I guess you mean you have change cylinders and not heads?
Unless you have Siebenrock cylinders and pistons it’s not advisable to keep the old cylinder-heads.
Siebenrock is identified by a “S” on the cylinders (upper side, close to engineblock). They are also marked “100” but that also goes for the standard BMW-cylinders.
When my Basic had a standard engine I once managed to get a top speed of 177 km/h (GPS) but it takes time… Up to 150 it accelerates pretty well.
If you have BMW-cylinders I would have changed the heads (or got them modified). The cheapest option might be to get some new heads on Ebay.
When it comes to carbs it depends on what you are looking for, if you want max-speed then bigger carbs are nice, if you want low end torque then keep the carbs you have.
I have ridden an original basic in 150-160 km/h for hours and it’s not a problem, but if you don’t have a good fairing you get tired and there is a lot of noise (wind-related).
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