How To Measure Fuel Volume.

I agree with above.

I did not see carb combo posted.

Could it be lean or perhaps too much timing (suggest 34 total
to start at Sea Level). Both of those would be more apparent
in high gears as RPMS increase.

With the 26 tire or even 28 = It might be happier with 3:91 gearing

I would be interested in more details on your combo to give you better feedback.