Converted from air box to K&N cone, now power breaks up at high RPM.

I've restored a 1970 Yamaha AT1 that serves as my around town scooter and occasional trail bike. One part that I could never find was a new air filter to fit in the stock air box, so after a year of riding around with a man-made hack job air filter in the box, I found a K&N cone that fit perfectly in the space once occupied by the air box. The bike starts and runs fine, that is until I have it at full throttle approaching the power band which is about 6,000 RPM, where it completely falls on it's face, any ideas on how I can solve this?

I would guess that you are getting more air flow with the K&N. I would try putting in a bigger main jet and see if it improves.

10yeareer is probably right that it's getting more airflow with the K&N. If you don't have any larger size main jets to work with you could try restricting the filter by covering say half of its filtering surface with tape. If the problem gets better or goes away then you've proved the theory.

:worthy: get rid of the K&N

prolly be better off with some coffee filters:smirk:

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