125cc 2stroke Tuning Guide

First things first, you need a good engine, as in no cracks and if you can't fit them yourself good crank bearings/oil seals (water pump where applicable).

The power listed on this guide is from experience not claims.

For sheer power for money and drivability the best setup would comprise of:

Malossi 172cc kit
Dell'Orto 25mm Carb (best match for the standard reed window anyway although gains and losses are present with any other carb fitted)
Gear up kit
Aftermarket Carbon Reeds
Runner 125 standard exhaust if you have a Typhoon

This package is really good for your money, it utilizes the stock pipe so it's pretty much stock looking and will still see a reliable 15+bhp in any fitment. It gives for over 75mph true speeds and will start first time every time. It should also last a long time as there's no real strain on the engine.

Upping the ante a little we can start looking at utilizing the transmission for best performance, also how to shift the power band of the engine to a taller peakier curve, power doesn't always make for a faster bike.

Next few things on the list would be:

Piaggio 180cc variator (Malossi MHR if you're looking to take things further)
Aftermarket Exhaust (matched stub goes without saying but is ALWAYS forgotten by amateurs)
Aftermarket Stronger Clutch Springs
Malossi straight inlet manifold
Stronger Aftermarket Reeds
Malossi CDI swap/similar timing advance

With the new pipe the engine will produce its power all at once in a certain rev range. This means we set the clutch to bite past the flat power at low revs and drop the roller weight slightly. This setup on top of the previous will see powers of up to 23bhp with correct setup although this is dependent on mainly the exhaust/match of the stub and setup.

Right then, now we get off the kiddie stuff and get busy with big boy cranks, bigger carbs and touch on porting slightly as well as other ways to gain power from the now "beast" like machine you've been creating...

I'd personally add to this setup:

Aftermarket Crank (BGM or Malossi)
New bearings obviously (although stock fitment bearings are good for 40+bhp and last longer than c4's... Also they rarely rev past 10k)
Malossi MHR or Polini Evo Reedblock (you can mod the stock to the same level as the Malossi or Polini item)
Bigger carb

With this setup on top of the previous 2 you can see 25+bhp but this is a fluke in most cases and will need case work to see the power reliably.

Aftermarket cranks are basically stronger, better balanced, higher revving, larger by volume version of the stock item with the exception of the big end bearing and different stroke cranks now available. A tried and tested crank good for 30+bhp is the Malossi although there are plenty of cranks out there now, adding a crank to this setup will give some performance increase but nothing really noticeable. It runs a lot smoother, revs better and can rev higher if your setup allows (the Malossi crank is good for 16,000rpm for instance) the webs in the crank are also filled (full circle crank) which means primary crankcase compression is raised. This means more fuel and air is packed into your cylinder, which means bigger bang, which means more power.

Stroker cranks are another way of playing with power. Basically they work by moving the centre of the big end bearing further out from the centre of the crank. Meaning the piston is moving faster at any given rpm. This gives you more low down power but takes a little messing about with. BGM cranks are pretty much plug and play but to make best of them they need port adjustment as well as matching of the special spacer plate to the barrel. With the 55mm crank there is a 3mm difference in the stroke obviously... Meaning the piston moves up and down by 3mm more. Merely moving the barrel up 1.5mm stops the piston hitting the head but you still have port bottoms that don't meet the top of the piston and aren't open long enough for the stroke. They usually make more torque over hp gains (loose hp in some cases) but torque is single turning force rather than running force. Meaning that acceleration is better, low rev power is up and wheelies are made a lot easier as torque keeps you in the air rather than hp.

Porting on the other hand is how you're really going to see gains, but most people don't actually know what porting is. Porting by definition is playing with the internal ports of the barrel so anything other than that is considered "flowing" work even though it's offered as port work by everyone who does it. "Porting" a crankcase is basically getting as much flow through an engine (measured in cfm or cubic feet per min) with the minimal amount of material taken out to keep primary compression nice and high. An unported crankcase will flow around 40-60cfm where as a properly flowed case will flow well over 110 cfm. As well the transfer feeds to the barrel are made wider so the barrel is fed through a bottleneck when the gas meets the port. The more you can squeeze a gas before it is compressed by the cylinder, the more power you have. That's how superchargers/turbochargers work. Flowing is a way of boosting the amount of air moved as well as the amount it is compressed before it hits the cylinder. Porting is the timings of the ports inside the cylinder. By timings I mean the degree of rotation the piston reaches before opening/closing the ports. This however has the most effect on performance as a barrel is designed with a port map to suite any need basically. You have to remember people designed these kits to stay cool, return good fuel economy, last as long as possible and not kill you if you sacrifice these points you will always go faster. There is lots of literature about covering this subject in a lot more detail. But with a bit of luck, I may have got those tiny little brain cells of yours firing a bit!