Prawn:

Need a little more info before commenting. What carb does it have? It could be black plastic, a flat sided aluminum block with "K/B" on the front, or it could be a ribbed casting with "K&B" in a oval cast into the front.

If yours is the black plastic, it could be either of two. From the front of the engine, looking down at the top of the carb, does it have one screw sticking up near the throttle lever and a second near the fuel nipple?

If it has the TWO screws it wont work without a pump, and the engine should have a Perry VP-30 pump built into the back plate.

If it has only the one screw near the throttle lever it will work fine with muffler pressure.

The "Ribbed: carbs have two variants, front or rear mounted needle valves, in use they can be considered the same.

The pictures should help the ID of the carb.

Bill.