Good morning, I am new to the forum, so let me get straight into the problem. I am working on an old scooter with small displacement (150cc, with about 9.6:1 compression ratio). After rebuilding the engine, I figured I have to put in a much bigger piston 60mm compared to the original 58mm since that's the only oversize piston I have, the cylinder sleeve is pretty worn so that needs to be bored up. In the process, I figure the compression ratio has also gone up since this 60mm has a higher dome. The problem comes from the vacuum fuel pump on this scooter. I figured I should change the pump, so I did just that. Since the fuel tank on this scooter is lower than the carburetor, it needs a fuel pump to pump fuel up into the carburetor's bowl. On a stock scooter, nothing bad happens. But when I put in the 60mm piston aforementioned, the carburetor tends to leak fuel out of the overflown tube, a lot. This is a fuel pump that uses vacuum from the intake manifold to push fuel up. What is the cause of this problem and how can I solve this? I figured the suction from the intake manifold has to be stronger for the fuel pump to pump too much fuel that the carburetor float cannot handle but if somebody is able to elaborate more, it will be much appreciated. Thank you. The small brass tube hanging out of the manifold is where the vacuum line from the fuel pump plugs into. This is the aforementioned vacuum fuel pump. The inlet line is on the total left side. the middle line is the outlet (into the carburetor float bowl) and the line on the total right side is the vacuum line that plugs into the small brass tube on the manifold pictured above.