poppin noise from engine

[VWCamperfan]

The idle jet will be in use up to 2400 rpm, at this point the main jet takes over and supplies the majority of fuel to the engine. So up to 2400 rpm its an issue of idle screw adjustment.

It may be in use but it will be doing damn all... flat out up a hill at 2,000, and it's on the 'idle' jet - can't agree, it'll be drawing most of its fuel through one of the intermediate jets and the emulsion tube will be controlling richness, as things progress.

There's no such thing as a carb being adjusted for an engine before it's fitted.

There is... all the fixed jets and emulsion tube can be fitted for that engine (original spec. for the country of delivery and expected altitude of use) + the idle jet can be set to a nominal 'as-delivered' state, to at least get you going.

There are too many variables that may require a different mixture ie. engine age and wear, compression, engine size etc.

Yes, that's true.
But it doesn't stop a supplier setting it up as mfr supply above to stand a damn good chance of running well enough to start with, if that carb was originally specced for those engines..

Few people go changing main and intermediate jets as their engine wears

'High idle' tunings for carbs are usually about 1300 rpm, the centre range of the idle jet. Thats why they do it.
I will admit, flat out at 2000rpm up hill won't use the idle jet very much but on a flat surface it will be doing most of the work.
As I said, you can't tune a carb for an engine before the two meet up... You can 'get it going' as you said but it will require adjustment when fitted, thats why they have adjustments.
The carbs for the CT engine don't have intermediate jets, just idle and main.

[HarryMann]

All the above then

and if it's not just on the overrun, but when it's hot and under power, might be burning an exhaust valve
That's worst case though, and a compression test would tell-a-tale in that case. Did the tuner guy happen to do one?

Thanks for the clarification Mark - I didn't know that.

[VWCamperfan]

I just hope that when I put me foot down it goes forward... Not if i'm in reverse though!
These carbs can be a bit of a pain, thats why I changed mine to something completly different and you can't get them anymore either, not the exact one anyway!
Theres a link here if you do want to have a play with your carb...
http://www.vw-resource.com/jets.html" onclick="window.open(this.href);return false;

But have a good read first if you don't want to muck things up too much!

Ps. A simple way to check your exhaust would be to hold your hand over the exhaust outlet (not when it's hot!), let the pressure build up a bit and listen for any blowing sounds from the exhaust system, on the left bank if possible! May be easier if there's two of you!