Hello from Wales again....

Big thanks for help with air box issue last time... I'm looking to replace twin carbs as having issues balancing them, what's the best single carburetor to get please folks....

Are you sure you do the balancing (carb sync ) right?
If you have issues balancing two carbs, you have to do a carb clean or rebuilt first.
Nothing is easier to sync two carbs to get a smooth idle.
Air leaks in the carb boots,diaphragms or vacuum tubes can make this simple carb adjustment impossible.
A picture of your carbs and environment around it will help us to fix your problem.
A service manual for your bike will also help you to understand what you do before you do it wrong.

No offense intended, but if you cannot balance 2 carbs you may not be technically equipped to convert your 2 carb stock setup to a single carb custom setup. My advice would be to correct the existing simple problem rather than create a new much more complex problem. Keep in mind that when your bike was new it ran very well with 2 carbs, so correcting the existing problem simply means finding the cause of the problem rather than trying to engineer a new carb setup. Ed

I don't know what motorcycle you are wanting to convert, but I am pretty sure there is no ready to install kit. The carb will be the least of your troubles.
The intake manifold will have to be fabricated, and that will need to be done by somebody who really understands the science of the thing. Velocity, resonance, volume, laminar flow and such will all have to be taken into consideration. The carb will need to be a mechanical slide carb with flow characteristics that compliment the engine and its new intake system.
Several members on this site have tried and failed.
I have read that some members over at Do The Ton have successfully converted a twin engine to single carb, so you might want to ask over there if you are serious about this. Be sure your wallet is well stocked.

On the other hand, since you already have a twin carb setup, why not let some helpful folks here help you balance them?

One other thing to consider is IF one converts a twin cylinder engine from a dual carb to a single carb setup the engine will usually make less power.

As an example, the single carb BSA Royal Star was the typical street version of the twin cylinder BSA 500, but the dual carb BSA Cyclone 500 was the high performance model that was usually used for racing. The dual carb Cyclone made appreciably more power than the Royal Star and much of that additional power can be attributed to the dual carb setup. Ed

Thank you, have ordered new diaphragms, as old ones very worn...

Thanks Ed, starting with changing worn diaphragms... Aliminate that first and see what happens, left side carb is the one causing problem...

Carbs cleaned, set up done, diaphragms worn so changed, hopefully sorted... Thank you.... Will keep you posted...