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.

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.

Quote from: chris900f on August 21, 2023, 06:11:44 PMI 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.

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.

Quote from: mr72 on October 06, 2023, 04:24:37 AMThis 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.

Quote from: chris900f on October 11, 2023, 07:34:06 PMYes 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.

Quote from: SK Racing on October 12, 2023, 12:51:32 AMThe 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.

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

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,

Quote from: moe_tunes on November 17, 2023, 12:48:37 AMGS 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.

Quote from: mr72 on November 17, 2023, 05:13:21 AMThe 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."

Last summer, while doing the valve adjustment, I decided to re-jet, dropping the mid-main down to a 62.5. I did this mainly to improve choke function (which it did). The bike still ran well, very clean, but there was a definite loss of mid-range power and acceleration. Not a flat spot, just generally lazier response. One notable difference was the complete elimination of decel-pop, which was present with the 65.

So having 65 and 142.5 mains sitting in the toolbox, I decided to drill some more holes. I reasoned that if you could tune for pods or lunchbox, a few more holes would be manageable.

Originally I really wanted to add one more filter on each side, over each carb; again I was limited by space inside box and the large velocity stacks ("trumpets")

So I settled on adding just one more filter, in a good spot where it didn't obstruct the big filter, cleared the stacks, and kept enough distance from the breather that it wouldn't get fouled with oil.

I re-did the original inlet tubes as well, securing them with screws and sealing the edges with silicone. I could not find short enough wood screws, so I had to custom grind what I had laying around. I'll upgrade those to something more flush in the future. The screws are really only there to make it easier to press on a new filter without displacing the tubes. Also note that because of its location above the carbs, the filter is secured with a zip-tie, so it can't work loose and get sucked into a carb.

Currently jetted 20/65/142.5 and the performance is pretty solid. (note tuning location is 2200ft ASL)
Main takeaways after 250kms.

--I have been running a 15t front sprocket (for years), because I felt the 16t made the bike too sluggish. Now I am running the 16t and all is well.

--More torque and roll-on power at any speed, in any gear. 6th gear roll-on from 100kph negates the need for a down-shift to pass in many cases.

--I haven't done a top speed run yet, but it hit 160 pretty easily before so...

--Not running rich, no decel-pop even with the 65 mid. I could probably run a 67.5 or even a 70, but 65 seems to be the limit, because the jet is dual-use with the choke-circuit. Although I no longer have the computer/O2 sensor installed to verify, I think a 145 main would still pull cleanly.

--The sound of the exhaust is deeper and throatier, and you can hear that there's more cylinder pressure (compression)

--Stock-bike smoothness. I'm not feeling any stumbles, stutters or flat-spots.

The velocity stacks inside the airbox are quite impressive. Clearly they are designed to improve the efficiency of air flow through the carb; but with a clamp-on style filter you lose that factory finesse. So I am biased, but I think that this is the superior set-up.

@herennow do you recall Buddha post says middle jet was only affecting choke. Here was my reference to try with mid main for popping.

@chris900f
Do you still have the wide and sensor, I think its great to be able to log afr and can give us a great deal of info for fine tuning.
If I wanted to to this mod(afr logging), I need to tap the header/soler a nut?
Which sensor do you advice to use with some kind of arduino/esp32 or Pic microcontroller. Don't know how electrical noise could affect it. Ignition

Quote from: Armandorf on August 06, 2025, 03:06:03 PM@herennow do you recall Buddha post says middle jet was only affecting choke. Here was my reference to try with mid main for popping.

@chris900f
Do you still have the wide and sensor, I think its great to be able to log afr and can give us a great deal of info for fine tuning.
If I wanted to to this mod(afr logging), I need to tap the header/soler a nut?
Which sensor do you advice to use with some kind of arduino/esp32 or Pic microcontroller. Don't know how electrical noise could affect it. Ignition

Aug 6...wow sorry for the late reply.

1) The mid-main is dual function. When the choke is on, fuel is pulled from the mid-main to the choke circuit. When the choke is off the MM operates as its name would suggest. filling the gap between the pilot and the main during part-throttle operation. Review my earlier posts with the O2 sensor datalog.

2) No I removed all the EFI stuff, I may try again as I have seen some new fuel pumps that have a send/return built in. That said, the MAP based EFI is not as tunable on a twin, because of the gap between intake strokes.

If you just want to monitor your engine AFR's with carbs, you could use a simplified version of my set-up. You need MAP, RPM and O2 inputs for the most basic datalogging. You could add the temp sensors, but they are not necessary.
I used a common narrowband O2 sensor from a Honda accord and tapped the crossover pipe between the headers. The bungs are available on Amazon...be sure to get the right size. Also, some muffler shops might be able to do this work for you. I had my Arduino powered by the taillight circuit, and didn't have any trouble with ignition noise in the circuit.

Hope this helps, Cheers.

Noticed that the bike was getting hot on long rides. A plug-check revealed some whitish ash on the plug tip so I decided to bump-up the main-jets one size from 142.5 to 145. My local shop only had 147.5's as the next size so I decided to try them out.

Here's the plug check after today's ride--about 20km at highway speed followed by 10km of city to get back home. Current jetting is 20/65/147.5



This is the darker side of the plug and shows almost no color. It was a cold day to be riding, at only 10C
but it still looks like I could go up another size. Anybody running 150 or 152.5?