Mike.

Are you using the Super Tiger carb?

In the 40 through 90 size Super Tigers, There is only one carb body. They will all fit in either engine.

The difference is in the spray bar. The 40 - 51 engines have a round sided spray bar. The 61 though 90 have a spray bar with flat sides on it.

Even the original stock carb for the 40 is rather large. If you have a ST carb with the flat sided spray bar in it, The carb will be too large and cause the problems you are experiencing.

I forgot to notice which muffler you are using. The economy muffler will produce more muffler pressure than the swing muffler.

Enjoy,

Jim