Quote from: AlphaFire X5 on February 23, 2006, 09:43:46 PM
How does more air pressure reduce air flow? I forget the physics behind it.

Air pressure isn't reducing the air flow, it's reducing the flow of fuel.  The airbox creates a low pressure zone from which the carbs breathe.  I think the physics that are relevant here is the Bernouli Principal--the tendency for fuel metering to actually richen as air flow increases through a constant venturi.  That's opposite to the common sense jetting theory which is usually dispensed here at the DIY level. 

What mitigates the richening tendency in a CV carb is the main air jet (MAJ) at the rear entrance--which introduces air into the fuel flow not only to atomize the fuel but also to lean the mix as the RPMs rise.

With the airbox removed the MAJ is drawing from a higher pressure source creating a premature lean ratio all else remaing equal and power is abruptly cut far short of redline.  Reduce the MAJ and upper end power can be restored without changing the MJ but of course that will create other unintended consequences.  Obviously the jet/needle/air variables are carefully selected at the factory.

Quote from: AlphaFire X5 on February 23, 2006, 09:43:46 PM
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...

The fact that Stage 3 pipe/filter/jetting matrix only nets about 3 HP at the upper end is a good indication that the big MJ is NOT delivering fuel in proportion to its increase in size over stock.  Actually, what we don't know without super-imposing before and after dyno charts is what effect Stage 3 has on low end & midrange power. 

After all that, I still say 147.5 is way to big, even if the stock jets are 130's. I've jetted plenty of bikes with high flow filters and exhausts. And they've never needed to go up 7 sizes from stock.
Also, I went up two sizes from stock with the same modifications as you, and it runs great. I think i remember reading something about there being a change in cam design at some point? But we are on basically the same motor. For one person to go two sizes bigger, and another person to go seven, is a pretty drastic change. Try 135's, mabye 137.5's and get back to us.

Main jets only kick in in the upper RPMs, and the mid-mains in the mid RPMs. The main jets peak power CAN be cut short by being too rich. That's why I feel the jetting matrix isn't doing a good job with the reccomendations. Running too rich will bring your power down considerably because there is too MUCH fuel and it can't combust, thus reducing power. Too lean and there is not enough fuel to combust, thus reducing power.

People here seem to think that the more fuel you put in the more power you get, not realizing that the fuel won't combust if it isn't atomized properly; this happens when there isn't enough air. That's one of the main reasons I don't like to follow with the "standard" beliefs of jetting on this board. My dad jetted his bike (FZ1) and runs 2 125 jets and 2 122.5 jets. That's a little bit more and a little bit less than stock on his. Ours is stock with 130. Do you really think that going up 8 jet sizes in the mainjet is REALLY the right setting?

Quote from: AlphaFire X5 on February 23, 2006, 11:21:11 PM
The main jets peak power CAN be cut short by being too rich. That's why I feel the jetting matrix isn't doing a good job with the reccomendations. Running too rich will bring your power down considerably because there is too MUCH fuel and it can't combust, thus reducing power.

Very true.  That's why when trial & error is employed you attempt to find the jet size ceiling where power begins to drop.  Being that it's tough to obtain jets larger than 150, the jet ceiling for the GS500 was never found--at least by the jetting Guru.

Quote from: AlphaFire X5 on February 23, 2006, 11:21:11 PM
My dad jetted his bike (FZ1) and runs 2 125 jets and 2 122.5 jets. That's a little bit more and a little bit less than stock on his.

Same story.  I've seen the recommended "stage 3" settings for an FZ1 and the MJs are +145 on stock needles.

Quote from: AlphaFire X5 on February 23, 2006, 11:21:11 PM
Ours is stock with 130. Do you really think that going up 8 jet sizes in the mainjet is REALLY the right setting?

I think the RIGHT setting is to leave the pipe/airbox/jets the hell alone save for minor fiddling with the pilot fuel screws.  Next best is Dynojet.  There ARE credible posters here who have reported good results using the DIY jetting matrix on 2nd generation GS500s.

Quote from: Gisser l[ink=topic=24297.msg250038#msg250038 date=1140764368
Very true.  That's why when trial & error is employed you attempt to find the jet size ceiling where power begins to drop.  Being that it's tough to obtain jets larger than 150, the jet ceiling for the GS500 was never found--at least by the jetting Guru. 

Yeah, I never agreed with the "Guru" either. All his were based off butt dynos, which are highly inaccurate I believe. I prefer factual readings; some good hard data.

Quote from: Gisser on February 23, 2006, 11:59:28 PM
Same story.  I've seen the recommended "stage 3" settings for an FZ1 and the MJs are +145 on stock needles.

Yup, and it's horribly rich. My father's jetting settings CAN be backed up with dyno runs because he has changed his settings based on the results of his dyno runs. He disagrees with the majority on his board as well. Like father like son, eh?

