no top end throttle response

[kalel3914]

Recently put on K&N Lunchbox, VH SS exhaust, and rejetted to (22.5, 67.5, 147.5) Mixture screws just under 3 turns out, and idle is steady at 1200rpm.  From 0-60 this bike is now a beast.  Much quicker and steady all the way up. So I took the bike on the highway today, and found out where it is lacking. I get up to some nice speeds and at 8000 rpm in the higher gears only, it kinda quits and doesn't want to rev higher, nomatter how much ithrottle give it.  Could the mains be too big/small, can i do more adjusting on the mixture screws to fix this?  Stock, it let off around 10.5k rpm in 6th, but that was because the bike had already given all it had.  Low end is a lot quicker, but my top end is suffering, Please Help!!! 

[scratch]

Hmm...did you move the clip on the needle?  Well, one thing, the needle helps roll-on acceleration.

[kalel3914]

no i didn't, it's at the stock setting

[scratch]

Well, one thing, the needle helps roll-on acceleration.  Have you taken your plugs out to get a reading?  What do they look like?  Of course, a true plug reading would have you put new plugs in, take the bike out and duplicate the problem at 8000rpm, as you said, pull the clutch and kill the bike.  After you have stopped, pull the plugs (they're gonna be hot!) and that will give some indication if you're too lean or rich.

[kalel3914]

the bike doesn't actually die at 8000 rpm, it gets there, the doesn't go any further, before i rejetted, it would continue to climb, now it just stays around 8000 and doesn't want to go any higher even with more throttle

[scratch]

Does it do this in every gear?  If it does it in every gear, it's something in the carburetor.  If not, I don't know

What does it do when it hits 8000rpm's?  Does it bog (just slow down)?  Or, does it hesitate (buck back and forth)?  Boggy is too rich, hesitant is too lean.

If it would continue to climb when it was stock, I would suspect going leaner than your current size main would help fix your problem. 

[kalel3914]

I'll have to get back to you on that one, i usually don't wind out my lower gears, only 4, 5, and 6 for highway speeds.

[domas]

If it would continue to climb when it was stock, I would suspect going leaner than your current size main would help fix your problem. 

I agree with scratch on that. 145+ jets are used with pod filters. I dont think lunchbox has the same airflow.

[mjm]

the bike doesn't actually die at 8000 rpm, it gets there, the doesn't go any further, before i rejetted, it would continue to climb, now it just stays around 8000 and doesn't want to go any higher even with more throttle

Take it up to 8000 at full throttle - when it sits there for a few seconds, hit the kill switch and chop the throttle - do a plug reading to tell you if it is lean or rich.  Adjust the main jet up or down accordingly.

If the flattening is at less than full throttle - and NOT at full throttle it is probably a need to raise or lower the needle.

[kalel3914]

so in neutral, it revs all the way up to red line.  don't have the tools to get the plugs out here, but will try to get a reading asap.  Just wondering if it is something other than the jets since in neutral it revs all the way up, but at speed, i can't get past 8000, brb gonna take a quick ride... Back. It red lines through the first four gears, Didn't check out 5 and 6 because it's cold and there are too many stop signs.  Maybe it just needed to be run for a while, i don't know?

[Gisser]

Just wondering if it is something other than the jets since in neutral it revs all the way up, but at speed, i can't get past 8000, brb gonna take a quick ride... Back. It red lines through the first four gears,

It don't take much power to redline in neutral and the 1-to-4 gears.  If top-end power isn't there it's going to show up in the higher gears. 

By lunchbox we assume you mean the one-piece filter which replaces the airbox and if so then 147.5 should be in the ballpark.  But let me tell you, the stock GS500 isn't going to wind out to 10500 in 6th gear.  It could be that 8500 is all she's got--which should get you to 105 MPH.   

[Chris_B]

Your main jet is way to big. Mine did this with a K&N and an exahust, the previous owner had installed 133 main jets which were way to big. I put in 127.5's and it runs all the way to redline in every gear. I dont understand how you guys are running such huge main jets and having it work well. Whats the stock jet size? 122-125? well 147.5 is about 10 sizes bigger than stock. And you shouldnt need to go ten sizes bigger for an exahust and airfilter. Try going 3 or 4 sizes up.

[Turd Ferguson]

Your main jet is way to big. Mine did this with a K&N and an exahust, the previous owner had installed 133 main jets which were way to big. I put in 127.5's and it runs all the way to redline in every gear. I dont understand how you guys are running such huge main jets and having it work well. Whats the stock jet size? 122-125? well 147.5 is about 10 sizes bigger than stock. And you shouldnt need to go ten sizes bigger for an exahust and airfilter. Try going 3 or 4 sizes up.

Hmm...that doesn't sound typical Chris.  If I'm not mistaken, the stock main size is 127.5 and 130 for 2004 and newer bikes.  Many people have reported their bikes to have shown lean when dyno tuned on the factory mains.

The mains do sound large though, I'm not totally sure if 147.5's are necessary.

-Turd.

[scratch]

He has an '02, as stated here in his original thread: http://gstwins.com/gsboard/index.php?topic=24222.0
Which uses different jets: http://gstwins.com/gsboard/index.php?topic=13146.msg110838#msg110838
and here: http://gstwins.com/gsboard/index.php?topic=12928.msg108863#msg108863

Kalel - I can merge your two threads if you wish.

Do you have the advancer?  If you don't, that also may be why you're hitting a wall at 8000rpm.

