Well, I just bought a 400ex about a week ago.
It has a new piston and a couple of new bearings in the engine.

The machine runs absolutely great (I have been a little easy for the bike because the new piston has to be broken-in).

Now I'm gradually giving it some more throttle.
But when I go down the road at speed and high-rpm I hear a rattle noise in the engine. (I had a Blaster before so not really used to 4stroke, maybe this is normal?)

When stand-still it sounds normal, doesn't even rattle when I rev the engine. It's only at high speeds and high rpms.

At first I thought it might be the chain rollers, but when I pull the clutch the rattling is gone, so it has to be in the engine.

Any clues?

Some extra info:

The bike hadn't been run for a while when I bought it
I use 98 octane fuel

 

i culd be the vavles mine are loose and are making rattling noises you should adjust them everyonce in awhile

 

Could be, but wouldn't I hear them rattle at stand-still too?
I will check the clearance though, although I doubt it's off because the guy I bought it from has a quad-store and he had the engine apart so I think he would've checked the valves.

 

Check any skidplates you may have - they are notorious for making bad noises that sound like something in the engine.

 

My headpipe comes loose every now and then, might check that too.

 

Well, I checked about everything, but found nothin'. I rode it again for about 2hours, and I think it's just the normal valve-noise all 4strokes have. Think I noticed because I was used to riding a Blaster without any valves. [img]i/expressions/face-icon-small-happy.gif[/img]

Anyways, thanks for the replies.