Need Rejetting Advice

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tricky

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The bike is a 2008 GS500. Last fall I put on a Jardine full system exhaust and K&N lunchbox air filter. I used this page to find a starting point for the jet sizing: http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting

For my bike configuration, it recommends 20/65/145 as the starting point, which is what I put in.

Now I think the bike is running lean... When I first did the modification, I thought it might be running a little rich because my plugs came out really black. Now that I've had time to ride it in normal temperature, I'm thinking its lean, and the sooty plugs are just from burning oil (I lose about 1L per 3000km. Suzuki says anything up to 1L/2000km is acceptable).

Using this guide to identify lean conditions (http://cgi.stanford.edu/~sanjayd/gs500/Upgrades/Rejetting-LeanOrRich),

Typical Lean Conditions:
- Poor acceleration; the engine feels flat. Yes
- The engine won't respond when the throttle is snapped open, but it picks up speed as the throttle is closed. (A too-large main jet also mimics this symptom.) This has never happened to me
- The engine runs hot, knocks, pings and overheats. Not to my knowledge
- The engine surges or hunts when cruising at part-throttle. Slightly, yes
- Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle. Tons of popping and spitting on overrun when downshifting, never when the throttle is being opened
- The engine runs better in warm weather, worse in cool. The jury is still out on this one
- Performance gets worse when the air filter is removed. Have not tried this
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Step 1 is confirm my theory that it's lean.. I will take it out on a really hot day and see how it performs.

I am in the process of reading this article (http://www.motorcyclecruiser.com/tech/motorcycle_jet_kits_carburetion/viewall.html) to get a more thorough handle on the black art of rejetting.

My questions are,

1.) The throttle response seems flat and numb at the lower rev range, and the surging/hunting on cruising (though slight) happens in the midrange. High end seems fine by me, so should I focus more on the pilot and mid jets? Or will I still have to change the main?

2.) With question 1 answered, how big of a sizing change will I be looking at? For example, would going from 20 to 22.5 pilot jet be a big enough leap? 145 to 147.5 main jet? I have no sense of what sizing I should be looking at

It is a pain in the *** to change the jets, so I would like to get it in as few tries as possible.

Thanks for any input.
 
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That will not be a big enough jump for the pilots. When trying to get the right jetting, always change one thing at a time. That way you know when you go wrong and where.

Those things run notoriously lean in factory spec, and any pipe/air filter combination exacerbates the problems immediately. Try and get the idle settings dialed in with different pilots and pilot screw settings, then worry about the jet needle setting, then deal with the mains. The GS500's are touchy too, adjust the pilot screws to far and the throttle will hang-up on the way back down, then you have to fiddle with the idle stop and go back and forth until you find the right settings.
 
Thanks you two.

Took Brian's advice, shimmed my needles 1.1 mm to richen, and also drilled out the brass plugs and adjusted the mixture screws, 3 turns out. Seems to be running better, it has the feeling back in the low end. Right on take off still feels a little sketchy, but that could very well be the dead space on my throttle that I need to fix. Might go to 3.5 turns on the mixture screws.

Helluva job, had to take an angle grinder to those JIS soft brass screws to cut in slot heads. Replaced them with hex bolts, and replaced the float bowl screws while I was at it, which were cut with a dremel last year when I changed the jets.

On the plus side, I scored myself a shop to work in. Rolled the bike into Kerr Hall at Ryerson, put it in the aero design team shop (I'm a member through the year). All the tools I need, with the sumptuous luxury of office chairs and proper working space.
 
Low end = idle jets & mixture screws, Mid = needles, Top end = mains. You can always preform a plug chop to see just how you are running. When messing with carbs remember to document EVERYTHING so you can put it back if what you try is worse.
 
skip said:
Low end = idle jets & mixture screws, Mid = needles, Top end = mains. You can always preform a plug chop to see just how you are running. When messing with carbs remember to document EVERYTHING so you can put it back if what you try is worse.
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Yea, the lean surging in my mid range seems to have gone, and the power feels smoother.. I also know the mixture screw adjustment really helped my low end. My top end is in good condition.

If I were to do it again (maybe in the future) I'll probably jump up a size on the pilot jet.

Theoretical question, if my mid-range is lacking, how would I know whether to alter the needle or mid jet? I think I have a handle on the pilots and mains.. am I right in thinking the needle effects a larger rev range then the mid jet?
 
