CBR250 exhaust upgrade help

[Hurrie]

Hey KB'rs

I'm considering upgrading the exhaust on my cbr250r (MC19) with a MC22 straight through race can, i know that there is little to gain in terms of hp but i would like the extra sound (18000rpm isnt loud enough :P).

my problem is that i was told that if i put in a straight through race can than the bike would run too lean so i would have to change the jetting and another person told me to get a highflow air filter.

i'm wondering if that's necessary or if putting it on is just good enough? or if there's anything added i should do?

i would appreciate any thoughts or ideas you guys may have and thanks in advance!!

[Gibbo89]

Hey KB'rs

I'm considering upgrading the exhaust on my cbr250r (MC19) with a MC22 straight through race can, i know that there is little to gain in terms of hp but i would like the extra sound (18000rpm isnt loud enough :P).

my problem is that i was told that if i put in a straight through race can than the bike would run too lean so i would have to change the jetting and another person told me to get a highflow air filter.

i'm wondering if that's necessary or if putting it on is just good enough? or if there's anything added i should do?

i would appreciate any thoughts or ideas you guys may have and thanks in advance!!

the jetting is pretty neccessary i thought, if you just put it on and do nothing with it, then why put it on at all?

get some quotes from casbolts or somewhere and see what rejetting it would cost perhaps?

[Hurrie]

the jetting is pretty neccessary i thought, if you just put it on and do nothing with it, then why put it on at all?

get some quotes from casbolts or somewhere and see what rejetting it would cost perhaps?

what does the jetting actually do though? does it just add more fuel to the mix so it isnt running lean anymore? and should i do that before or after i put on the new exhaust?

[Grasshopperus]

Have a read up on carburetors on the net.

Jets are little nozzles inside the carb that the fuel passes through to come out as a fine spray. There are different jets in the carb for different throttle positions but the only one you'll probably need to change is the main jet.

You can do this yourself pretty easily. You need to remove the carbs (they come out all connected together so it's not that daunting), remove the trays at the bottom of each carb (known as bowls) and then you can unscrew the main jets using a small flat head screwdriver. Find out what the current size is (it'll be written on the jet itself) then go into a bike shop and ask for 4 new ones that are one size up. You'll probably have jets sized 138 and the next size up will be 141 or something like that, they increment by 3 each size or something.

Find a manual and you'll be sweet. Take heaps of pictures along the way and don't force anything.

You could always just do the exhaust by itself and see how it goes though.

[Hurrie]

yeah i tried reading up on the net but it sounded a bit daunting, a friend of mine told me that since my exhaust headers are the same size i wont need to rejet, i have no idea what that means or how that affects the jets

[Grasshopperus]

I'm probably going to get a major smack down from a mechanic for this because I'm guessing here.

If your exhaust has less restrictions (usually has less restrictions if it's louder or more performancy) then the pressure and speed of the combusted gases leaving the engine via the exhaust valve is lower, this reduced velocity means that vacuum pulling new gas and air into the cylinder is not so great meaning that less gas will get sucked through the jets meaning you'll run lean.

OK, ignore all that crap I just wrote, just take it from other people that you may need to re-jet after changing the exhaust.