flat spot

[tt_four]

it's a 91 gs, and the guy we bought it off of had installed a vance and hines exhaust, as well as a jet kit, but what i don't like about it, is it has a horrible flat/sputtering spot between 5-6k when you give it gas, once you get up to 7000 it's fine, and just the whole engine in general feels weaker to me than i'd think it should, i know there isn't a ton of power, but with a little bit of effort i could get the front wheel of my old ninja 250 off the ground an inch or two, i could get it up quite a bit if i popped the clutch, but her gs i can't get to budge off the ground, i'd think a gs should be able to outpower a 250, and the last thing, is when you're over 5k and just let off the gas, and engine braking kicks in, there's just huge backfires out of it, i'm pretty sure the guy who installed the stuff just didnt' really know what he was doing, but i was just wondering if you guys have any ideas about what i can look into to see if there's a problem, i don't know a ton about engines, but i figured maybe you guys would have an idea

[yamahonkawazuki]

lets start with the kiss principle (keep it simple stupid) its a saying i live by and apply to my bike. first things first check the plugs and/or the air filter.

[miket]

Backfiring when closing the throttle is leaness due to the vacuum created by the engine. Some of it is normal. If too much, either you have a vacuum leak or you need to open up the pilot mixture screw. Try 1/4 of a turn at a time.

The flat spot... hard to tell by what you say, could be too lean or too rich, I'd guess you're too lean. Try moving the clip on the needle (which you should have if you have a kit) a couple of notches higher.
However, to to identify the carb component involved, you should look at throttle opening rather than RPM.

[sprint_9]

Id almost bet that it is too lean. Mine is stock and backfires around 5,000 rpms when you let off the throttle, although mine isnt that bad. Since stock is lean and mine has a similar symptom I think yours is lean. Also what type of airfilter is on it, you may want to figure out what is on it for sure and buy a jet kit for it, that way you know where your at. Just a suggestion.

[Rashad]

Unlike the usual attacking the carbs that i would usualy do.. check the air filter. Its night and day with my bike haveing the filter in and out. Its so clogged, that with the Vance and Hines pipe and jets.. its breathing out better than in... i figure. So when i pull the filter all the sputtering and surging and crappiness goes away and i have a crazy revving machine!

[The Buddha]

Open the throttle and it falls on its face = slide lifting too fast.
5-6K sputtering = nedle could be lifted a bit.
I'd strat with lifting the neeldes. If that dont fix it then block the hole in the slide.
Cool.
Srinath.

[tt_four]

thanks a ton guys, i don't know much about engines, so i don't have a clue what any of the stuff you're talking about means, but i'll just have to figure it out, ha, thanks

and where's the pilot  mixture screw you speak of?

[miket]

Look at my pictorial page. It should help you figure out where things are.

[mjm]

Try moving the clip on the needle (which you should have if you have a kit) a couple of notches higher.

I think you mean put the clip a couple notches LOWER which lifts the  needle HIGHER and richens the mixture.  If the needle does not have slots for the clip, then use small machine washers (can't remember the size off hand but they are sold at Radio Shack is a kit with several different sizes) under the clip to raise the needle.

[miket]

Yes, thanks for the correction, I got it the other way around. That's what I meant: lower the the clip to rise the needle.