Tuning for K&N drop-in plus slip-on, and an airbox mod

[chris900f]

After I got my V&H slip-on installed I decided to get a K&N drop-in filter--so I needed to re-jet. I was able to view the output from my o2 sensor at each step. What I saw was that the slip-on made things slightly leaner, and the combination of filter and slip-on really affected the part-throttle cruise, with very lean readings and somewhat weaker performance. I didn't do too much full throttle running with this set-up as I was worried about damaging the engine.

What I did see was that idle was still pretty close to stock readings but the off-idle, partial throttle cruise and mid-range were most affected.

Consulting the wiki for jet suggestions I bought 20 pilots, 62.5 and 65 mid-mains and 142.5 mains. After reviewing my datalogs I got some 137.5 mains as I felt 142.5 would be too rich.

I ended up installing: 20 pilots, 65 mid-mains, and the 137.5 mains.

This set-up gave me a very rich idle/off-idle and I set the fuel screws to 1.5 turns out. Even then the bike only needs about 1 minute of choke before you need to cut it back to 1/2 choke for another minute before turning it off completely.

Running throughout the range the performance was quite good. I did notice one rich stumble, if you revved the bike out in 3rd gear it would choke out a little at around 8000rpm, but the problem wouldn't show up in 4th gear. The mid-range running from 4000 to 7500 was really good with great throttle response and acceleration. Still overall I could tell I was still a bit too rich. Full speed runs felt dull and slightly more sluggish.

Back to the datalogs I saw that the mids were close to ideal with lots of 12.5:1 AFRs but the idle was still too rich at 10.6:1 and I was even seeing 10's in the high rpm range, despite only running a 137.5 main.

I went back and forth about re-jetting again: 1) I hate cracking open the carbs too often, I got away with re-using the bowl gaskets but should probably replace them 2) what should I do. Should I go back to stock pilots and give up the much-improved off idle (I can gently release the clutch in 1st and get moving with NO throttle) If I reduce the mid-main will that alter the pilot-zone and the main-zone? Should I just go back to stock mains or buy yet more jets (135s)? Should I just resolve to shift before 7000rpm and leave everything alone since it's pretty good?

In the spirit of "more is more" I decided that the best course of tuning was to get more air into the mix. I have a 54mm UNI foam filter that I thought I could squeeze in, but after pulling the air-box I realized I had over estimated the available space inside. I looked at adding an Emgo cone filter, but that meant waiting for parts that might not work.

Drilling holes in the air-box is not really a recommended procedure--I realized that if things went south I would looking for another one on eBay or living with duct tape.

Here we go:


This is the filter on my 175cc Toro lawnmower. They do a great job of keeping the intake clean in very dusty, dirty conditions.


It's about 4.5" long and 1.25" thick The holes are are 28mm and seal up tight to a 30mm intake tube. The rubber is pliable. I get these 6 at a time on Amazon for about $5.00 each...note that on the mower only one intake hole is used the other side is just a solid plug.


I swapped a fresh filter into the mower and cut up the dirty one to make a template.


Test fitting for location, slightly offside to allow for open space on all sides of the filter material. Another choice would have been upright on the back wall of the box. I decided that the back floor would give better flow as it was just under the main filter. The back of the box is a cool-air zone, without much turbulence.


For intake tubes I discovered that the big syringes I used for oil and as temporary fuel supply were exactly 30mm. The flanges were a plus, as I didn't need to used any epoxy or silicone to keep them in place and the seal is really good--the vape is important too...it took me large amounts of nicotine to get the nerve to drill those holes I cut the tubes longer than they needed to be, so I guess I could trim them down to get more flow.

The main filter just barely lays against the top of the secondary filter so there is no chance of it moving once the big filter is installed. Also note that the stock filter fits just fine as well, so if you are brave, you could save a fair chunk of cash, getting more flow without shelling out for a K&N.


needlessly complex computerized non-injected GS500F.


Pretty much invisible air-box mod.


Results: right away I could see the idle AFR's lean out--changed the fuel screw settings to 2.5 turns out. The cruise and part-throttle are all cleaner. The rich-stumble at 8000rpm in 3rd is gone. Apparently it's not cool to mention speed in top-end runs so I'll just say it pulled strong to 9000rpm in 6th gear and was still pulling when I saw the magic number and chickened-out. The datalog shows a cleaner, leaner mid-range and the top-end is in the 11's instead of the 10's---overall I call it a success.

[mr72]

This is very interesting. My GS has a weird stumble at part throttle roll-on in midrange rpms, but pulls and runs great at all other conditions. I figured it was my best compromise after doing a whole lot of tinkering with jets and needles and all that. I wonder if my UNI drop-in filter has anything to do with this...

