The a55 is easily the highest performance factory engine that tomos has produced. Although reverting back to a piston port inducted cylinder from the previous reed valve inlet a35, the a55 produces better torque and midrange than it's predecessor. Additionally, the a55 has enormous top end potential, and is a very capable 50:50 engine.
The a55 is also the most sought after tomos engine when it comes to high end performance, due mainly to the massive cases and case transfer area that is available from the factory. This increased case area enables large cylinder kits with massive transfer area to be fit without fear of needing to weld additional material on for proper gasket matching and sealing. This large transfer area isn't just for massive cylinder kits however. Even the stock cylinder benefits from large transfer area, and is one of the reasons why a55s are so fast when set up properly.
a55's are restricted from the factory through gearing and exhaust. by switching out to better gearing and a better exhaust, you can go over 45mph quite easily. i personally (mattology) made 48mph on radar with an a55. stock ports. there is lots of potential in that cylinder.
Any performance a55 build should include a balancing of the clutch drum or replacing it with the a35 variety if it is determined to be out of round.
Stock A55 port map and porting info
- This kit will fit the 2007 and up Tomos A55 motor that has 2 bolts holding on the intake manifold. If your moped has 4 bolts holding on the intake manifold to the cylinder is our reed valve cylinder kit part #505040 (does not come with wrist pin bearing). If bought, I recommend the 17.5 Dellorto PHVA carb to get power out of this kit. $180
- A55 cases have much larger area for transfer ports compared to A35 and A3 cases. This makes them prime candidates to install the larger Puch kits on them, such as 74cc gilardoni, or metrakit cylinders.
- Technigas Next R. Requires custom mount.
- Biturbo. Easy fit. Silencer tube commonly breaks.
- Tecno Bullet. Similar to Biturbo, doesn't break as easily.
- Motomatic, MLM, WEAK-ENDS, Treats, custom. Much better quality, more $$$.
Stock A55 sprint/st/lx gearing is 26X28 to limit top speed. You can change to 26X22, the stock A35 gearing, and go 35+ with no other changes. You will still have OK acceleration because it will stay in first gear longer.
Front sprockets can also go 24, 26, 27, 28(from treats) and 31tooth ( need to modify or run without side case for this one and it wont fit with the stock flywheel )
Rear sprockets 28, 26, 24, 22 and 20 tooth (Need to bevel the rear wheel for chain clearance for this one folks)
Some Tomos Revival, Streetmate, and Arrows use a different front sprocket and the Arrow uses a different rear sprocket from all other Tomos'(scooter wheel)
Carburetors and Air Filters
Tomos A55 mopeds come stock with a tunable 14mm PHVA carburetor. These use the larger 6mm Dellorto main jets.
If you run it without the air box, and with a bigger jet, it goes maybe 1/2 mph faster, but looses a lot of low end torque, and climbs hills worse. Put back the stock air box, but cut the “elephant trunk” off of the rubber connector between the carburetor and the air filter, and round out the rubber into a bell, instead of a reverse bell. The still air in the middle of the frame is somehow easier for the engine to breathe in, than the moving air outside. Flip the stock air box 180 degrees, so the outlet is facing out, instead of into the hole in the frame, and the bike looses power, about 1 mph. Anything else you do, like drilling holes in the air box, making the inlet tube bigger, or running a hi-flow foam filter, causes a noticeable loss of acceleration and power. Apparently Tomos has found a shape and size that silences the intake, houses the air filter, and somehow helps the engine breathe better at low to mid rpms 
Please see Racing transmission fluid for your Tomos.