A 26/26 carb has a real diameter of 24,95 mm (as SIP specifications says).
Thats not too big, but this carb is not easy to jet.(as I know from own trials)
I kicked it out again and fitted the original 24/24 on my T5
By the way, you tell us some things, but not the relevant ones.
So due to the lack of information, here are 2 general advices:
1) fit the original carb.
- Perform a classical jetting procedure.
- Maybe you will find out , that a main jet 2-6 numbers higher than the original jet will give the best result.
-> have you done a jetting procedure already?
2) check the ignition timing as a longer stroke than original means definitely: different timing
-> Have you done this?