Odd running behaviour

Ok guys , this is driving me nuts , what do you make of it ?
It’s a 3 .4 mk 7 , starts easily and runs on 4 5 and 6 only . Putting on or removing 1 and 2 plug leads , while the engine is running has no effect , but when no. 3 is connected , the engine will noticeably falter and slow . There is a good spark and plugs are new . Even tried another set of new ones . A sniff of ether in the rear carb initiates a momentarily response . If no . 3 plug lead remains connected , I get popping and banging . Head and carbs were rebuilt by 'professionals ’ , could the fault be in the rear carb ? Help please .

I would start with the rear carb, check fuel bowl is filling.

But why the faltering when 3 plug lead is connected ?

3 is probably getting a bit of fuel from the front carby.

Check for a vacuum leak at the vacuum take off below the rear end of the inlet manifold. Had this problem with the pipe to the booster on my Mk 7. Could also be that it’s leaking at the booster.

Make sure the both carburetor spindles are connected tightly together, maybe only one carb butterfly is opening.

Either you posted that before or someone has had the same problem recently, I think I thought it was the rear carb not working right with number 3 feeding off the front carb a little?

If it runs smooth with ether to the rear carb (or a leak in its area) that’s definitely what it is, if it just pops and bangs then go through ignition leads and do a leak down or compression test?

Is there any chance you have some plug leads in the wrong order?

Tried that Rob . Gonna do some testing today .:pray: