how useful they are as a diagnostic tool and not just for checking the timing.

As for the battery, I had a new one just over three years ago that failed just over a year ago, when the replacement failed too. Supplier claimed it was out of guarantee as this remained three years from the original purchase even if replaced. When I pointed out that the battery itself was clearly labelled '3-year guarantee' he eventually backed down, but saoid they wouldn't replace it again. We shall see - again. At this rate the plugs are outlasting the battery!
Paul Hunt

This is exactly the problem I had with my car in a previous thread. You say you have replaced ignition components but not to what extent. I replaced all the leads and then the plugs as well which eventually cured the problem. As Paul says the plugs looked perfect. If you haven't done this it may be worth trying. On the n/s two of the leads have to be fitted so that they are not adjacent to each other in the plastic clip (I think it is 5 and 7 without checking my book). I don't know why this is but I'm sure Paul can enlighten us. Good luck

Thank for ideas and suggestions
I have replaced all ignition components i.e-Coil,Dist Cap, Rotor arm, Points and condensor,HT leads, and plugs, Actually when car ran best (last September) I remember not having to
use the choke to start car from cold, now needs full choke for first few minutes before being able to tick over and even then tickover very low 300-400 rpm and stalling,This was after setting with colourtune and engine tickover at 850rpm last night(normal op. temp)as per original posting.I have now richened up the mixture approx half a turn d/side carb, and appreciably more p/side carb, car now pulls extremely well and very little sign of hesitation at light throttle-in fact when richening up p/side carb revs rose by 2-300rpm so this is one engine that likes a drink!
I am thinking of trying to weaken mixture again, say 1/8 turn each before daily runs until car starts misfiring and then richening up slightly, not very scientific, I'll also try Martins suggestion of moving HT leads, Martin I used to drive around Shrewsbury a few years ago, A49 to Leominster especially good road I remember,
lots of nice bends near Condover.
N.T Griffiths

I didn't have the book in front of me when I replied to your first. Here is what it says

It is essential that the ignition cables to the sparking plugs in the left-hand cylinder head are correctly positioned. In particular the ignition cables to number 5 and number 7 sparking plugs must be clipped in the position shown and not adjacent to each other'

The diagram shows the leads clipped into the plastic clips as follows starting from the front of the engine and ignoring sparking order. The first lead is clipped into the first position, the third lead into the second position, the second lead into the third position and the fourth lead nearest the bulkhead into the fourth position.

I don't know the reason for this because they cross one another at the distributor anyway but evidently BL thought it necessary to make a special point of it in the handbook.

Yes the A49 is a nice driver's road if you can get on it when it is free of traffic as it used to be a few years ago. Unfortunately nowadays it is just a crawl behind slow moving lorries for miles as there are precious few passing places. A story only too familiar in this country now I fear.


I have commented on this topic in previous threads, of interest to those of us with SUs.

It has taken me some weeks to be convinced that my new SU electronic fuel pump is faulty and I am returning it to the distributor today.

The car has been shocking when cold and I have been searching for choke type problems and am close to taking at least 1 head off, as one cylinder has lower compression than the others.

Anyway back to this fuel pump: It took me a while to realise that it doesn't start pumping immediately when the ignition is switched on, even if the car has not been used for a week or more. If you disconnect the fuel hose from the carburettors it still will not pump for a time, but when it does start it stops and starts if you remove and replace your thumb as per normal.

It seems the pump is reluctant to get going, but when it does it is OK for hundreds of miles. Nevertheless I sometimes feel the engine is not quite right and wonder if the fuel bowls are being kept topped up by the fuel pump. I am confident the float levels are correct.


Ian G Buckley

The reason for 5 and 7 not running close and parallel for a long distance is that unlike any other physically adjacent cylinders these two are also adjacent in the firing order and spark induction from 5 could cause 7 to fire way too early.

Ian - exactly the reason why I went back to a mechanical SU from an electronic - when the electronic starts doing that you are totally stuffed until it decides to start working again. Mine doesn't have the vent tubes like the mechanical one does, and much later I did wonder if it has a small vent hole on the body somewhere that was getting blocked and heavily damping the diaphragm, as that was the symptom.

Went to the NEC Autojumble yesterday and boaught a battery cut-off switch as it suddenly occurred to me that maybe the relatively little use the V8 now gets is poisoning the battery with the constant smalldrain of the alarm, even when not set in the garage. The roadster with alarm unplugged doesn't seem to have the same problem even though used even less in the winter.
Paul Hunt

Fuel pump - I fitted electronic contacts to the fuel pump after constant trouble with the points, and so far - no problem. (Tempting Fate).
The battery problem is a continuing pain in the *rse with low use. I bought a Unipart Samson in 1982 for my GT with a lifetime guarantee. I've probably had five or six replacements since then, but I see in a thread elsewhere that 'Unipart'are now arguing the toss as the replacement for the B is not listed in their catalogue. We'll see.
A couple of years ago I bought a couple of battery management units from Brown & Gammons. You'll be aware no doubt - always connected, they regulate the state of charge, and since then, they've always been kept fully charged, and no sign of problems (again I'm tempting fate). The V8 batteries are about five years old. The Samson is 6 years old.
I don't think a gentle slow drain will harm the battery providing it doesn't go totally flat. Some 'exercise' is recommended during lay up.
Finally, HT leads are very vulnerable on the V8 because of the length and close proximity to metal parts. Couple that with extreme heat & vibration and it's easy to see why they break down.
I wonder if the plug problems are related to engine spec? I have a pretty much standard spec low compression V8 and have not suffered too much with plug failures - always use NGK.
Dave Wellings

