so i am having to replace my fiancee's exhaust due to a series of unfortunate events. how drastic of an impact would there be by installing a stock exhaust with carbs jetted for a full yoshimura system and a K&N lunch box air filter?

thanks in advance.

Your jetting would be too rich and it wouldn't run right...not sure why you would want to do that.

Wait.... you are keeping the K&N lunchbox filter, correct? Just swapping a Yosh exhaust for a stock one?

If so, your jetting probably won't be that far off. The intake has a much more dramatic effect on jetting than your exhaust. I'd run it on the rich side and jet down at my convenience.

Quote from: Big Rich on May 14, 2014, 05:53:45 AM
Wait.... you are keeping the K&N lunchbox filter, correct? Just swapping a Yosh exhaust for a stock one?

correct.

i have access to the stock air box and such if i need it. the stock jets are still on hand as well, so they can be replaced if called for. i would just rather not. having been only familiar with FI bikes where the computer does all the work, i'm not sure how boogered up things will get if we swap the exhaust.

Sorry don't agree that the air intake has a bigger effect than exhaust...they work together. If you restrict one or the other then you will effect the other...air that goes in has to come out. Your jetting will be too rich.

Ok. I'll agree, they do work together. Restricting one will certainly effect the other. And the jetting will be too rich after putting the stock exhaust back on. But a quick comparison of the components will show my train of though:

Look at the stock air filter compared to a K&N lunchbox filter. The opening on the stock filter is what, about 1" in diameter (working from memory)? The K&N filter has enough surface area that I feel confident in saying it barely restricts air flow into the carbs.

The stock exhaust and a Yoshimura exhaust have "roughly" the same flow characteristics. I say roughly because I haven't seen them on a flow bench in a direct comparison. But just by looking at the two systems, there's no mind blowing differences. Sure, the Yosh exhaust will flow more air......that's what it is designed for.

Looking at the wiki chart on jetting (http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting) for the early carbs, a K&N lunchbox filter with stock exhaust uses a 140 main jet (that's 7 sizes up from the stock 122.5 main). Installing an exhaust system with the K&N filter bumps the main up to only 150 at most. That's why I said the intake has a greater effect on the air / fuel mixture.

My '09 was jetted 17.5/60/130 from the factory. When I installed a K&N drop-in I rejetted to 20/60/142.5; after adding a new performance exhaust I went to 22.5/62.5/142.5

The filter is the bottleneck on max airflow into the engine; and as such has the most pronounced effect on the ideal main jet.  The restrictiveness of the exhaust will be most apparent at low airflow conditions and as such the exhaust is the main factor that affects the necessary pilot (and mid-main) size(s).

Quote from: dennisgb on May 14, 2014, 07:29:16 PM

Quote from: Big Rich on May 14, 2014, 05:53:45 AM
The intake has a much more dramatic effect on jetting than your exhaust.

Sorry don't agree that the air intake has a bigger effect than exhaust...they work together. If you restrict one or the other then you will effect the other...air that goes in has to come out. Your jetting will be too rich.

Granted the intake and exhaust work together, but they affect the airflow in different ways.  As I outlined above, Big Rich was correct in saying that the intake had the most dramatic effect on jetting; the exhaust has a less drastic impact on the jetting.

with the common knowledge that suzuki originally jetted the gs lean to begin with, we went ahead with the stock exhaust swap. we kept the current jetting (whatever that may be) and the K&N lunchbox. since i've noticed a lot of first time owners being directed to increase jet size asap after buying their bike while keeping the stock exhaust, i figured we would end up with a similar product. i wish i could tell you all what it's jetted at, but if i don't have to pull the carbs, i'm not going to. i searched the PO's old posts, but i can't seem to find anything regarding jets.

we searched for a similar replacement yosh system, but i refuse to pay 200 + bucks for an exhaust for a 20 + year old bike. it just wasn't going to happen.

first off, upon starting it up, the damn thing just sounded better. idle was rock solid and only took a teeny bit of choke to get started. i figured the this was negligible and the real test would come during a ride.

so the ride came and went. the quietness threw me for a sec. to be honest, the bike is running and performing better than it was with the yosh system. throttle response and power delivery are on par or even better. not once did it feel bogged or stressed, whether it was on city streets or the highway, even in the hot, humid, windy florida weather we've been having lately.

my fiancee' uses this as her every day commuter. i will have her give me reports daily to see how things are going.

That's great, glad it turned out to be OK 
I'd still at least take a look at your spark plugs after, let's say using a tank of gas with the new setup.

