i have a aftermarket pipe on my 89 lt250r and a rad valve,how should i rejet the carb?it
bogs alot in the lower rpms

 

You need to rejet. Make sure your filter is clean and buy some plugs. Start with the main.
You have stock carb?
It should be around #260 for that set up, and once that acomplished work with your jet needle.
From experience, 1/4 to 3/4 throttle is always rich on these bikes. You need to lower the needle by raising the E-clip to the 2nd groove from the top.