The finer points of tuning GS450S carbs.

[IanF]

Wow

Thanks for all the responses guys Didn't expect this much response to a GS450 question since most of you ride four-bangers. Yes, the needles do have one slot and clip, making them non-adjustable without adding washers/spacers equal to some fraction of the OE nylon spacer, which would lower the needle, leaning it out and bringing it back toward stock. I have nothing in there right now, wanting the bike to be a little on the fat side at closed to 1/2 throttle openings. My frustration was at very small throttle openings where the pilot circuit and main circuit are both working and in transition from one to the other. Trying to open and close the throttle at 5k to 6k rpm was giving me lots of stumbling and surging.

I am pretty embarrassed. I completely forgot to check the pilot screws, thinking they were still where I left them, at approx. 1.5 to 2 turns out. They were not. The left was at 2.5 and the right at 1 - Doh! No clue how they got that far off. After setting them both back to 1.5 the symptoms are not completely eliminated but the engine is much more responsive to small throttle openings and seems to be its normal self again, although not perfect.

I remember someone saying it took him a lot of fiddling to get his 450 running the way he liked it with decent fueling at all throttle openings and throughout the rev range. I was never after that much perfection so lived with it the way it is now. It's certainly ridable as is especially for a newb.

Thanks again for all the suggestions. Still going to work on the airbox and get it sealed and experiment with different pilot screw settings and some thin washers. On my VX800 the rear carb has the same nylon spacer leaning out the carb and I ended up taking a dremel to the spacer and grinding it down to about 60% of its original thickness which richened it up enough to change the pipe from blueish to goldish. No sense in running too fat.

[IanF]

correction

Correction - I went with the float height specified in my factory service manual which was 22.4mm but I just remembered the number incorrectly. Thanks again guys

[Nessism]

Thanks for all the responses guys Didn't expect this much response to a GS450 question since most of you ride four-bangers. Yes, the needles do have one slot and clip, making them non-adjustable without adding washers/spacers equal to some fraction of the OE nylon spacer, which would lower the needle, leaning it out and bringing it back toward stock. I have nothing in there right now, wanting the bike to be a little on the fat side at closed to 1/2 throttle openings. My frustration was at very small throttle openings where the pilot circuit and main circuit are both working and in transition from one to the other. Trying to open and close the throttle at 5k to 6k rpm was giving me lots of stumbling and surging.

I am pretty embarrassed. I completely forgot to check the pilot screws, thinking they were still where I left them, at approx. 1.5 to 2 turns out. They were not. The left was at 2.5 and the right at 1 - Doh! No clue how they got that far off. After setting them both back to 1.5 the symptoms are not completely eliminated but the engine is much more responsive to small throttle openings and seems to be its normal self again, although not perfect.

1.5 turns open is too lean for the pilot screws in my experience - that's why I recommended 3 turns in my other response. More open means more fuel so if the bike is lean, you need to open them more. Also, what about the carb sync? You can't fine tune the carbs until the basic maintenance is complete.

[BassCliff]

Hi,

After Ian gets his airbox boots and other bits needed to tighten up the intake system, I'll have him over and connect up my Carbtune. We'll tweak those carbs to life.


Thank you for your indulgence,

BassCliff

[pete]

Now there's an offer you can't refuse Ian!

[IanF]

1.5 turns open is too lean for the pilot screws in my experience - that's why I recommended 3 turns in my other response. More open means more fuel so if the bike is lean, you need to open them more. Also, what about the carb sync? You can't fine tune the carbs until the basic maintenance is complete.

Yes, 1.5 turns out were not enough. 2 seems to be working much better. The bike is now running better than I remember it, but I haven't had a chance to give it a nice long workout or run it on the freeway at a constant 65-70mph. If I went to 3 turns out would it be smart to shim the needles down a bit, in the interest of mpg? I have a throttle lock which has a setscrew conveniently located to give me an index mark to easily tell if I'm at half throttle or wherever and I noticed that that if I'm not on the freeway I generally use the motor in one of two ways, either loafing along under 5k with the throttle as closed as possible or looking for more of the power, and using lots more throttle, over halfway open. Well, engine braking would be number three. So, the height of the needle may not be as much of an issue when it's only a transitory stage between small and large throttle openings. I still don't know how the bike will do when asked to run many miles at a constant 6k or more.

As far the rest of the maintenance, I've never disturbed the sync on these carbs. I did the valve clearances, sparkplugs and oil just a hundred miles ago. On my VX800 I just did a "bench" sync, adjusting the butterflies until they clicked shut in unison. That was good enough for that bike, but I'm sure it wouldn't hurt to get more precise with the sync on both bikes. I think once the airbox is sealed up it will be even better. The GS450 seems quite usable and relaxing to ride now just with getting the pilot screws in the ballpark. Maybe I should trying syncing the carbs at 6k since that's where it runs on the freeway, but that's a lot of revs with no load and a lot of noise. I guess I'd have to use a fan and earplugs. The engine is smooth at idle and all the way up over 5k as is.

Thanks again for all the interest gentlemen! I'm honored.

[Nessism]

Just for grins you might want to try a getto sync (good for twins): set the idle speed up to 2500 rpm or so and then take turns pulling off one spark plug cap to see how far the rpm's drop. Set the sync so the rpm when running on one cylinder is the same for each side. After doing this double check with Cliff's gauge. Might be interesting...

