Tuning the Vespa PX scooter involves a number of steps that must be performed together to achieve the appropriate increase in engine performance. This tandem work involves the carburettor settings, the engine cylinder and the exhaust system. Though lesser improvements can be achieved by using one of the three, the full potential of scooter tuning will not be realised.
Disconnect the engine from the scooter by separating the electrical wiring by hand and cutting the control cables with a cable snipper. Use a socket wrench and crescent wrench to release the engine bolt, the rear suspension bolt, and the rear wheel and hub. Use a socket wrench and screwdriver to remove the carburettor and old exhaust. Pick up the freed engine and place it on a workbench. Use a screwdriver, crescent wrench and socket wrench to disassemble then engine down to the bare engine cases. Place all the parts in a bin for safekeeping.
Take the cylinder gasket from a new performance cylinder and trace the added port area that will need to be removed from the cylinder side of the engine cases. Use a Dremel or similar grinding tool to open up the engine case cylinder ports. Perform the same opening work on the intake port on the top of the engine if matching a larger carburettor and using the new carburettor gasket.
Wash the engine cases clean. Pop out the old bearings and seals. Install new bearings and oil seals with an appropriate size socket and hammer (freeze the bearings first for easier installation). Reinstall all the removed engine parts back together using a socket wrench, crescent wrenches and screwdrivers. Apply new engine gaskets when closing up the engine cases. Slide the new performance cylinder onto the engine after placing the cylinder gasket in position. Tighten down to factory specifications with a socket wrench.
Reinstall the engine into the scooter, connect the swingarm bolt and rear suspension bolt. Reconnect the rear wheel and hub. Reconnect the electrical and control cables using new cables. Use a socket wrench to install a new spark plug and connect the ignition cap to it.
While the rebuilt engine sits in the scooter body, discard the old exhaust. Take a new performance exhaust and line it up with the exhaust manifold sticking out of the bottom of the engine cylinder. Use a rubber mallet to tap the exhaust mouth onto the manifold. Use a socket wrench to attach the performance exhaust bracket onto the engine body elsewhere under the scooter. Tighten to satisfaction and tighten the clamp securing the exhaust mouth to the cylinder.
Use a screwdriver to remove the filter off the carburettor. Identify and loosen the main jet assembly in the carburettor. By hand pull the assembly out of the carburettor when loose. Use a towel wrapped around pliers and carefully loosen the jet itself from the assembly if unable to do so by hand. Replace the jet with a larger one designated by the number on the side of the jet (this may require multiple tests to find the right size for the new cylinder).
Put the reassembled jet assembly back into the carburettor, and screw it securely with a screwdriver. Reinstall the filter. Use a socket wrench to bolt the carburettor and its cover to the top of the engine intake. Reconnect the fuel line to the carburettor.
Start the scooter engine by kicking it over until the fuel flow reaches the cylinder and starts the combustion process. Adjust the idle jet on the side of the carburettor until the engine rotation ticks over reliably without sputtering or dying out. Take the scooter off of its kickstand and outside for a test drive. Use a socket wrench to check the spark plug regularly while testing to confirm a chocolate colour on the tip (optimum combustion of fuel, air, and gas).
Have an experienced Vespa mechanic or friend show you how to rebuild your scooter engine the first time if you are not sure how to perform a rebuild and tuning. Keep in mind tuning involves the use of aftermarket parts and changing the original use of the scooter. Mistakes can happen and may require replacement of ruined parts.
If the spark plug removed when test riding looks goopy or oily black, the engine is getting too much fuel and not enough air. The main jet in the carburettor will need to be replaced with a smaller one to cut down the fuel flow. Alternatively, if the spark plug shows a powdery white tip, not enough gas is getting to the engine, and it will overheat.