Welcome!
What?
the rear pulley is longer than the engines shaft..
doesn't make ANY sense, sorry.. post a picture
the driver pulley (the one on the engnes shaft) IS smaller in diameter than the DRIVEN pulley (the other one!)
that is wanted AND needed that way.
it has nothing to do with the pulley ratio as of now, since it is a Torque Converter (means the pulleys are both variable and adjust as per your needs as good as they can!)
What your issue sounds like to me
is the number one reason for TC issues..
WRONG BELT!
You MUST have a TC belt (not an automotive V belt) in your case very likely a series 30 belt,
that's specifically asymmetric one side appears to be flat, that needs to point towards the engine.
Most Dingos use a 5959 belt length you can buy at any go kart shop..
I suggest this:
https://www.gokartsupply.com/5959belt.htm
incorrect belt length can cause friction that in turn tries to move the kart.
(as does incorrectly set idle speed.. that must be set to less than 1500 rpms!)
but post pictures, that way we can surely identify the problem
and give much better than just "general" advice.
'sid
PS It's always a good idea to do proper TC maintenance at this step,
cleaning the sheaves (babysmooth) and fixing all that needs to be fixed (worn buttons, worn spring or weights)
and add dry lube to the driver pulleys inside so the weights have it easy to expand and retract.