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Central Scooter Club > Engine Derestriction, Performance & Tuning > How to modify a 50cc scooter

Title: How to modify a 50cc scooter

JDi - March 1, 2008 07:45 PM (GMT)
Hi guys,

I thought I'd create this post to lump together various answers from many threads, which all have one thing in common... how can I make my 50cc scooter go faster???

Let's start with the legal stuff... if you are 16 years old you can only ride a moped. The definition of moped is a bike with an engine capacity of up to 50cc and a top speed restricted to no more than 50kph (31.5mph). If you carry out any modifications to a moped including derestricting it you are breaking the law, due to the following:

Your moped is now classed as a motorbike and your licence does not allow you to ride one.
Your insurance is valid for a moped - it is now invalidated.

What does this mean? It means if you get caught you could face a hefty fine, points on your licence, and your scooter could be siezed and crushed. You'll then face a good few years of stupidly high insurance premiums, if you can find a company who will cover you.

There you go - you've been warned!!!

If you're 17, you can ride a scooter or motorbike up to 125cc with an engine power up to 14.6BHP, providing you've passed your CBT. So it kinda begs the question - if you're old enough to modify a 50cc, why not just buy a 125cc? A standard 125cc will run rings round a tuned 50cc all day every day with total reliability, plus it will handle better, stop better, ride better and be a bit bigger.

Hey, it's an option!!!

But... let's assume you're over 17, you've done your CBT, you have a 50cc scoot and you wanna do some modding, damnit!!!

Okay, let's start with the basics.

Firstly, running in - if this has already been done then skip this bit. There's loads of different ideas and methods on how to run in a brand new scoot, ranging from "ride it like your granny" to "ride it like you stole it". This is my method, which is based upon the method I use to run in freshly rebuilt car engines, as taught to me by Dave Baker of Puma Racing fame. First step - change the oil, the thin green slurry that these bikes ship with won't even lubricate your gran when granddad gets his weekly viagara. Put in some half decent 10W40 mineral oil - brand is totally unimportant, you'll be changing it again later the same day. Also change the transmission oil for something decent, you'll never have to change this again. There are how-to guides for both of these on this site. Also, change the spark plug for a decent one - NGK CR7HS get a lot of votes. Now take the bike out on the road, and give it 10 seconds worth of full throttle followed by 20 seconds of coasting. Do this ten times. What you're doing here is forcing the piston rings against the bore walls so they wear to a nice tight fit. Now go for a decent ride, say around 100km, and during this ride you want to steadily increase throttle usage and speed until you're using bursts of top revs and full throttle, but with plenty of cooldown coasting. Now you've worn off around 99% of the high spots in your engine, so change the oil again. The engine is now ready for anything you can throw at it, and over the course of the next 400km speeds will increase.

Servicing - the most important thing of all is regular oil changes. Change the engine oil as soon as you get the bike, straight after running in, and every 1000km. After 1000km fill up with fully synthetic 10W40 motor oil, use mineral oil before this point to help the engine run in properly. Change the gearbox oil for some fully synthetic gear oil as soon as you get the bike then forget about it for 20,000km. There are how-to guides for both of these on this site. Change the spark plug for an NKG CR7HS, clean it every 1000km and change it every 10,000km. Make sure the air filter is cleaned and oiled. Do all this and you maximise the performance of any mods you carry out, and you maximise reliability.

Derestriction - there's various restrictions depending on your model, there's a whole section of this site dedicated to it so go read. Don't derestrict until you've done the running in procedure. This gives you an instant 10mph increase in top speed to 40mph.

Modifications... this is the bit you wanna read right??? If you don't read anything else on this page, please read the servicing paragraph. Also please remember these scooters were designed and build to be cheap economical no-frills transport, not sports bikes so if you really must have the fastest scooter ever go buy a second hand Gilera or Aprilia. I'm only gonna go as far as what I would class as "Stage 1" mods, which is really as far as you can go without changing any major components or increasing the capacity beyond 50cc. Above this stage, things get costly and it's impossible to justify the costs of heavily modifying a 50cc when you can just go and buy a 125cc. So... head on over to and from their Chinese Scooters page order the following parts:

* A set of 4.5g rollers
* A #90 or #85 main jet
* A freeflow air filter

You can get the while lot for less than 20 shipped. Fit these parts (there are how-to guides on this site for all of these), and you greatly increase acceleration by letting the engine rev higher from standstill, and you increase overall performance by giving the engine more air and fuel. This is good for around 45 - 50mph and is about the performance limit for a Chinese 50cc. Any faster than this and I have serious safety concerns about the suspension, brakes and handling and to modify these would be an extremely costly excercise when a 125cc bike would have upgraded components as standard. Plus fuel consumption will suffer.

So there you have it! Please note that this is not meant to be a definitive guide, nor is it a guaranteed "how-to" bible for every make of Chinese 50cc. All of the above is in my opinion onlybased upon my experiences with a Baotian BT49QT-9H and is made available on a no liability basis - if anything breaks, don't blame me!!!

Admin - worth stickificating?


### UPDATE 10/02/2008

I thought I'd do an update to this thread, and share some of the experiences I've had trying to modify the engine of a 50cc scooter. I think I can sum up all the condensed wisdom I've gained in just one key sentence...


The ONLY mod worth keeping has been the roller weights, which have had a massive positive impact on acceleration and hill-climbing ability. All the others have been a waste of money, time and effort for absolutely no performance gains whatsoever.

Fitting a freeflow filter and bigger jet sounded like such a smart idea, and it made a lovely noise when revving!

And then it rained. HARD. And the filter got waterlogged. Thankfully, I have breakdown insurance... the RAC man took one look under the seat and said "Your filters waterlogged mate, seen it a million times!", chuckled a bit, strapped the bike onto the flatbed and away I went in the big twuk wiv de flashin lights.

Now I know I could have fitted a filter with a shield, but to keep them waterproof they are just as restrictive as the stock airbox so why bother?

Performance... no difference at all. No increase in acceleration, no increase in top speed, marked decrease in fuel consumption and on a long ride the loud BLAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHH coming from under the seat soon does your nut in. probably sounds great on a motorbike with varying throttle and revs but on a one-note scooter it's annoying.

Plus, I found that an #85 jet was a fraction too lean causing the bike to run hot, and a #90 was a fraction too rich causing the bike to bog down slightly on full throttle. if I had the time, a set of metric drill bits in 0.1mm increments and a pillar drill with a vice bed then possibly I could get it spot on. But quite frankly I couldn't be arsed to do all of that for a negligible net gain so I'm back with my stock airbox, the original #79 jet, no decrease in performance and over 100 miles to the gallon again!

My next mod is going to be fitting a set of +2000rpm clutch springs and a +2000rpm torque spring in the clutch assembly to give me higher revs on initial take-off, when this has been done I'll post another update.

andyscooter - March 9, 2008 08:14 AM (GMT)
what a write up there

i found a freeflo with a mesh and metal cover

worked a treat and didnt get waterlogged

111kurt111 - March 9, 2008 09:17 PM (GMT)
Cheers i think thats cleared everything, to notch well done.


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