Quote from: Gisser on February 23, 2006, 11:59:28 PM
I think the RIGHT setting is to leave the pipe/airbox/jets the hell alone save for minor fiddling with the pilot fuel screws.  Next best is Dynojet.  There ARE credible posters here who have reported good results using the DIY jetting matrix on 2nd generation GS500s.

Credible in the knowlege of carbs though? I'm not saying I'm an expert, I'm just sharing my views on the matter. You're right though, the best bet is to just leave it the f%$k alone. The factory obviously did enough research to get it good enough to run well enough to sell thousands upon thousands of units worldwide. I had a DynoJet kit in my old GS and it ran like a dream. I had no problems with it whatsoever.

I will dyno run the bike in before and after when I get the lunchbox when I can though. I'm going to be jetting it at 20 pilot, 62.5 mid main, and 137.5 main. I am probably going to have to bump it down to 135 though. I'm not sure how much the lunchbox will affect the midrange on it, and I think it will cut out some of the bottom end. I might keep the pilot jet in the stock position too. Not sure yet.

Is changing mid jet one size the same as moving needle clip?

Im assuming that when you guys talk about the mid jet your refering to the needle jet. So yea, raising the clip and going a size bigger would have basically the same effect.

Quote from: Chris_B on February 24, 2006, 01:41:01 PM
Im assuming that when you guys talk about the mid jet your refering to the needle jet. So yea, raising the clip and going a size bigger would have basically the same effect.

No, we're talking about the mid main jet. The '01+ carbs have a pilot jet, a mid main jet, and a main jet, whereas the old style ones have a pilot jet and a main jet only.

Oh I see, I have a '99. I've always wondered what the hell the mid main was. Thanks! Now I know.

Quote from: Gisser on February 23, 2006, 03:11:09 PM

Quote from: kalel3914 on February 23, 2006, 02:16:04 PM
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,

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.   

yes, by lunchbox i do mean the clamp on.  I'm telling you from experience that the stock gs does do 10500 in 6ht gear.  stock it would get to 10000 - 10500 rpm and give me about a 110 indicated.  since the rejet, i get to 8000 and all i get is 90 mph.  it's good, but i figured by rejetting installing a new filter and exhaust, i would get more top end as well as better low and midrange, hence my post to figure out how to remedy this.  checked my plugs and the look good, maybe a bit lean, but by no means starving.  I'm now thoroughly confused as to what to do next.

Try to cover a part of air filter with duck tape. This should reduce the air intake and richen the fuel mixture. If it is too lean then the bike should go better. If it gets even worse, try instaling old jets (smaller) that will make the mixture leaner. If it gets better with smaller jet, then you are running too rich and should go for smaller main jet.

If you run out of all ideas, take the bike to the carb mechanic or  dyno and the specialists there will tell you more about your problems.

Keep in mind that you can be lean at some points, and rich at others, while still getting great performance from your engine. Carburetors are not precise, linear devices (multi-port fuel injection is a lot more precise, but qualifies as overkill for all but the most modern engines); depending on RPM, you can see both rich and lean conditions at different throttle settings, which is perfectly normal.

My own personal preference is to be richer than leaner whenever possible; I will gladly trade the last 5% of performance to keep my pistons cooler by being slightly rich. I've seen pistons turned into slag from being run too lean (especially on air-cooled engines), and if you have any true affinity for your hardware, such a sight will make you cringe. The level of damage can vary, from burned/holed pistons to warped (out-of-round) cylinders, to all kinds of destruction (a broken con rod sticking up through the case, valve heads welded to piston crowns...); never anything pretty. Admittedly, my only personal experience with extreme damage has been with stockers-turned-racers, not well-maintained (OK, sometimes pampered) near-stock streeters, but that doesn't make it any easier to accept.

Though this will probably make me sound like a motorhead, I tend to develop emotional attachments to my motorcycles (probably not healthy, but I don't care); I have no doubt that I comprehend the four-stroke cycle as well as most automotive engineers, and engine tuning is one of my passions. From fuel atomization to cylinder scavenging, if something goes wrong in the process, my first inclination is to break out my wrenches and try to locate the problem before the engine suffers a catastrophic failure. An ounce of prevention...

Anyway, domas has made a suggestion that I understand in principle, though I disagree with it in practice; he wants you to make a change (partially block airflow to the filter) which will tend to make the mixture flowing to the engine richer, rather than suggest that you do something like run the engine without an air filter, which will make the mixture MUCH too lean. Being too rich ALL THE TIME won't really tell you anything; restricting air filter intake flow will make your system rich at all RPM levels, and you'll probably have a difficult time knowing when the increased richness is helping you, and when it is detrimental.