I really like the lawn-mower filter mod and keeping the airbox. Great idea. I am about to try swapping a whole different kind of carb into my scooter and I am seriously considering also switching to a lawn tractor muffler and adapting a lawn mower air filter.

[SK Racing]

I ended up installing: 20 pilots, 65 mid-mains, and the 137.5 mains.

This set-up gave me a very rich idle/off-idle and I set the fuel screws to 1.5 turns out.

Your GS looks very similar to mine, which is a 2004 Spanish model. Do I have to drill out the caps to reach the fuel screws? I really need to get to know my carbs. The backfire problem that I have lately is driving me nuts.

[chris900f]

Yes I had to pull the welch plugs to get to the fuel screws. The good thing is that Mikuni thoughtfully pre-drills a tiny hole in the centre of each plug. I drilled out the hole a bit and drove in a woodscrew (prepped with its tip ground off) and a pliers to pull them out. They actually popped out without too much effort.

[chris900f]

This is very interesting. My GS has a weird stumble at part throttle roll-on in midrange rpms, but pulls and runs great at all other conditions. I figured it was my best compromise after doing a whole lot of tinkering with jets and needles and all that. I wonder if my UNI drop-in filter has anything to do with this...

I really like the lawn-mower filter mod and keeping the airbox. Great idea. I am about to try swapping a whole different kind of carb into my scooter and I am seriously considering also switching to a lawn tractor muffler and adapting a lawn mower air filter.

I have A set of K&N pods and a single UNI (all 55mm). I used to have a very powerful Shopvac at work and its hose was a good fit for the 55mm flanges. When I tested the flow, it was obvious from the sound that the UNI was not on the same level as the K&N. The K&N didn't cause any change in the sound of the vacuum; the UNI sounded similar to putting your hand over the vacuum inlet. How is it with the stock paper filter?

With the exception of the fatter mid-main, my jetting is pretty conservative compared to what I read in the Wiki: 20/65/137.5 with stock non-adjustable needles, that's with the K&N, plus the lawnmower filter and a straight-thru glass-pack slip-on. My o2 sensor indicates I could probably add another mini-filter and still not be "too" lean. Right now I think I'm "optimally rich", I still see some 14-15:1 AFRs in the cruise zone and lots of 12-12.5 everywhere else, but not I'm generating any soot in the exhaust and the plugs are clean.

[SK Racing]

Yes I had to pull the welch plugs to get to the fuel screws. The good thing is that Mikuni thoughtfully pre-drills a tiny hole in the centre of each plug. I drilled out the hole a bit and drove in a woodscrew (prepped with its tip ground off) and a pliers to pull them out. They actually popped out without too much effort.

Thank you, I will do that soon when I take the carbs off. The bike is difficult to ride because I can't brake on compression for fear of really bad backfiring. I still don't know went went wrong.

[mr72]

The bike is difficult to ride because I can't brake on compression for fear of really bad backfiring.

Crackling on decel is normal and not a sign of running lean. It's actually a sign of running rich if anything, but it means you are getting unburnt fuel into the exhaust while decelerating. EFI systems prevent this by doing a full-on fuel cut on decel, but even my Bonneville will crackle on decel if you give it just a tiny bit of throttle during decel to bypass the programmed fuel cut. Carbs can't cut the fuel 100%.

Backfiring is a whole different thing, that's when it's igniting fuel in the intake tract and it's very loud almost like a gunshot because there's no muffler on that side to mute it.

[moe_tunes]

Here is a vid of a honda mechanic adjusting the idle mixture screw to fix up a backfire problem that happened when a bloke opened up the pipes on his bike. The helpful part of the vid is near the end where he demonstrates how the idle mixture is right with how the engine drops straight back to idle. The idle drop test as he calls it.
Point to note: he mentions it in the vid but the carbs he is working on have idle fuel mixture screws and the GS has idle air mixture screws so on the GS turn the idle mixture screw in to add more fuel..

EDIT : Click on vid for the link

[moe_tunes]

Here's another vid where the same bloke gives some tips for the home tuner on how to set carbs up for pod filters. For those new to changing stock bikes I feel it could be useful. the_buddha has done a lot of work on carb settings for the GS so do use the search function here.

Hope it helps,

[mr72]

GS has idle air mixture screws so on the GS turn the idle mixture screw in to add more fuel..

The reality is exactly opposite of this. Glad I turned off hide long enough to see this post. There's a link in my signature with better information on setting the idle mixture on a GS500.

[Bluesmudge]

The reality is exactly opposite of this. Glad I turned off hide long enough to see this post. There's a link in my signature with better information on setting the idle mixture on a GS500.

I was going to say...that seems wrong. I'm also fairly certain that screwing the mixture screws in makes it leaner. If there is no air to pull in fuel, then there is less fuel. "turn the idle mixture screw in to add more fuel" would be like saying, "use less throttle to add more fuel."