I have a 1916 with a mild cam, standard heads, solid rocker shafts, heavy valve springs and twin 40 idf webbers. I am finding recently it is missing and back firing at mid to high revs when it is under load especially after idleing for a bit, sometimes to the point where it is not drivable. Also i don't know if it's related but i am finding my Passenger side J pipe is getting much hotter than the other? Not even hot enough to burn the wd 40 i am spraying on the webber manifolds to check for air leaks. Any thoughts?

Have you checked all the basics - points gap, timing? Missing is usually ignition, often a condensor or old plugs that 'throw in the towel' when asked to work harder than idle the engine

What colour are the plugs??

Have you checked all jets for blockages??

What condition is the coil. When they get old they can do what you describe

Coil and blockages are the only things i haven't checked. I thought the carbies may have been out of balance because of the J pipe thing, a leak in the balance pipe?? but i checked that and it's all good. I actually didn't have any hose clamps on part of the balance pipe but i fixed it and it's still the same. After the new spark plugs it ran great for about a 2 hrs. I'll check what colour but they have never given me problems before, i have been driving the car for about 6 months. Would you think if it were blockages it would have cleared out by now? it's ok on the highway. Is there any way to test the coil? or just put another one in and hope for the best?

You can look at the colour of the spark

It needs to be "BRIGHT" - white/blue

If it is orange/yellow it is shot.
For $40 i would put a new one in anyway and rule it out.

Same problem happened to me a couple of years ago.
Greg Ward found the problem .... coil breaking down under load.

how hot should a coil get? should it be very hot to touch?

haqve you set the tappets could be a bit tight as for carbs just turn in the mixture screw count the turns than set both to the best one and change plugs and points and condensor
i had a coil go on me once it was really bloddy hot than it started to miss and do the things that you r having if you can get a loner try that to

Checked coil and the spark seems ok. It's a nice white/blue. But i did notice when i was ckecking the spark, at revs the spark was missing every now and then. Could it still be a coil problem even though the colour of the spark is ok?
Condenser is new, rotor button is new, points are new, spark plugs are new, Tappets were reset before christmas, They sound ok. no different than before.

cant you beg / borrow / steal one??

Yeah i can get one, But i couldn't yesterday because my parteners car was broken too! we had no way of getting anywhere so i had to fix one of them and i knew what was wrong with the hyundai. I should look under the house, i probably have one.

i had all these probs on my 2276cc
i ended cleaning out the carbs and getting the jetting right and buying a u beaut compufire dis x ignition, now running like a charm.

what jetting do you have in the idf's, there could be a prob there too

The jetting was all done by vw & volvo centre and it has been ok till now. i'm hopefully gonna try a new coil today.

coil, coil, coil !!

Checked coil and it's not the problem. I think the problem might be no1 cylinder. when i pull the lead off it doesn't run rough like it should. I dont have another lead to test it. i will get one tomorrow and try it.