Downgrading from stage 6 70 to 50

Pretty sure pistons have gone ( got low compression and wont start) so gonna 50cc it instead of the 70cc can i put a 50cc into the 70cc head as its a £300 kit

 

Just fit the 50cc barrel /head down jet a little.

 

Stage6? Single ring piston? Put a new ring on, unlikely you need a full kit

 

It is just a single ring but all the components are fine just got low pressure for some reason