MGB Register Forum

A good point Roger!
Use the all plastic ones (usually a dark green scourer with a light yellow foam backing), and put them between the leads with the engine off. Then restart the engine to see if it runs any better.
Also if checking in the dark for leads sparking between each other, only look! Turn the engine off before putting hands into the engine compartment. Although the late cars with the radiator moved forward (like the LE) have an electric fan and better protection from getting fingers in dangerous places, always switch off first.

Just a quick update !
Nationwide recovery have just sent another technician out to try and get the car sorted and in fairness he was absolutely superb , he checked the leads , cleaned the plugs and set the gap,
Took the rocker cover off and adjusted ( not sure what though ) and then took the carbs apart and cleaned them out and refilled with oil , so I must say hats off to him for getting stuck in.
The car now starts and idles well . Still a tad lumpy but I am more than happy with it again.
He has restored my faith in the roadside recovery agents.
Going to leave it alone now and wait for the first decent day to put a few miles on it .

New update , it was quite a nice day yesterday perfect i thought for a quick spin in the mgb, it started up fine and settled into a slightly lumpy but consistent idle , however as soon as i got into third gear the
misfiring and spluttering started all over again , should I be running it on normal unleaded ( 95ron ) or super unleaded ? only i put normal unleaded in it but only a couple of gallons.
it is due to go into a garage for investigation after the new year so i am keeping my fingers crossed that they are able to find out what the problem is .
it is a shame that the car is so nice but i am unable to drive it

Hi Roy,
Sorry to hear your problems are ongoing, but you seem to have had quite a lot of areas investigated so far. I've had some more thoughts as to the cause of your symptoms!

I had quite a problematic misfire similar to yours with a Mk1 Mini Cooper on a long distance tour of 8 days in France. It misfired quite badly up to 4000 rpm, sometimes immediately after start up, other times after 100 miles or so. When it misfired I could drive through it and above 4000 - 4500 rpm it cleared and ran fine. All the ignition side was changed by either myself or the travelling mechanic several times. Many other people on the tour gave opinions from a leaking inlet manifold to condenser failures. In the end I drove it home to Staffordshire from Valence (100 miles south of Lyon) like this, which was not enjoyable although on the autoroutes/motorways with was manageable. A few days later it did misfire locally and I got home and started to investigate. A few minutes after switching the engine off, I had my head under the bonnet when I heard a soft 'clunk'. It turned out one of the SU dashpot pistons was sticking in the dashpot as it tried to drop down, so one carb was running very rich below 4000 rpm compared to the other. I had some old carbs so swapped the dashpots and it never occurred again. I had by then remembered having a similar problem with an MGA 40 years earlier! The problem solving was hindered by opinion being that a misfire is always electrical.

So some more things to check:
1. Can you drive through the misfire? Feels 'cruel' when trying it, but if runs right at higher rpm you might have an answer above. Do this first otherwise you might never know if you have cured the problem!
2. If you take off the air filters before you take it for a run, when it misfires stop and turn off the engine, open the bonnet and check if both dashpot pistons are fully dropped down (and blocking the inlet tract) by feeling down the carb inlets. This would be a strong clue they are sticking, but they might drop down as you open the bonnet so it is not conclusive if they are fully down by the time you get to inspect them!
3. Once back home, (even though the rescue service stripped the carbs down) take off the dashpots (3 screws around the base) and gently lift them off leaving the piston sitting in the carb body, and run your finger around the inside. If you feel any roughness or high spots, use some fine wet and dry (600 grade is ideal) and gently rub the area(s) of roughness. Clean the inside of the dashpot (carb or brake cleaner is good) and refit. Don't fully tighten the screws individually, but tighten until you can just move the dashpot by trying to turn it to make sure the dashpot is centralised on the carb body, then tighten them fully. If the air cleaners are still off try pushing up the pistons with your finger and let them drop down. Both carbs should be the same. Finally refit the air cleaners.

You can detect 1 thou of an inch with your fingertip so that is the only measuring device you need! Take it for another run to see if it is better.

On the question of fuel, I think the MGB was designed to run on 4 star leaded petrol, so myself and I would guess many others prefer to run on super-unleaded which is 98 Octane (Tesco is 99 Octane).

I hope you find the reason soon so you can start to enjoy the car!

Thank you again Peter , I will try out what you have said as soon as I get another chance and let you know how I get on.
Will I need to change anything if I decide to run it on premium unleaded , ie timing etc ?
Regards
Roy

You will only need to adjust your wallet!
I think if it is running on the standard ignition settings it should be fine with super unleaded, and it might feel a little more responsive - when it's not spluttering that is!

I purchased my 1980 car at the weekend and have had the same issue. Sorted with 98 octane in the end. The guage was showing empty but the tank only managed to take three gallons before it was full !! So it had had plenty of old fuel that had been in for over six months. After 444 miles and lots of new fuel all seems well. This is my first visit to the Forum so I am "chuffed" to have found this article.
Please can someone explain my Chassis Prefix? GVADJ1AG ??

James
Here is the answer to your Chassis Number
G = MG
V = MGB
A = RHD Roadster
D = Roadster
J = 1800cc B series engine
I = RHD 4 speed Manual
A = 1980 model year
G = Assembly plant MG Abingdon

Hope this helps, if you need to buy a book get Andres Clausager's "Original MGB" it is the MGB Bible