It could be an air leak around the base of the intake manifold. That happened to me a couple weeks ago. Make sure the manifold gasket is good, and the two nuts bolting it on are tight. Then, and this may sound obvious, make sure the screw clamping the carb onto the manifold is tight. Then, if neither of those suggestions help, buy one size smaller jet (52), and one size larger jet (56) and try them both. In my experience, running with or without the air filter has absolutely no effect on performance, so don't worry if you don't have a snorkel, but that's just my experience. The amount of air going through is _primarily_ determined by the size of the venturi. If none of those things fix it, check your timing (should be 2mm btdc) and your compression (should be 120psi-140psi).
Hope this helps,