I've had the same set of HT leads since just after buying the car 7 years and 65k miles ago. They are the silicone-cored type from the MGOC and show no physical deterioration at all. The only thing wrong with them is that they came with all straight connectors at the dostributor whereas those leads for the right bank should be right-angled. Didn't experience any problems despite the right-bank leads being in contact with the bonnet. Many years later I took the right-angled connectors off the old leads (if you haven't found a use for something yet you haven't kept it long enough) and was able to use them as an 'adapter' between cap and straight connectors. I had been using NGK leads for many years but because of the repeated short life bought Bosch last time. But now I've changed the cap I won't know if long life (if they get it) is because the Bosch are better or there had been an incipient fault on the cap for some time.
Paul Hunt


My new electronic SU pump has 2 vents which I have plumbed into the boot as recommended. I will be interested to hear what the Australian distributor of SU products has to say. I bought the SU because I cannot stand the noise of the facet square pump I had before. I was told the later electronic SU pumps are more reliable than the earlier ones. Hope so.

When my car was a 4 cylinder I would rebuild the original SU pump and it was good for 8 to 10 years so I am prefer to stay with SU.

As a long term owner I have not had real battery problems. I appreciate this country is kinder on batteries than yours. My single 12 volts have lasted on average 4 years. I truly had one once that lasted 10 years!!

I fitted a battery isolator behind the driver's seat 25 years ago. I was always terrified if there was a fire I would have no way of disconnecting the battery.

My 3.5 has 10.5 to 1 comp and I was previously plagued with plug fouling problems. I have tried many of the solutions recommended on this and similar threads and my greatest success has been changing from NGK 6s to NGK 5s. The plugs now appear to be staying clean.

I think Dave is right, higher comp murders plugs. Many years ago I had a P6 Rover 3500S with 10.5 to 1 and this really ate plugs. I always blamed the local fuel.

Ian G Buckley

I have swapped lead position in clips for 5 and seven(yes they were side by side) but no difference to misfire, I again used the colour tune( I will buy a gas analyser -one day)set both 7 and 8 cylinders to correct colour, again p/side bank of cylinders not happy, I checked all inlet bolts/nuts etc all seem tight, problem found on P/side carb throttle shaft, I can move it approx 1/8 inch side to side, other carb no movement, could this be the problem?, In a previous thread someone suggested spraying carbs in WD 40 as this temp. stops inlet leaks on throttle shaft, I have previously tried this with no effect.Anyway took car out for a drive, and working on the principal -probably misguided that mixture on both carbs now equal, stopped every few miles and adjusted mixture 1/8 of turn each carb, richening as I went along.
Firstly power increased as before, pulling very strongly but with still a hint of hunting just at point between over run and part throttle condition, I suppose dropping down from higher speed scavenges the cylinders completely and then when on part throttle, weak mixture more evident-after a further three stops and richening mixture each time, car now runs smoothly and no hesitation evident,I guess when, and if it starts and runs smoothly tomorrow I will know problem is sorted.
Question- Can I adjust side to side play out of carb shaft?
N.T Griffiths

1/8" sideways play in a throttle spindle is a huge amount! And it can't be adjusted out as such. Spraying something on there isn't supposed to temporarily block the leak, more the opposite. If you use something like carb cleaner it will make the revs rise as it is more volatile. However that is usually to detect a manifold gasket leak. With a spindle leak half is on the vacuum side and half on the air side, so depending on which side the 'hole' is it may not make any difference.

The only way you will be able to use an exhaust analyser is *after* you have balanced the carbs using some other method, you then use the analyser to adjust *both* carbs by the same amount in the same direction to achieve an overall CO figure, but there is only any need for that in a post 31st Dec 1974 car in the UK. Prior to that the MOT check is for visible smoke only so you can have it as rich as you like for best running.
Paul Hunt

The Australian SU pump distributor telephoned me today and advised me that he had found the fault with the electronic pump I returned and he is sending me a new one. Apparently the body casting is porous and there is another fault which appears to explain the symptons I described above.

They sell 30 to 40 of these pumps a month in this country and returns are not uncommon he said.

While waiting for the pump to be returned I have removed my fuel tank so I can solder on a fitting for my fuel return system. I have decided to install a small brass fuel tap in the return line so I can (hopefully) regular the circulation.

I left my fuel tank out for a few days in the 100f temperatures of late to evaporate the remaining fuel. I hear the weather is a little different in the UK!

Ian G Buckley


I did some welding to my fuel tank. A lot of people advised me not to do that but i also received a very good tip.
Fil the tank with water. I did this and everything went well.

By the way, my plugs already last 20.000km and no problems at all with starting or misfiring
Good luck


I do weld but have never welded a tank!! I have a good friend who has a radiator repair business and they can do so much with solder, I have never found the need to weld.

My last post above is incorrect with regards the fuel pump fault. The diaphram was porous and not the pump body.

I repainted my fuel tank amd let it cure in the recent 40c degree heat. This weekend I can start the motor and test my adjustable fuel return system.

Ian G Buckley

It will work. Fill tank with water. Afterwards flush two times with fuel, and a last time with oil. Fill with water just beneath the place where you want to weld.

Frank de Groot

This thread was discussed between 03/01/2003 and 15/01/2003

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