Simply put, the problem I forsee from using his method is that you will make the engine richer at ALL times; in fact, you might not even be able to ride the motorcycle from blocking the airflow that way. You could foul the spark plugs long before you were able to warm the engine properly and get it up to temperature; then you'd have a NEW problem to overcome, and that might confuse you even more. You'll most likely be better off running rich SELECTIVELY, under limited circunstances, which I'll explain:

Logically, working from an OCCASIONAL too-rich condition (which might foul the spark plugs after a while, though you can probably avoid that by not doing it excessively) is much better than working from a too-lean condition (which can wreak all kinds of destruction in very short order); fouled spark plugs are cheap to replace, while pistons, cylinders, and cylinder heads are not. Remember the PRINCIPLE domas has suggested;, you want to work from RICH(er) to LEAN(er), and never the other way around, unless you understand and are willing to risk the chance of causing severe damage. Out-of-round pistons don't move to well within a cylinder block, and the potential piston-ring breaking / cylinder-wall scoring / engine-breakdown type of damages can accrue quickly.

My own method for diagnosing this will require you to ride the motorcycle and simply add choke at the time you encounter problems (~8K RPM, or thereabouts, in fifth or sixth gear); if the engine bogs down, you're OBVIOUSLY too rich, and you should install a one-size smaller main jet. If the engine runs BETTER when you add the choke, your main jet is too small (HIGHLY DOUBTFUL, based on your previous descriptions of the problems you've encountered to this point in time). I'm sure you realize that trying to reach high RPMs will require WFO throttle settings, so there won't be any doubt that you're running primarily on the main jet, which is the right way to approach this.

Think about this for just a minute; logically, you WANT to work from large throttle openings / the main jet before you move on to the mid-main, pilot jet, or needle settings, and NOT the other way around. Everything you've posted so far indicates that your system is too rich at the top end; adding choke when you hit the wall at 8K in fifth or sixth gear can help confirm that; if the engine sputters and bogs, you need to switch to a smaller main (install one size smaller and retest; don't make BIG changes to your main jet size, it is best to work with incremental changes only). If the motorcycle accelerates from adding choke, you're too lean, and should install a larger main jet (again, HIGHLY DOUBTFUL); I agree with Alpha and Gisser that you're probably too rich on top, and increasing the choke while you're running at ~8K in fifth or sixth gear will probably confirm that pretty quickly.

Let us know what effect making the mixture (selectively) richer has on your engine, and we should be able to get you pointed in the right direction. Ask questions if you need any clarifications, and make good notes about the effects your throttle / choke level changes have. Focus ONLY on correcting the high RPM problems at this point in time; don't be concerned about anything else (a low-RPM stumble, a mid-RPM surge)... work with large throttle openings at high RPM, diagnose the fuel condition at those levels (too rich / too lean), and report your findings here. You have a community of knowledgeable people willing to help you, so as the old saying goes, "Use your friends wisely..."

Quote from: kalel3914 on February 25, 2006, 06:34:59 PM
I'm telling you from experience that the stock gs does do 10500 in 6ht gear.  stock it would get to 10000 - 10500 rpm and give me about a 110 indicated.

I'm calling you out and saying you're completely wrong there.

I was thinking about this when I was riding on the highway today and decided to double check your claims. My bike is stock, too (took out the K&N). I stuck the bike in 6th gear (I checked multiple times) and watched my RPMs and speed. at 6500 RPM, my speedo was reading right at 80mph. At 7000 RPM, the speed was at 90mph indicated. That means that for every 500 RPM in 6th gear, you get a 10mph increase in speed. According to you, you were running at 10,500 RPM in 6th gear, which means you would have been travelling at 160mph indicated. You wern't in 6th gear. 110 is pretty much where our GS tops out, sometimes 115/120 depending on wether you have a tailwind and are going down hill, but 110 is pretty much your upper limit.

Yep, GS500's theoretical maximum speed is listed as 140 MPH @ 11000 RPM on the gsfaq.   Working back from those numbers...

8500 RPM = 110 MPH
8250 RPM = 105 MPH
8000 RPM = 102 MPH

It's not certain poster has a problem.

Still, there may be another 500 RPM in it depending on MJ.     A plug/chop should point the way for further fine tuning.   

Or, maybe a 15t front sprocket?  http://gstwins.com/gsboard/index.php?topic=4200.msg245082#msg245082

Quote from: scratch on February 26, 2006, 08:05:56 PM
Or, maybe a 15t front sprocket?  http://gstwins.com/gsboard/index.php?topic=4200.msg245082#msg245082

Nah, I don't think so. It wouldn't raise it that much. From what I've read the 15T sprocket raises RPMs by about 500 RPM. 10,500 RPM would be 150mph indicated.

maybe there are other problems with my bike then, because at 8500 rpm, I was pulling 90mph, and when it got in the 10000-10500 range, i was at 110.  now i'm gonna check my tach cable.   this page is turning me into a hypochondriac gs owner.