PS the backfiring was due to a crook sparkplug. The problems with the carbies were that it was running rough.

 

I actually think you will find it was a combination of lean running on acceleration & a duff plug.

 

Quite possibly Glyn.

 

Indeed, lean mixtures need a much more energetic spark. The problem is then partly fuelling and partly electric and, as you have experienced, very difficult to diagnose.

 

Yep! SU's have no throttle pump & depend on slight opening of the jet & delay in piston rise to create depression over the jet & thus to achieve enrichment & provide the throttle pump effect. Having an air leak beyond that on the shafts would nullify this not helped by a duff plug. Thus running lean on acceleration & backfiring.

My experience noted previously in this thread with a split pipe on the LHS tank/pump was hell to diagnose as we appeared to have good air free fuel flow at the front feeding the carbs. We had a number of good minds trying to diagnose including an old technical chap that used to work at Brown's Lane (said Lyons could be a difficult man to work for.) It flummoxed all of us until we found the split pipe by a process of elimination. We made the classic mistake of jumping to conclusions ~ e.g. It must be the Pertronix Ignitor ~ let's go back to standard points which at least eliminated that as the cause. And let's check the valve timing which I knew to be correct.

It's one thing having the knowledge ~ but quite another applying it to diagnosis and doing a physical repair with the required skill in that regard.

e.g. I would go to a specialist to ream the carb bodies to accommodate oversize butterfly shafts. I would not trust myself to do the job, neither do I have the required equipment.