Looks from your profile like you just bought these--I just bought mine on Ebay too, and found out somthing interesting that might help you.
They put a bigger-than-normal carburetor jet into Batavus mopeds when they were new to prevent them from seizing during break-in. These big jets make them run rich, which makes the bike go slower. So you might want to check your carb jet to see if it's the right size.
To check it, unscrew the big brass bolt in the side of the carburetor and take it out. The jet will be screwed into the end and will have a number stamped in it. The correct sizes are 50 for a 25 mph bike, 52 for a 25 mph bike and 56 for a 30 mph bike. If your jet is 2 sizes bigger than the numbers listed above, you need to switch to the correct size.
They use the same jets as Puch maxis, which are pretty easy to find. Mine had the bigger jet in it when I bought it--I switched them and went from a top speed of 22 to 28.