now i need some help with my jump timer

Um... If you're using a Yvel# why aren't you just applying a gravity to it?

ie.

Graphics 640,480

y# = 400
yvel#=0
gravity# = .1
groundlevel = 400

Repeat
Cls

; draw the ground
Color 0,100,0
Line 0,401,320,401
Line 320,301,320,401
Line 320,301,639,301


Color 255,255,255

Text 320,0,"Press SPACE to jump, ESC to exit",True

For x=0 To 639
	y=y+yvel
	yvel = yvel + gravity
	
	; change of floor level
	If x<320 Then groundlevel = 400 Else groundlevel = 300
	
	; hit the ground
	If y > groundlevel Then
		y = groundlevel
		yvel = - yvel * .2 ; bounce
	EndIf
	
	Plot x,y
	
	If KeyHit(57) Then yvel#=-5
	
	Delay 10
	
	If KeyHit(1) Then End
Next

Forever