There is no such thing as a mid jet. The needle position is the mid range. Fuel for the needle goes through the main. Vacuum slide up (high load on engine) main jet governs. Less load, needle governs.
 
Brian P said:
There is no such thing as a mid jet. The needle position is the mid range. Fuel for the needle goes through the main. Vacuum slide up (high load on engine) main jet governs. Less load, needle governs.
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But my carb has 3 jets.. pilots, mids and mains, plus the needle... 01 and earlier GS500s only had pilots and mains, is that what you are referring to?
 
That other jet is the "starter" jet. I have seen that mid-range issue, it can be a combination of pilot size and needle position depending if it is a low-mid-range or high-mid-range. There is always a bit of an overlap of affected rpm ranges for the different circuits in the carb.
 
Iggy76 said:
That other jet is the "starter" jet. I have seen that mid-range issue, it can be a combination of pilot size and needle position depending if it is a low-mid-range or high-mid-range. There is always a bit of an overlap of affected rpm ranges for the different circuits in the carb.
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Could you elaborate on what a "starter" jet is? To help with the transition from pilot to main?

Found this post in GSTwins, apparently no one really knows exactly what range the mid jet covers... http://gstwins.com/gsboard/index.php?topic=43853.0

Pilot---- 0-1/8th throttle. Typically take off rideability is adjusted with this. Ilde is adjusted with Turns out ... but they both control that 0-1/8th throttle.

Mid-Main--- Who knows ... they came up with this BS possibly after we decoded the needles. Cynical me.

Main---- 3/4 throttle and up

Needle Shim--- (washers on needles) Needle controls 1/4 to 3/4 throttle, shimming them lets a smaller diameter serve as an obstruction in the emulsion tube. So it stays at 1/4 to 3/4 throttle for effect, but acts like its a hair more open than it is without the shim.

Turns out--- Idle and take off, and adjust it for idle but take off also can be affected.
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Starter jet is exactly what it sounds like: it meters the fuel provided to the starter circuit.

That definition you found is pretty close, nothing glaringly wrong. However the pilot jet and screw combination also affects decel quiet a bit as well.
 
just had it out on the highway, something really funky going on around 7.5k rpm.. feels like fuel starvation or crazy lean surges. Thats main jet territory... I'm a little confused

Sent from my SGH-I747M using Tapatalk 4 Beta
 
If everything was working fine and suddenly things started to go wrong, it might be a clogged jet or clogged air/fuel lines. Check your tank breather and fuel filter for kinks/obstructions and clean your carbs. Did you spray carb cleaner in all the fuel/air circuits when you were changing jets? Blow them out with compressed air too if you have access to that.

If fue/air lines and carbs check out then it might be ignition related.

In response to the 'mid jet' comment: The three jets in the float bowl are the starter, pilot and main jets. However the main jet is also connected to the needle jet (not to be confused with the jet needle) which contributes quite a bit to low throttle operation (although not necessarily low rpm).
 
Iggy76 said:
Starter jet is exactly what it sounds like: it meters the fuel provided to the starter circuit.
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Otherwise, commonly but mistakenly referred to as the "choke" circuit.

Motorcycle carbs built in the last 30 years don't actually use a "choke", they use an enriching valve (which opens and adds extra air and fuel to raise the idle speed and run the engine slightly richer during and after cold starting).

Bottom line, as noted above, the 3 jets are the pilot/slow jet, the main jet, and the starting/enriching jet. Fuel that goes through the main jet is blended with air from the main air bleed and then that emulsion of air and fuel goes through another orifice that varies in size based on the needle position. The vacuum slide that positions the needle goes up or down according to the amount of air going into the engine. Changing the clip position changes the relative position of that needle to the vacuum slide.
 
Tricky,

Being an old school bike rider I am quite familiar with carbs.

A trick I use that helps if your on the lean range to gauge how far off you are is duct tape heh. I know right. This is what I do.

Basically whether you are running pods or flat filter or whatever, what you want to do is just tape off half of the air filter element/pods/ filter. Take it out for a run, you should go right past the flat spot or point you felt hesitation. If that is the case, you need to richen that area where you feel the flat spot or hesitation.

