has it just started happening for no obvious reason or after other work on the engine ? popping thru carb usually due to running too lean, but could be many things or a combination - check any cut off valve is working (pull off & replace electrical connector with ignition on - should hear it click) & check for airleaks at carb mount, head flanges etc. & check that autochoke is connected and working. any fuel leaks from carb spindles? have you checked that the rocker gear is all intact & valve clearances are good ?