I am having problems with my 450r. It was all stock then I had the hrc kit and only installed the end cap. Ran fine and then it is having problems in the higher rpms. It is just spittering and sputtering. Well changed plugs and still did. Then I took the end cap off returened it to stock looked it over made sure no wires where disconneted and drained the gas put new in and still does it. It has no problem idiling or anything just wont go it high rpms. If anyone with some solutions would help would like to get riding again thanks

 

When you uncorked the exhaust, did you rejet???? If you didn't, you were running lean (lean means hot!), and may have done engine damage! Honda jets lean at the factory to meet emissions. What you did makes it even worse........

What did the plug look like? Whiteish???

 

seems a lot like jetting. probably main jet