need help please!

[skinindustries]

I have a 99 yz 250 and I just put a new top end in it I have not rode it for about 2 weeks now when I went to start it up everything was normal till I started ripping through the gears it started to make this god aweful rattle like you dropped a nut in the cylinder and ran it like that just rattling and pinging like no end I don't know what is wrong with it I did the top end changa by the book so if anyone could help that would be great..... Thanks brian

 

[IndyMX]

You're going to have to open it up to find out..

No one is going to be able to tell you what's wrong, without looking in the engine. Everyone that will say what's wrong on here is just guessing.

 

[AV8R]

Old gas?

 

[whenfoxforks-ruled]

All you have to do is look at the plug. Lean, detonation? If you keep riding it, it could soon break, and be easier to find the issue, and cost more to fix.

 

[2strokerfun]

Did you warm it up before you started "ripping through the gears?" Did it only make the sound when you were accelerating??
I'd probably start with pulling the head off and looking at the piston crown to inspect for damage. If crown is damaged, you have to determine if it's detonation, or parts of a bearing being hammered. If it is detonation, you may have an air leak, too lean of jetting or really bad gas. If it's a bearing, well, lots of work will be needed.
If no visible damage to crown, pull jug off and inspect piston, cylinder, rings, top rod bearing, see if crank rotates smoothly, check lower end bearings and rod bearing.
And Indy is right, I'm guessing, but guessing in a somewhat logical progression (to me, anyway).