One more thing...... Does the motor have a high performance fuel pump?
If it has too powerfull a fuel pump it may just over power the needle and float inside the carb fuel bowl. This would flood the carb and motor.


So again - It is either too much fuel or fuel not getting burned completely.
The unburnt fuel could be ( most likely) weak ignition or incorrect timing.

If the above is found to be ok then we are back to the carb.

I am tempted to say that the carburetor is defective rather than think that someone used too large as jet, metering rod etc. Without knowing the intenetions of the builder tuner and or history of the motor it is very hard to solve the problem.

Good luck and let us know how it turns out