Your description of how easy it started and ran without much choke indicates that it is running rich. You can't hurt the engine if it is, but you may foul your plugs very rapidly. I would do a plug chop and see the color...but that's just me.

Quote from: radodrill on May 14, 2014, 09:11:44 PM
My '09 was jetted 17.5/60/130 from the factory. When I installed a K&N drop-in I rejetted to 20/60/142.5; after adding a new performance exhaust I went to 22.5/62.5/142.5

The filter is the bottleneck on max airflow into the engine; and as such has the most pronounced effect on the ideal main jet.  The restrictiveness of the exhaust will be most apparent at low airflow conditions and as such the exhaust is the main factor that affects the necessary pilot (and mid-main) size(s).

Quote from: dennisgb on May 14, 2014, 07:29:16 PM

Quote from: Big Rich on May 14, 2014, 05:53:45 AM
The intake has a much more dramatic effect on jetting than your exhaust.

Sorry don't agree that the air intake has a bigger effect than exhaust...they work together. If you restrict one or the other then you will effect the other...air that goes in has to come out. Your jetting will be too rich.

Granted the intake and exhaust work together, but they affect the airflow in different ways.  As I outlined above, Big Rich was correct in saying that the intake had the most dramatic effect on jetting; the exhaust has a less drastic impact on the jetting.

If you put the head on a flow bench and test it without any manifold or exhaust then put the stock exhaust on you will see a dramatic drop off in the air flow through the head. The volume of air that can be drawn thru the engine is controlled by the components that are on the engine. If the exhaust is restricted you pull less air, irregardless of what air filter you have on the bike. There may be slight difference between the stock filter and the K&N but the amount of air volume will be controlled by the exhaust in this case.

Also there are other issues that can happen such as unburned fuel trapped in the pipe...but then that doesn't seem to be a concern.

Proper jetting is very important to get optimum performance in any engine. It is not easy to get it right. Just saying it will be alright doesn't set well with me...but to each his own.

Quote from: dennisgb on May 15, 2014, 06:39:45 AM
If you put the head on a flow bench and test it without any manifold or exhaust then put the stock exhaust on you will see a dramatic drop off in the air flow through the head.

I think you misunderstood my point.  I was saying (from personal experience) that when comparing the stock exhaust (properly jetted) to a slip-on or full replacement kit (e.g V&H or Motad) they typically are close enough that only small changes to jet sizes are necessary to compensate (same filter in both cases); while going from a stock filter to say a lunchbox often requires more significant jet changes.

I wasn't saying that the exhaust has no effect whatsoever; and as you said there is indeed a huge difference in how it runs between no exhaust and full exhaust.

Quote from: dennisgb on May 15, 2014, 06:39:45 AM
Proper jetting is very important to get optimum performance in any engine. It is not easy to get it right. Just saying it will be alright doesn't set well with me...but to each his own.

I agree; I much prefer the behavior from a properly jetted bike and any change to the intake or exhaust should be followed with checking/adjusting the jetting for optimal performance.

Quote from: radodrill on May 15, 2014, 08:43:42 AMI agree; I much prefer the behavior from a properly jetted bike and any change to the intake or exhaust should be followed with checking/adjusting the jetting for optimal performance.

Many years of experience with building high performance racing engines here.

Too many people go with bolt on performance components without realizing the effect on how the engine runs and what it needs to perform properly. On so many forums you see the question asked "What jetting do I need for this exhaust or air filter?" It is not that easy...published charts on mods and jetting may work for one bike and not another. The use, environment, altitude along with other differences between each bike can make a huge difference. proper carb tuning is an art or science depending on how you look at it. In this case if this were my bike I would be putting an 02 sniffer up the pipe to see what the actual burn is and change the jets accordingly.

These mods are not "plug and play" and in some cases severe engine damage can occur because of not doing it right or caring if it is right.

Sorry to sound so strong but I don't like to see such a casual approach to something that is so important in the performance and life of the bike.

well...i see this has stirred quite the debate. anyway...

the bike has experienced two solid days of riding (approximately 100 miles) with no qualms in performance. it has been in everything from crawling traffic, regular city traffic, and a decent amount of higher RPM highway riding. the wind has been a bit nuts here lately, so the standard crosswind-power drain is the thing only negative that has been noticed.

i got home first today, so i literally stood there in the garage, wrench in hand, to pull the plugs as soon as she pulled up.

and wouldn't you know it. they looked essentially identical to the "ideal" spark plug picture featured in the back of our manual (we have clymer and haynes. i forget which one it was in.) i still plan to check them on a regular basis as things progress. i was a little concerned before, but the plug check has me more confident that the bike won't somehow spontaneously explode.