I have my cup model in Paris at the moment which has been fun, see www.tomsgonetoparis.blogspot.com/ for details if you are interested...
However,it has developed a missfire at speed, which has got gradually worse over the last couple of hundred miles. I'm pretty sure that it its electrical as it seems to vary with advance and retard (better fully advanced or fully retarded, worse in the middle), and is worse when the car is hot.

I suspect the problem lies with the rather old sparking plugs KLG M50s I think, or the coil.

Do A7 eat plugs? the engine is fast road spec, what plugs do people use? Also, is there such a thing as a high performance 6v coil - I've seen high power oil filled coils for 12v, I wonder if they would make a difference to a 7.

Hi Tom, A7s dont eat plugs usually.but if they are elderly fit NGK we would have to know the state of tune to advise the exact one..It does not really sound like a coil , but you can get sports versions. question. What carb? what distributor?, and what cylinder head?etc..

Hi Tom

You describe it as being affected by the advance/retard position? Moving the advance /retard will affect the relative positioning between the rotor arm and dizzy cap at the point of sparking. Have you looked inside to make sure all is well?

Steve.

Worse when hot might also suggest condenser? Whatever you do, only change one thing at a time, otherwise you will never know what caused it in the first place...

I've just ordered a new condenser and a set of plugs, I'll stick them in and see what happens.

It just could be a question of slight fuel starvation, I had all sorts of strange goings on with my car which I thought might have been the coil or condensor. Transpired to be a partial blockage in the fuel pipe.

I had a very similar mis-fire, the car would only run when flat out with plenty of advance. I eventually found that the spark was tracking down the front face of the rotor arm and shorting to the body of the condenser when the brass strip of the rotor arm was over the condenser. The high speed 'partial cure' was a shorter time over the condenser and the more advance the position of the rotor arm over the condenser changed sufficient for the spark to go the right way to the cap and plugs.

I tried everything to cure it whilst on the journey, I didn't however change the rotor arm. If I had then I could have travelled home in comfort.

It may not be your problem but it is worth a look at the front plastic face of the rotor arm under the brass contact.

Malcolm

I thought you ran everywere flat out with loads of advance anyway?

Steve

It is well said that when you think it's petrol, it's electrics and when you think it's electrics it's petrol but, in this case, given that it runs OK at maximum or minimum advance but not 'in the middle', the problem is , most likely, related to the rotor arm or distributor cap.

No doubt it will now prove to be a blocked carb jet!!

STEVE

PS: Just check you don't have an induction air leak!

Just in case you are running with newish "aftermarket" parts inside the Distributor, there are Rotors at least which are notorious for becoming conductive after some use.

I will be surprised if it turns out to be condenser as these much villified items are usually trouble free and generally, once they fail, very quickly totally upset things and identify themselves.

I don't suppose your contact set is not really ancient and has lost spring tension.

Worn shaft bearings (even very slight wear) can do all sorts of terrible things at highish rpm.

I sort of gather that you have a manual advance distributor, so I will not wax lyrical about the mechanical advance components.

Regards

Barry R.

My RP used to suffer a similar problem, and after trying everything else I changed the distributor for the non-vacuum advance Bosch one. Since then, absolutely perfick.Guess it was shaft wear in the distributor that was the source.

Hi, now to get a real non technical approach! I still don't know your hardware set up (carb. head and distributor)or if the problem goes away when cold? What petrol are you using? If you have a standard Austin 7 set up, and are using unleaded (!!!???)(some of us are!) and you get backfire when the engine is hot, try a little paraffin in the petrol. You are now allowed to use up to 10% so that the rating is lowered to one star, which is what these engines were designed to run on! I haven't gone as far as this much, and my very knowledgeable friend (moderns) hates the idea, but in mid summer when crawling up a long hill there is a differance!
How old is your petrol? By bitter experience, if you stop using your car in September, and wake it up in April, you will find all sorts of problems. Put new petrol in and drive expecting the same problems you will find your decrepit 7 is now jet propelled! I have had all sorts of arguments with modern car drivers who cannot believe that modern petrol goes 'off', but check the internet. Petrol companies won't admit it but it has been proved!
I heartily agree to try one thing at a time, but sometimes it might be better to go back towards as it was originally, rather than add 'modern' improvements. After all, the old basic 7 still holds speed records for its class, despite efforts to break the records using modern bits!
No, not having a dig, its just that the car was designed in 1922!
Best of luck
Tatty Box Bill (yes I'm still around!)

Bill

you are correct about modern fuel "going off"

I work with small machinery, and we suffer from stale fuel all the time. It will go off very quickly and you cannot start the engine. We have found that some jollop down the carb inlet will get it up and running and sometimes if you are lucky a hot engine will then carry on running on stale fuel.
there are products available to add to fuel to prevent this, but its not worth the bother realy, if you have some spare just put it in a modern to use it up! (or go for one last drive in the Austin!!!)

The first thing we do when a machine won't start is put in fresh fuel, it is that important!

Might not be the problem but its something else to consider I guess?

Steve

Hi again
As a very non mechanically minded A7 owner, I tend to go over very basic things! I would concur over new things. Once fitted a brand new rotor arm, and couldn't get any life from it. Put the old one back and joyful running was restored!
What sort of speed are we talking about?
Is there any play in the distributor shaft? Talking of which, my modern thinking friend prevailed on me to fit a later automatic distributor, and although I missed the joy of manually selecting advance or retard, it did improve the going up hills. But then so did the later 4 speed gearbox we squeezed in! (Should have changed it years ago!)
What is the compression like? Might it be a valve beginning to 'leak'?
Told you I was a mechanical idiot, but I have experienced a lot with my 7 over the years, but can still learn!
Best wishes
Tatty Box Bill

The car is set up as follows: - bored out to +80 with pheonix crank, big inlet valves, double valve springs, pigstye fast road cam, and rosengart ally head, the carburettor is a Solex 30mm updraught with a fabricated inlet manifold, and a standard exhaust system (with straight through silencer).
The missfire seems to occur above about 45mph in top, and at the corresponding RPM in the indirect gears. The petrol is brand new so I doubt that is the problem.
I've just got some smart new plugs and a condensor in the post, so I'll try them this weekend. I know its better to change one thing at a time, but I don't have the time for a process of elimination, so I'll just have to try everything at once, and keep fingers crossed...

Hi
Sorry, you've lost me! You got technical!!! Do you ever get the feeling you wish you'd never asked? You mean these cars do more than 40 mph? Just to let you know I'm not being nasty. I'm notorious. For years I had a long chassis Box, and kept the 3 speed gear box. Consequently, every hill I got to would be a crawl. I used to get left behind on road runs. I'm the one that years ago, we got to a hill in Sussex, and had to reverse the car to get up it! I now have a 4 speed sync gear box. Should have done it years ago.
I did tell you I was the mechanical idiot!
I still wonder if the modern petrol is getting too hot for the engine. If your new bits don't improve the situation, try a little paraffin. I know my modern thinking friend hates the idea but I found in my bog standard engine it did help. I would suggest a pint to a full tank? It just lowers the grading of the modern petrol.
When you find the cure do let us know. You now have this mechanical idiot curious!
Oh, and before you think I'm one of those 'purist' fanatics, my box is the wrong colour, and my latest car is a 2 seater Austin 10 complete with A series engine gear box and back axle, and it's like driving an A40! This was done in the 60's when they were being dumped (it came out of a quarry).
Best of luck
Bill