Lumpy idle - S2 UK spec - suggestions please

[db7gtgrigio]

Symptoms

- takes a while to start from cold with choke but once warm will restart immediately just using the key and no throttle
- idles at 700rpm but feels and sounds lumpy. When I put my hands over the exhaust pipes the sound from the RH pipe is a nice quiet even humming sound whereas from LH pipe it makes a loud regular pft pft pft noise.
- has started running rough at higher revs with occasional backfire
- car has had very limited use over last few years (fuel is new)

Points checked - gap correct, faces clean
for peace of mind I changed the old orange lead condenser for a brand new condenser from Distributor doctor -
wire in distributor from points back ultimately to coil looked dodgy so just replaced that
plugs relatively new NGK BP5s
Relatively new copper core plug wires with NGK caps all giving 3 ohms resistance
Just noticed a small exhaust leak between REAR manifold and downpipe

Does the fact that one exhaust pipe is nice and even and the other is rough point more to carburation from cylinders 1,2,3 as opposed to ignition? That would be middle and front carb

any suggestions kindly appreciated

[Allrand]

Process of elimination, try to determine which cylinder is misfiring by disconnecting or shorting out one plug at a time. Then do a compression test, it will tell you if there's a bad valve or rings. If that's all good, try swapping out the plug lead with another & see if the misfire moves to the other cylinder, that'll tell you if it's the plug or the lead.

[DWW]

I had that happen to me a while ago, cylinder 1 was misfiring, once I cleaned the spark plugs it return to its proper idle, I found the rear carb was running slightly rich and after mixture correction all was fine. If the car was was running ok previously and just not in use for a long while normally giving it a good run may cure the issue.