Whats wrong with this?

Question

i have just bought a yamaha aerox 50cc and i was test driving it today and noticed it will only go up to around 20mph? but sometimes it will go all the way past 40 i do full throttle it both times when i try to achieve maximum speed but i did notice if i was going 40 plus and when i brake it will not do 40 again unless i hold the front brake and pull the throttle all the way back then take off again? anybody know anything about this? it has also just had new pistons fitted aswell......not sure when they were fitted though thanks alot =) also how could i fix it?

Answer 1

A car, or a bigger bike, the engine is big enough that it is very seldom completely flat-out.  A 50cc bike is USUALLY runnning flat-out, total maxed.

So factors that you'd hardly notice in a car make a huge difference in a 50cc bike.   A headwind that you might not even notice if you were walking might cost you 10-15 mph on a 50cc bike.  A very slight uphill grade, again you might hardly notice it on foot, you wouldn't notice it in a car at all, but on a 50cc bike it will cost you another 10 mph.

This is just one of the characteristics of an engine that small.  It will always get you there sooner or later, but it won't always reach the same top speed.

Answer 2

Your brakes aren't retracting after you release the lever. Or your oil/gas isn't mixed good. You get a speed boost when more gas than oil is sucked through, and lack of performance when oil is more. Don't mix it in the bike. mix it in a container first, then pour it in.

Answer 3

When you say that having applied the brake and then it won't go as fast it sounds like the brakes are binding after they are applied, if you try pushing it with the engine off is it difficult to move? Are the discs very hot after you have ridden for a while?

If that is is the case you will need to get the brake(s) serviced, it is something you can do yourself – but only if you are sufficiently experienced. Your bike should only have one piston in the engine (and one in each of the brakes).