When I have my airbox lid off I've noticed a knocking noise that occurs when you give my 400ex some throttle. It sounds like a power knock but occurs at higher rpm's than a I would think it should. If i put the lid on it doesn't do it near as bad. My bro has nearly the same setup as I do and his does the same thing. Where does this knock come from?

I have a 400EX big bore 440 with powerbomb header and white brothers E-series slip-on. I have a DynoJet 165 main. fuel screw 2.5 turns.

 

Do you keep the same jetting with the lid off as when the lid is on? If so,you are running lean with the lid off. You ALWAYS need to rejet when you remove the airbox lid!

 

Yes I have re-jetted. DynoJet 165 main. Plug looks good with box lid off. Putting the lid back on it should be running rich. I guess I can double check the plug again but I'm thinking jetting is fine. Thanks for the suggestion.

 

Surly someone knows whats going on, I've got to wheelers doing it here. Maybe I need to explain better. Let me give it a shot.

Basically when either my or my bro's 400ex is under a load at any speed it has a loud knocking sound. It sounds just like a power knock. My concern is that with the air box lid off the power knock exist at a much higher rpm than with the lid on.