So I had a bad leaning out problem on my streetfighter at around 7000rpm. It could free rev to redline but under load would sputter and cut out at 7k. Under the advice of some folks on customfighters, I routed the carb vent tubes into holes I drilled under the air box to equalize pressure. Now she pulls up to 10-11k but still cuts out. I have a mystery brand can, stock air filter and I think stock carbs. It runs great throughout the rpm range. Would simply putting 140 mains on it solve my problem? As in, would upping the jets fix the top end problem while leaving the lower rpm range alone? Thanks in advance for the help

 

First thing I would think of, unfortunately, is pull the carbs and make sure there isn't a jet kit already installed..... If there is, well it gives u a place to start on adjusting it. If not, then the best idea would be look at the jet kit website, and choose one based on your mods. As close as you can. Then the not so fun part of tubing begins. Check the carb cleaning and tuning sticky for a good link on tuning .

 

Thanks good advice, I've read through the 101 guide a couple times now, and checked the link from factory pro on tuning. They suggest selecting the best main jet first so thats what I'm going by

 

Yeah, u do want to do that. Give factory pro a call or email. They can give you a good base to start with in jet sizes etc.

 

Like gtcole said, you need to know what's in there, and also, you didn't mention whether or not the carbs are squeaky clean... dirty carbs behave in a lean fashion.

By the way, stock mains are 135, and if you're running an OEM air filter, going to 140 mains will probably make you run way too rich.

Anyway, I would do a "reconnaissance" mission, to scope out your current carb settings, and give it a thorough cleaning while you're in there.

 

Definitely going to clean everything out by following the 101 guide when their off. I'm pretty sure everything is stock, but I'll find out when they're apart. What if I ran basically a straight pipe with an oem air filter, would that warrant 140s? Or would jumping to 145s be a good idea at that point?

 

There's no reason to spend any money on jets, unless you need them, even though they're not real expensive.
If you're running an OEM filter, there's no reason to go above 135, unless you find that the bike has a stumble at full throttle in the upper rev range (about 7500 to redline), and even then, you would step to a 138 first, re-test, repeat... that's just carb tuning.
Unless you've punched this thing out, or are running higher-than-stock lift/duration cams, then there is no way at all you'll need 145s.

Now, an exception to the rule on jet selection: I made several changes at once, to my first F2, and upon riding the bike with 138 mains, as a starting point based on my changes, I found that it began to stutter a lot once I got a little above 7000 RPM, and it would NOT rev past, say, 9200 RPM or thereabouts.
I knew from the way it behaved that it was lean, and since it was not just that it "didn't pull as hard as I thought it could", but rather that it fell flat on it's face, I knew that moving just one size up to 140 would be pointless, so I jumped to 142, and it ran like a scalded dog, tach needle sweeping from 6000 to redline in a flash!!
Bear in mind, though, this is with a freshly cleaned and oild K&N filter, race headers, and a shortened race slip-on... if you put these carbs on a bike that was otherwise stock, it would barely run, I'm sure.

 

I have a k&n and an older yoshi slip on. I went to this website :
Jet Kit Calculator Techincal - Jet kits for Carburetors

and entered my info, they suggested 145 mains, 40 pilots and 2 shims on the needle to start. Seeing as how I've never heard of them, I took that info and my bike info and emailed factory pro to see what they suggest. I seem to trust factory pro more. Hopefully someone else with more knowledge on properly sizing jets will chime in. It gets expensive and time consuming guessing at jet sizes and swapping them out repeatedly.

 

That sounds almost exactly like what mine does, I took it out today and it wouldn't go past 8k or so. Also, I thought it had a stock header until I looked at my friends f3 and saw a 4-2-1. Apparently mine has a vance and hines 4-1 header. It's possible my carbs are gummed, but it runs like a champ below 7/8k. I'll try 140s to start and report. Thanks for all the advice guys, the jet kit should arrive tomorrow. One more question, I was reading factory pro's Carb Tuning Procedures page. They say the main jets affect lower rpm operation as well. How much do the mains affect lower rpm operation?

 

What did factory pro tell you to do? I ordered my jet kit from carbjetkits, they say I should run 140s or 145s because of my header and unrestrictive slip on