Also, you should be tuning from your largest jet you have backwards. I wouldn't worry too much about your pilot jet too much, stock will usually suffice even on a modded motor. Its nice as well as you can get a much better mileage at steady state cruise(slightly leaner). My GSXR1100 has pods, stage 3 jet kit, ported head, full exhaust, advancer and pistons and I still run the stock, 35 pilots with no problems. On the highway I can almost hit 300km before switching to reserve.

I know it sounds simple but try it, you basically just temporarily richening the mixture using a caveman method.

Let us know.
 
Brian P said:
Otherwise, commonly but mistakenly referred to as the "choke" circuit.

Motorcycle carbs built in the last 30 years don't actually use a "choke", they use an enriching valve (which opens and adds extra air and fuel to raise the idle speed and run the engine slightly richer during and after cold starting).

Bottom line, as noted above, the 3 jets are the pilot/slow jet, the main jet, and the starting/enriching jet. Fuel that goes through the main jet is blended with air from the main air bleed and then that emulsion of air and fuel goes through another orifice that varies in size based on the needle position. The vacuum slide that positions the needle goes up or down according to the amount of air going into the engine. Changing the clip position changes the relative position of that needle to the vacuum slide.
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Brian is correct here, my bike will not start without choke plunger being pulled. Looking into my carbs that portion is basically small holes that let more fuel in when the plunger is pulled.

As far as changing the clip, it doesn't come into play as much until you start rolling on around 1/3 the way through. Lowering the needle in the hole(moving up on the notches) makes it leaner, raising the needle(moving down on the notches) makes it richer.
 
tricky said:
just had it out on the highway, something really funky going on around 7.5k rpm.. feels like fuel starvation or crazy lean surges. Thats main jet territory... I'm a little confused

Sent from my SGH-I747M using Tapatalk 4 Beta
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Full throttle 7500 rpm, or part-throttle-cruise 7500 rpm?

At full throttle, it will be on the main jet, but part-throttle cruise is on the needle. Keep in mind that by design, there is a wide range of overlap between which thing affects what. If there is a problem at full throttle 7500 rpm, it is quite likely that your full exhaust system and open air filter are interfering with how the engine runs in that RPM range. If you took out the stock airbox, at a minimum you are going to need bigger main jets (because removing the airbox reduces the vacuum signal that the carbs see). It is quite possible that the engine can't be made to run smoothly without the airbox.
 
Brian P said:
Full throttle 7500 rpm, or part-throttle-cruise 7500 rpm?

At full throttle, it will be on the main jet, but part-throttle cruise is on the needle. Keep in mind that by design, there is a wide range of overlap between which thing affects what. If there is a problem at full throttle 7500 rpm, it is quite likely that your full exhaust system and open air filter are interfering with how the engine runs in that RPM range. If you took out the stock airbox, at a minimum you are going to need bigger main jets (because removing the airbox reduces the vacuum signal that the carbs see). It is quite possible that the engine can't be made to run smoothly without the airbox.
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Part throttle, which is why I'm thinking the needle is the culprit here. WOT through that band doesn't give me an issue. The exhaust and airbox are both common mods for my bike, in all my reading up on it I've never heard of someone saying it can't be made to run smoothly.

It could possibly be rich where the needle and main jet overlap... before I shimmed the needle, the upper rev range was fine, after shimming to richen it, the problem popped up.

I have ordered several different main jet sizes, some bigger and some smaller than the one in there already (145 is in there, 130 is stock I think). I will first determine if it's lean or rich by checking the plugs or patching up the filter like ANDRASTA said, then I'll try out different main jet sizes and/or different needle shims to see if I can't get it sorted out
 
All needles are also not made the same. There are different tappers to them. See if you can scrounge up a different shaped needle as well.
 
Tricky

When i had my gs500 i shimmed the needles with 2 washers and i went up one size on each jet, i dont remember the jet sizes now its been a while.

Mine had 2 jets not 3 like yours but same difference.

I will bet anything that you are WAY lean.

I only had a k&n drop in filter, and i was was barely runnin normal with my set up. Without the the 2 shims to raise the needle the bike was super lean at idle and roll off. And without the jets.. "Forgetaboutit"

The gs is super lean bro. If you put the filter and the exhaust you best make sure you shim the needles (2 washers) and maybe go up two sizes on the jets

And then i would tinker with the mixture screw until you will feel the crisp response and take off
 
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