i was ridingmy yamaha aerox yq 50 up a hill the power just cut out and would not start again we have cleaned all the carb out and all is fine what could it be please help

adam, December 2010

cheers i add it rebored the other day set it all up today evrythink bak togeather all good put mixture down plug socket brand new battry added i turned it over started once then as i tried to pull thottle cut out and wont start again trying really hard to fire will only fire when i put mixture down plug socket hole fires up full trottle then soon as i release thottle a little it cuts out and i tries to kick it over again and it wont fire but still trying i can here it in the exhaust do u have any idea whot this could be(please help im pullin my hair out here ) thanks adam

adam, January 2011

if there is no stamp on it mate then its 40mm and std,so go for a 40.5 mm piston kit and get it rebored to suit the new piston.

adam bike doctor(of milton keynes motorcycle centre), January 2011

i forgot to add there is no stamp on it so i could of been replaced befor not sure so i need a piston kit then hopefully should be bk on the open road

adam, December 2010

ok thanks for the info so if order a 40mm that should be ok or will it have to be a bit bigger as there is grazeing on the old piston down the one side thanks

adam, December 2010

you need to loomon top of the piston thats in it and it will have a oversize stamp on it if its not standard,for instance a barrel that has been ebored to 0.50 mm will have the .50 mark stamped on the top of the piston.i think the standard size for a yamaha is 40mm but ill check and let you know,the correct place to measure a piston is at the bottom of it and from the back to the front as a piston is in fact oval and not completley round and aslo a piston is tapered its bigger at the bottom that it is a the top i know as i can operate a boring bar and have done thousands of rebores over the years.

adam bike doctor(of milton keynes motorcycle centre), December 2010

hiya thanks yes that is the problem but what size would need to get for a yamaha aerox yq 03 plate i mean wot size piston plz could you let me know thanks

adam, December 2010

Well it will need a new piston and rings probably. you will more than likely need a rebore as well. It's not a terrible job if you take your time. The only thing to do is strip top end and see what state your piston and barrel are in.

bb, December 2010

ive just been told its got no compression some 1 fitted a racing exust befor i brought it is this ,major could some 1 tell me plz

adam, December 2010

hiya thanks but ive tried both them things you sugested but still not start still tries to fire from kick and electric but just wont fire but we cleaned all the carb out and pins / jets all fine electics spark all good

adam, December 2010

You need to check for spark and fuel as a first point of call.
Spark. Take plug out and pop in HT cap. Put end of plug on metal frame and start as normal. this will earth it and should spark! Just replace plug then you know it is ok but still check for spark.
Fuel. Pop a bit of fuel down bore hole when plug is out. pop plug back in and start as normal. Does i fire for a few secs?

bb, December 2010