Sounds like your accelerator pump is not delivering fuel properly when you stomp on it. That is why it backfires or lays down. I would buy a Barry Grant rebuild kit some carb cleaner and do a full cleaning.
Make sure the accelerator pump adjustable piston is set to make contact with the accelerator pump actuating arm. I set no more than .010 clearance on that puppy.
If your 650 is a double pumper, it may be delivering too much fuel.
Hope this helps. Email me at
jim_howard_pdx@yahoo.com with a full description of your carb, your intake, and how you use the car.