[kalel3914]

main jets in my bike were 127.5, don't have an advancer, was planning on doing the diy project sometime in the next 2 weeks. The reason i went with 147.5's i sbecause the rejetting matrix recomemded 147.5 - 150 jets for an 02 with kn clamp on and vh exhaust.  I'll do the ignition advance over the weekend and see if that cleans up my 8000 wall.  I       will also take the bike to work tomorrow and see if it just wasn't the carbs readjusting to the new settings since today it hit red line in up to 4th gear.  I'll report back tomorrow and tell you what i've found.

[Alphamazing]

main jets in my bike were 127.5, don't have an advancer, was planning on doing the diy project sometime in the next 2 weeks. The reason i went with 147.5's i sbecause the rejetting matrix recomemded 147.5 - 150 jets for an 02 with kn clamp on and vh exhaust.  I'll do the ignition advance over the weekend and see if that cleans up my 8000 wall.  I       will also take the bike to work tomorrow and see if it just wasn't the carbs readjusting to the new settings since today it hit red line in up to 4th gear.  I'll report back tomorrow and tell you what i've found.

Personally I think that rejetting matrix is crap. Utter and complete rubbish. Pretty much every reccomendation on that thing has you running WAY too rich. 150 mains? Freakin' please! There is absolutely NO reason that you need something that big on our bikes. It seems to be based off people's misconception that bigger is better, and therefore richer is better. But that's crap. Complete and utter crap.

Try bumping your mains down to 140 or 137.5. I'm going to be putting a lunchbox filter on mine sometime soon and I am hoping that 137.5 will work with it, if not I'll be taking it down to 135 or less. Stock on my bike is 130, btw.

[Gisser]

Personally I think that rejetting matrix is crap. Utter and complete rubbish. Pretty much every reccomendation on that thing has you running WAY too rich. 150 mains? Freakin' please! There is absolutely NO reason that you need something that big on our bikes. It seems to be based off people's misconception that bigger is better, and therefore richer is better. But that's crap. Complete and utter crap.

No, you underestimate how much the jetting gets f*cked up when the airbox is removed.      The large MJ recommendations are actually based on trial & error.  Problems arise when posters don't understand the difference between stage 1 and stage 3 K&N filters or they are jetting with Factory or Dynojet needles which are incompatible with the DIY so-called matrix.

[Alphamazing]

Personally I think that rejetting matrix is crap. Utter and complete rubbish. Pretty much every reccomendation on that thing has you running WAY too rich. 150 mains? Freakin' please! There is absolutely NO reason that you need something that big on our bikes. It seems to be based off people's misconception that bigger is better, and therefore richer is better. But that's crap. Complete and utter crap.

No, you underestimate how much the jetting gets f*cked up when the airbox is removed.      The large MJ recommendations are actually based on trial & error.  Problems arise when posters don't understand the difference between stage 1 and stage 3 K&N filters or they are jetting with Factory or Dynojet needles which are incompatible with the DIY so-called matrix.

There is no such thing as a "Stage 1" or "Stage 3" K&N filter. There are stages of JETTING, which are used in conjunction with what level you want. Stage 1 is meant for the street and is used to smooth out the torque and power curves while still retaining gas mileage. Stage 2 gives you a bit more performance, but at the sacrifice of low end AND gas mileage. Stage 3 is meant for race/track only. As for your "trial & error", it's all butt dyno. I have yet to see a single air/fuel mix roving these jetting setting are correct. In FACT, the air/fuel graph I've seen on a stock pre-'01 GS showed that the GS was RICH in the top end already, and lean in the VERY bottom end, but overall it was jetted fine throughout the midrange.

Yeah air flows better when the airbox is gone, but not enough to need THAT big of a jump in main jet size. That's absurd. When I get my lunchbox in and rejet I'm probably going to get mine dynoed so I can properly adjust my jetting settings.

[Gisser]

There is no such thing as a "Stage 1" or "Stage 3" K&N filter. There are stages of JETTING, which are used in conjunction with what level you want.

What really defines stage 1 from stage 3 is whether or not the airbox gets ditched.  Period!

As for your "trial & error", it's all butt dyno. I have yet to see a single air/fuel mix roving these jetting setting are correct.

No, I don't trust butt dynos either.  In this case trial & error means trying a variety of jets and seeing which one makes tach & speedo needles go furthest to the right on the dial during flat-out runs.

In FACT, the air/fuel graph I've seen on a stock pre-'01 GS showed that the GS was RICH in the top end already, and lean in the VERY bottom end, but overall it was jetted fine throughout the midrange.

I agree!  I'm not one who holds an assumption that the stock mainjet is lean on a stock GS500. 

Yeah air flows better when the airbox is gone, but not enough to need THAT big of a jump in main jet size. That's absurd.

It's not the airflow screwing with the carburetion; it's the air pressure.  Fuel draw in the venturis is greatly diminished when the air pressure rises.  In the absence of an airbox it  takes a huge jet to achieve the same fuel metering that the stock jet delivers with a stock set-up.

When I get my lunchbox in and rejet I'm probably going to get mine dynoed so I can properly adjust my jetting settings.

Good idea.  Get a before and after comparison.  Be the first to back up the usual wild claims with hard data!  

[Alphamazing]

Yeah air flows better when the airbox is gone, but not enough to need THAT big of a jump in main jet size. That's absurd.

It's not the airflow screwing with the carburetion; it's the air pressure.  Fuel draw in the venturis is greatly diminished when the air pressure rises.  In the absence of an airbox it  takes a huge jet to achieve the same fuel metering that the stock jet delivers with a stock set-up.

How does more air pressure reduce air flow? I forget the physics behind it. I know that air pressure is good for low end torque though because it forces air into the intake tract, but high speed air velocity doesn't work because of the pressure. Do you know the physics behind it? I always thought that without the airbox airflow was a lot better, especially in the upper end. And without the restriction of the airbox airflow was increased dramatically, but not by enough to require 147.5 mainjets...