More fuel,up one size of jet as described in operators manual.unless you have the following,
It could be the throttle butterfly bearings are leaking air if worn, try Viton o rings I sourced mine at simplybearings.co.uk they have these high temp seals in all sizes, you will need to groove the shaft on a lathe, be careful, do a test run on a piece of stock bar before the original is machined try it in the body, if it's correct it will try to suck your finger in the hole (piston like).
Check the butterfly blade sits nicely in the body, if you have had it apart, is it back the right way round ? there will be wear in the contact areas here, good to get it back as it was, bedded in from previous service.
David Jones supplies spares, refurbished and exchange I believe here in the UK, a good supplier and fount of knowledge on these great instrument.
1948 4G Square.