If the carburetor fits, does that means it works!

I have had my Honda Dax ST70 since I was a kid. Thats around four decades. Now that my own kid is old enough to rid it, I brought out of storage, changed the oil, filled up the gas tank, and it started after just few kicks. For few days it ride fine. Then, the carburetor started to leak all over. I couldn’t find a rebuilt kit or same model carburetor. So, I had to buy a different model KEIHIN that I was told fits fine. It has similar bolt pattern (48mm center to center) but the intake is 2mm larger. When installed, the bike rides fine except when accelerating at full throttle or speeding over 40km it stalls. Is this caused by size difference between intake manifold and the carburetor? Or, is it just no good carburetor?
Thanks for the help

It may be running too rich ie too much fuel.
The main jet in the carburettor may be too big or you may be able to adjust it by lowering the needle.

I may be telling you something you know but if you take the slide out you should see the needle valve hanging below. They are ususly held into the slide with a tiny circlip.The needle should have 3 grooves. Put it back with the circlip in the top uppermost groove. If that doesnt improve it you will have to get a smaller main jet

I did lower the needle all the way but its still doing the same.
I'll try replacing the main jet I guess!
Thanks Higgysdream for your help.

I would recommend rebuilding the original carb, I think you can buy a 'carb overhaul kit' from CSNL.
The bike wont 'cut out' thru a little excess fuel, just 'burble' somewhat.(like too much choke)
It will cut out and die thru lack of fuel ie.- excessively lean mixture