Replacing an exhaust with a carburettor-type engine is more than just bolting on a new pipe. Whether it's a car or motorcycle or scooter, you are likely going to need to make sure your carburettor is running with the correct jetting for the new exhaust.
Carburettors are designed to mix air and fuel together as the engine runs and speeds up. This mixture is necessary because in combustion-type engines both fuel and air are required for ignition. The combustion then translates energy to the engine pistons which in turn drives the crankshaft and the car as a whole.
Carburettors provide the right mixture via jets which are brass parts with small holes in them. The holes regulate the amount of fuel that goes through the carburettor. Bigger jets allow more fuel and smaller jets allow less. The rejetting process figures out which size jets is needed to keep an engine running with the right mixture. Too little fuel and the engine may starve or overheat at high RPMs. Too much fuel and the engine could flood and bog out.
To Change or Not to Change
If you're just swapping the exhaust with a new one of the same type, there's probably no need to rejet your carburettor at all. Where it becomes an issue is when you are installing a stronger or tuned exhaust. Doing so raises the running temperature of your engine and more fuel is needed to compensate. This requires a larger jet. Ergo the rejetting tuning the carburettor to match the new exhaust's demand.
Don't Modify Existing Jets or Swap Them
Jet parts are so inexpensive that there is no need to try to modify one yourself at home. First, these are machined parts fabricated by exact measurements. You will never be as accurate. Second, the home-modified jet will not run correctly to factory specifications, causing engine problems inconsistent with factory settings.
Additionally, jets by different carburettor makers come in different ratings. It is not possible or smart to assume a jet for a Mikuni carburettor will work just fine on a Dellorto unit. Use the jets provided by the carburettor maker for your unit.
Many manufacturers are concerned about making sure they pass emissions tests. As a result, the lower end jets for idle and low gear driving tend to be leaner than they should be since these are the engine stages where the most pollution is produced. Second, because carburettors can be used in the mountains of France or the flats of Ohio, manufacturers will also ship the upper end jet bigger than it should be in some cases. If it runs rich, then the maker can at least be assured it didn't ruin an engine.
Rejetting usually requires opening up some part of the carburettor. Make sure to use new gaskets when reassembling the parts. Failure to replace the gasket can lead to leakage or grit getting into the inside of the carburettor.