Simple question
 

I have done some searches and didn't come up with a definitive answer, and if I should have posted this in Tech related I apologize. Here goes: Assuming that my 1991 f2 is running stock jets, will I have any fuel problems if I add a K&N filter? The only modification I currently run is a Yoshi slip-on.

i wouldn't think the air filter would be a cause for re-jetting, not as much as the slip-on anyway.

Problems? not necesarrily, but I promise you that you will lose optimum power in ALL rev ranges if you install the K&N filter without re-jetting and tuning the carbs. For example, I also have a Yoshimura slip-on (RS-3), this won't change much of anything - but after implementing the K&N filter, I had to go to 142 mains, to get max power in the top end (about 7500 RPM to redline), raise the jet needle one clip position for mid range (roughly 5000 RPM to 7500 RPM), and adjust the fuel screw to 3 turns out for the lower rev range and steady cruise.
These aren't "magic" numbers, since every motor has subtle differences, but I just wanted to give you a "for instance".

Hope this helps.

Awesome! So I'll assume that all four carbs run the same jetting. And more than likely if I go to 142's all the way across it'll run well enough that I wouldn't notice the difference? What about the slow jets? Any guess on the size of those? I found a place that sells keihin jets in a set for all four carbs and it seems to be a reasonable price. Take a look at the site

Jet Kits 4 Carburetors :: Honda CBR600 F2 1991-1994 - Bike Jets Keihin Honda Types Low Prices

and tell me what you think I should order. My guess was 140's, 40's, 2 shims, and the D-tool just to make life easier. Thanks much for the feedback too! I got a killer deal on this K&N but didn't want to install it without a bit of reference on the changes it could cause.

The key is tuning, and being prepared to pull the carbs several times, maybe even as much as 5 times or more, to get things all sweet. You have to get the fuel circuits/rev ranges dialed in one at a time, from high to low.
MotorcycleSuperstore.com, and I'm sure several other sites, have 4 packs of Keihin round main jets for around $10 a set, if I remember correctly, and I honestly wouldn't begin working on it without having an extra set that's one size higher than your starting point, and one set lower - it's just part of the game - if you want to start at 140, for example, you should have a set of 138s and 142s on hand, so if 140 doesn't feel like it's got the power, you can go up or down, depending on HOW it runs wrong, and see if it gets better, or worse.
After the top end yanks your arms off under acceleration, then you gotta go up or down to get the needle jets dialed in, for the mid-range (by the way, 1 shim on the needle jets is equivalent to roughly half of one clip position), and lastly: the fuel screws (slow jets aka pilot jets).
Now, about the pilot jets, you probably won't need to go up a size in those, unless you find that when you get around to that last rev range, the lower range, you have to back the fuel screws out too far, in order to get the right fueling - I think when you get to 3 1/2 or maybe 4 turns out, is when you should jump a size - I'm fuzzy on this bit, but you should be able to find the answer.

Here is a link, that will help you to sort all this out:

CV Carb Tuning Procedures

Ignore the portion regarding float position, as our floats are non-adjustable.

Now, finally, unless you're a "tinkerer", like me, and just friggin' love to do this kinda stuff, I would almost recommend you stick with OEM type paper filter, and just keep the jets stock - I'm just saying, this can take A LOT of work!!
You'll get opinions from A LOT of people out there, that swear that OEM filters are the ONLY way to go...there's definitely a division of folks out there!

Maybe I shouldn't fool with the K&N right now. I'd gladly pull the carbs 5 times if I had to, but I don't want my bike down that long. Besides, as soon as I get the carbs for the 900rr engine I'm swapping into it I'll have to jet the hell out of them and that engine will probably need an aftermarket filter to reach it's potential, as it was a race built engine with the head ported and polished and who knows what else done to it. The only thing that will cripple my project is if the compression has been bumped up so high that it will only run on race fuel. I'm not trying to have a street bike that operates at a cost of $7 per gallon.

Well, whatever you decide to do, good luck! You'll find help here for a lot of stuff!

You guys have already been more help than you can imagine! Thanks to everyone on here who takes time to share their knowledge!

I'll tell ya what man I am a tinkerer. I buy broken down bikes just to fix them up. I'm a Technichian for a living but ill be damned if I didn't just decide to say to hell with this K&M filter. It was in the bike when I bought it so I figured I'd tune the carbs to it seeing as the kid I got it from knew nothing. It seems like every time I get it dialed in close it'll be good for an hour

I'll tell ya what man I am a tinkerer. I buy broken down bikes just to fix them up. I'm a Technichian for a living but ill be damned if I didn't just decide to say to hell with this K&M filter. It was in the bike when I bought it so I figured I'd tune the carbs to it seeing as the kid I got it from knew nothing. It seems like every time I get it dialed in close it'll be good for an hour or so then start to act up again. I've got it pretty close to dialed in now but still I just don't like it. I can't seem to get that crisp pull through all stages of the carbs. I suppose if it was winter I probably wouldn't be so bugged by this and keep at it and possibly bring it to the dyno, but the weather is nice and I want to ride more and tinker less plus I have my next project sitting collecting dust. I actually love this CBR so much I'm going to keep it and have more projects lined up for it but I need another running bike before I tear into this one anymore.

Sooo long story short I just ordered my stock filter off of Amazon :)
If all goes well ill be tuned perfect before next weekend.