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Will bigger valves in my 125cc pit bike cause issues?

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Please keep in mind this the first time I have ever opened a motor and tryed this sort of thing,,, I did do my home work before starting.,.,.,Ok,,I have three 125cc motors, 1 works mint no issues near new another is brocken casing and the GPX engine needs new oil rings.....

,Using the Pitster Pro 125cc GPX motor,,, Its been smoking real bad due to needing oil rings replaced,,,Now instead of getting a new piston kit for it I decided to take the head off my other my other pit bike im a cheap skate that way.......

Realizing the cylinder and piston are smaller by about 5mm on the replacement head I ended out having to swap pistons as well now when doing this I noticed the replacement head has much smaller valves on it so I decided to leave the original top part of the head (with bigger valves) on there only replacing the cylinder and piston.... Now slapped all back togeather,,, Boom she fired on first kick was awesome but runs hairy like its being held back or something not sure if this is due to having a larger top end valve whatsy on it and its getting drowned in petrol or if its just needing to be tuned correctly....

I want to know if having the bigger valves on there is causing this and what I have changed has created this problem or is it simply a matter of setting the timing up correctly and tweaking the carb????

Cheers to anyone who posts usefull info.....

Edited by Mattrix

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