I'd start by measuring what psi your fuel pressure is. If it is too high your car will run really rich.
A common cause of high pressure is if the car has been sitting for a while the return line can block, blowing it out with compressed air sometimes frees it up.
depending on the size/ flow of the walbro it could be to much for the stock pressure regulator to handle.
start by checking those and let us know how you go. Faulty air flow meters or map sensors can also cause the same problems