i have a stock 08 yfz 450 i am going to put on a motoworks sr4 slip on and wondering if i have to rejet it or will it run alright with stock jets not really wanting to change jets if i dont have to but what size jet would i have to put in to get it to run decent

 

Are you opening the air box too? If you leave the air box lid on, a slip-on alone generally does not require a re-jetting.

All machines are a little different. My z400 for example has a Motoworks slip-on and I did not need to re-jet. I did however leave my stock air box lid on, and I run a foam filter such as Twin Air or Uni.

I did install a Power Now intake ram, which basically picked up any slight slack that might have been on the low end of the jetting with the slip-on added in. But the z400 has a bit of a big and sloppy intake and those carbs benefit from the Power Now product anyway.

On a YFZ I don't think you will need to do anything. They have a pretty good intake design to start with. But it also depends on how lean yours was jetted from the factory. Depending on where you live the dealers will sometimes be instructed to adjust the jetting for your climate and elevation.

Just to make sure run it for a while and pull the plug to look.

 

im leaving the air box lid on i mostly wanted the pipe for the sound eventually ill get a hi flow filter and re jet it but just wondered if it would alright if i just changed the pipe hows that pipe sound on that 400

 

i have a blaster with a full fmf exhaust and i run the air box top off and its the stock jets
the only problem i have with it is if i dont hit the powerband in 5th gear it will bog out in 6th but the racing store across town told me its because my airbox top is off, i dont lose power with the stock jets and it runs fine
*knock on wood*

my buddy's bro has an 08 yfz and he has a whie bros mx4 slip on installed on his and he runs it with a twin air air filter and i think the box top is on (unless when he is racing) but the only problem he has is that it backfires when it idles but thats about it
i was on in 2 weekends ago and my neighbor has a done up banshee he put about 9 grand into it and i blew it away this was on dirt and road so theres the proof of it not losing power

 

It sounds good..... Everyone seems to like it.... Its not crazy loud but its not quiet either, it is a little louder than my buddy's YFZ with the GYT-R system.