Hi all, I have recently changed a piston on my yamaha aerox and now it will only start with the choke on and with full throttle but will only rev up to about 2000 rpm but as soon as you release the throttle it dies again, all jets are clear , float is ok, new spark plug, new air filter and a new ht cap. any suggestions please, thanks.

Is your piston in the right way around?

Yes I have got the arrow pointing forwards at the exhaust port, I'm sure that's correct ? But now it will not fire at all and is unregularly backfiring ?

Try clearing the fuel from the exhaust by shutting of the gas,pull the plug and crank it over. Then a new plug and only turn the gas on long enough to fill the carb and see if it will start and run untill out of gas. sounds like dirty carb flooding the motor.possibly float stuck

Piston arrow to exhaust=right

so you've had the barrel off, perhaps you have unfortunately left a torn piece of the original cylinder base gasket trapped under the new one?

That's a good suggestion thanks alot but I'm quite sure I did a good job of cleaning the 2 surfaces and the old gasket did come off in 1 piece, also there is fuel getting to the cylinder as after a few kicks there is fuel on the plug and I have a good spark it just won't go.

a few questions...

Why did you replace the piston?

What size was the old one, What size is the new one?

I suggest a compression test can you do one?

I changed it due to poor compression and then realised the ring had snapped when I took the old piston out, the ring fell off the piston into my hand, and I'm not sure I ordered it from pedparts.com it said it was for any water-cooled aerox so I really do not know what else I can do ?

But how do you know you bought the right size one?

Have you still got the old one?

Take the barrel to a rebore mechanic. They will supply a new piston and re-bore the barrel to match it.