Below suggestions are about two stroke scooters that came from the factory with disabilities, that can, luckily, be cured for the most part. It is important to understand that to get the best out of your bike, you need to make several changes and updates and you simply cannot gain a lot by simply changing one component. I have listed below some ideas things where I’d focus (in the order of priorities) to make the scooter faster and gain in top end speed.
First things first – get your scooter derestricted. This means checking your exhaust, CDI and variator.
1. Exhaust
- Difficulty 2/10
This is often the easiest thing to check or replace. This would give you gains you feel instantly. Scooter (even 4-stroke scooters) exhaust come with restrictions to reduce speed, noise and make the bike environmentally friendlier. If resources are limited then use a tools to derestrict the exhaust. Find a small washer welded in the pipe, or remove the dummy pipe.
Getting a performance exhaust would be a best option. Keep other potential tuning plans in mind (e.g. larger cylinder) and get an exhaust meant for the proper CC.
Exhaust system change is simple procedure and will take you probably 10-20 minutes to replace with right tools.
2. Variator derestriction + Oversized CVT
Difficulty 2/10 (for derestricting)
Difficulty 4/10 (for replacing the CVT)
Factories install restrictive washer between front pulley‘s. This is to avoid the plates to push the variator belt to front pulley’s outer diameter and limiting the maximum speed of the scooter. This is like you are riding a geared bicycle and cannot change to the largest sprocket in front. By derestricting the CVTit affects positively bike’s acceleration and top speed.
Again, if you have more resources, consider oversize pulleys to the front and rear (full CVT set) with a longer belt and you feel more effects.
Getting proper CVT roller weights is often trial and error to find a proper balance at which RPM the engine would “shifts gears”. If rollers are too heavy then acceleration would suffer and engine would change to higher gear faster (e.g. try accelerating with the car in 5th gear). If rollers are too light then the engine would work at very high RPM to change to next gear and you might happen that you do not use the engine potential properly. As rollers do not cost much then get couple of sets and try them out.
3. CDI update
- Difficulty 1/10
Off the shelf CDI-s come with rpm restrictions. Find yours, and replace with aftermarket option and you feel instant gains.
4. Carburettor change
- Difficulty 5/10
Carburettor plays an important part in the combination if the inlet is restricted and/or too small you cannot get sufficient gains. Often 50cc engine comes with 12-13mm carb and this needs to be replaced. Get yourself larger inlet diameter carb, choose correct main jet and get yourself going.
When choosing the main jet then keep in mind simple calculation – carb inlet diameter multiplied by 5. E.g. 21mm x 5 = 105 for the main jet or 19mm carb x 5 = 95 for the main jet. In motocross you would multiply this in addition by 0,95 to get the engine a bit to the leaner side (e.g. 21mm carb x 5 x 0,95 = 99 or 100 for the main jet) but this would already require variable curve ignition to work properly and safely.
Also, keep in mind that bigger is not always better! For 50cc-70cc engine 19-21mm carburettor is sufficient. 70-90cc engine consider 24mm. Why? If you choose for example 32mm carb for the 50cc engine then simple physics come into play – the cylinder does not create sufficient vacuum to suck air/fuel mixture to make the carb work. With too large carb you will get no or very rough idle it is very difficult to get the jetting right. Smaller size carb would benefit your acceleration and throttle response.
Replacing carb takes a bit more time to get sorted as you have to get the throttle, fuel and oil lines over, new gaskets, etc. Keep in mind that would probably need a larger inlet manifold to fit your new carb.
5. Bigger cylinder
8/10
If you have finances for larger cylinder then go for it. Aluminum and nikasil coated would be an option to consider. If you have less funds then larger bore cast cylinder will do.
If you consider doing it yourself then youtube is full of instructional videos how to do this. From experience couple of pointers:
- Keep your work area clean
- Make sure to get correct thickness base and head gaskets. These are important to get your squish right. Measure several times to get this correct or otherwise you will seize your engine.
6. HPC crankshaft
Difficulty 10/10
Scooter engine primary and secondary compression are out of balance. What it means is that the crankcase volume will accommodate way too much fuel mixture for the cylinder volume. This does not create sufficient enough pressure and engine cannot work well at higher RPM.
HPC, i.e. High Primary Compression (HPC) crankshaft will reduce the crankcase volume and thus increase the primary compression. This will increase the pressure in crankcase while the piston is moving down will improve the speed of mixture to move into the cylinder.
To replace the crankshaft it requires removing the engine block and open the crankcase. For this you need several special tools (to remove stator, crankshaft bearing installation and removal, etc) and I would suggest you to find an expert to do this for you.
It will take a few good hours if you plan to do this yourself having all the relevant tools.
7. Better ignition
Difficulty 8/10
You would have to
The microprocessor controlled CDI unit offers the additional advantage of a variable ignition timing. With increasing rpm the timing shifts towards top dead centre. Thus you win around down clearly in torque and relieve the engine thermally in the upper speed ranges. This is clearly felt in the power-band well and
8. Reed valves and petals
Difficulty 3/10
Lets start with petals. You have an option of glass fibre or carbon fibre reed petals. You rarely find metal petals nowadays and they are not really an option. But, which one to choose – carbon or glass fibre – and does it affect performance? In short, yes it does affect performance and there is a bit of science related to it.
Three aspect that are important:
- weight – less weight means faster response time
- material stiffness – more rigid petal would mean less resonation and more stability at high rpm but too stiff will mean lower throttle response time
- adhesion to the valve – airtight seal at the down stroke situation.
Valve direction and size are also important.
9. Cylinder porting
Difficulty 10/10
A simple thing that anybody with dremel tool can do is to drill out imperfections from the cylinder channels. But if you truly want to gain, then keep in mind that there is a whole science behind it. This is where it is important to make things work in concert from the inlet, to cylinder, to exhaust and ignition and figure out what you want to achieve – is this a motocross bike, scooter racing, dragracing. It all affects what cylinder should be used and what the ports should look like. Talk to expert.
10. Buy a different bike
And this is an honest reply I have had to give some of my clients. From time to time you get clients with some random engine which crankcase is limited in size, inlet system is custom made for just that bike, etc. For example one TGB R50x engine I had on the table recently. This is a old Morini type engine with its own modifications – you could not install larger CVT system, change crankshaft, inlet tract is oddly built, etc. So the only thing I could suggest – let’s change CDI, exhaust and install a larger carb with proper jetting. So we managed to turn this bike from 45km/h into a 75km/h top speed scooter. But the acceleration and throttle response were really poor. To improve that would have meant custom built crankshaft, adjusting case, installing ignition, etc. This would have cost a fortune and it makes much more sense to buy a faster scooter or an engine that can be tuned more easily.
Happy tuning!