I have just finished re-jetting my Batavus Starflight. I de-carbed and cleaned the exhaust port and exhaust, set timing, cleaned carb and cleaned and lubed all the moving parts. In other words, I tried to get it back to original condition befor I started. I really need a new drive chain but it is not too bad.
I removed the air box/cleaner and still four stroked at max speed, which was about 20 mph on the speedo.
The carburetor had a reduced diameter between the slide and the manifold. The intake side of the carb and the manifold were the same size and larger than this "restricted" section. I drilled this section out to the same diameter as the rest of the intake. The immidiate result of this was that I ran really rich at idle. Besides smoking, the most obvious symptom of this was that if I turned off the gas at idle, as the carb gas level fell, my motor would rev way up before dying. Looking at the carb slide, it had a slot in it, positioned directly over the jet, which allowed the mixture to get past the "step" created by the restricted section of the carb. When I removed the restriction, the whole length of this slot was exposed and sucked fuel out of the jet like a shop vac. I filled the slot with epoxy gas tank repair putty and left it lean initially.
I have the advantage of owning a clock bushing tool and reamers which allow me to size jets easily so I proceeded as follows:
With the air box off, I reduced the main jet size and tested the mixture by opening the throttle wide and using the choke, fortunately the old fashioned flap over the intake, to control the mixture on accelleration. With too small a jet, I had to use some choke at whatever speed I peaked at. I increased the size of the main jet a little at a time until when I reached maximum speed, with the throttle wide open, any amount of choke would slow me down.
Then I opened up the slot in the slide a little at a time until the idle mixture was rich enough to maintain an idle.
I made a foam air cleaner and verified that the mixture did not change.
When I ride now, I get best acceleration by rolling on the throttle as rpms increase to maintain a good mixture. The ped tops out at 25mph on the speedo and when I go somewhat downhill, it will hit 30 without 4 cycling.
The Batavus is still a little slower than my Motobecane Model 7s which are also stock except for air box and jetting. I suspect the exhaust on the Motobecanes is a better design.
I hope this helps.