The pilot screws should only effect low throttle position use, but the needle should have a broader effect. I don't know if you would have to drop the needle if running at 3 turns...that's fodder for some trial and error iteration. Easy to do without a lot of drama.

[KEITH KRAUSE]

The jet needle assembly must be correct for the bike to run well. If you're missing a thick spacer that's supposed to be there then the mixture won't be even close to what is needed. Sounds to me like your problems are related to the throttle position range that the jet needle regulates. I see no point in adjusting other parts to compensate for what seems to me an obvious problem. Compensating never works when dealing with the jetting.
AFTER replacing the missing jet needle spacers I'd clean the carbs too. Sitting unused for 4 years suggests they aren't clean enough. The smaller holes involved in jetting parts will get a layer of varnish (old gas) and just add to the problem. So will carbs that need to be synched. Just typical riding will eventually cause the carbs to loose their synch. You also mention possible air leaks which compromises the mixture and also the vacuum on your type of carbs.
Add up all the above and you can have problems exactly as you describe.

[IanF]

The jet needle assembly must be correct for the bike to run well. If you're missing a thick spacer that's supposed to be there then the mixture won't be even close to what is needed. Sounds to me like your problems are related to the throttle position range that the jet needle regulates. I see no point in adjusting other parts to compensate for what seems to me an obvious problem. Compensating never works when dealing with the jetting.
AFTER replacing the missing jet needle spacers I'd clean the carbs too. Sitting unused for 4 years suggests they aren't clean enough. The smaller holes involved in jetting parts will get a layer of varnish (old gas) and just add to the problem. So will carbs that need to be synched. Just typical riding will eventually cause the carbs to loose their synch. You also mention possible air leaks which compromises the mixture and also the vacuum on your type of carbs.
Add up all the above and you can have problems exactly as you describe.

Thank you, Keith. Just to clarify, the bike hasn't been sitting for four years. I purchased it in March 2005 with 8k miles and put on 14k the first year, then bought the VX800 since I was piling the miles on so rapidly. Since then the GS450 has been ridden at least 30 miles per month and the fuel treated with either Stabil or Seafoam to prevent varnishing. The spacers, washers and airbox parts arrived yesterday so I will be installing them soon to see what effect they have. The air leaks are on the airbox side not the intake boot side. I had already replaced the intake boot o-rings and swapped the fasteners for Allen heads.

My friend Cliff says that most of the experienced guys here say to put everything back to stock and then deviate as necessary. However, I have read from numerous sources that all these GS bikes were set too lean at the factory to meet EPA regulations, which caused driveability issues especially when cold. My original intention was to sand these spacers down to about 60% of the original thickness like I did with my VX800. Now I am not sure what I should do first. It seems pointless to put the bike back to stock if indeed that will make it run worse!

[cowboyup3371]

Ian,
Putting it back to stock only means putting the stock equipment (airbox, carbs, exhaust, etc) back on it and then tuning it from there. If you do everything right you won't make it run worse, it will be exactly the way it needs to run smoothly. Follow the advice here and have fun at it. You'll love how these bikes run.

[KEITH KRAUSE]

My friend Cliff says that most of the experienced guys here say to put everything back to stock and then deviate as necessary. However, I have read from numerous sources that all these GS bikes were set too lean at the factory to meet EPA regulations, which caused driveability issues especially when cold. My original intention was to sand these spacers down to about 60% of the original thickness like I did with my VX800. Now I am not sure what I should do first. It seems pointless to put the bike back to stock if indeed that will make it run worse!

If you know the carbs are reasonably clean and there are no leaks on either side of the carbs or airbox body, then my concern is the missing factory spacer for the jet needles.
Now, since you believe the bike comes a little bit lean with totally stock jetting then here's what I think would work for you.
You just want to richen it up a little. I doubt the main jet needs changing so that leaves the pilot circuit (to help cold-start/warm up period) and the jet needle.
Minor adjustments to the pilot circuit can be made at the mixture screws. The mixture screw assists the pilot jet and is intended to be the fine tuning for each cylinder. The mixture screws should be capable of giving you that little extra richness you want. Turning them out (counter-clockwise) richens that cylinder. How much to adjust them depends on how much on the lean side you actually are. I'd try an additional 1/2 turn out from where they are, provided they are currently very close to the factory setting (but I think you said you moved them). GENERALLY, the factory would set them about 3/4 to 1 turn out. Based on that, I'd try the additional 1/2 turn and then test. If your screws move freely and the carb body threads aren't gummed up with old factory sealant, etc, then I'd record the settings you have now so you can always return them to where you started should adjustments give poor results. Remember to only seat the screws lightly. I see no reason simple richer screw adjustments won't be enough to make your pilot circuit a little richer as you want it.
As for the jet needle adjustment, that's a little more difficult. Replacing the spacer is necessary but how much to remove is the question. Again, it depends on how much on the lean side you really are. You're only trying to richen it up a little to make up for the factory leaning things out to meet restrictions as you believe. You're not richening it to allow for significant intake or exhaust changes. There's no pre-determined amount to remove from the spacer because each bike is a little different. Without a Dyno or such equipment all you can do is guess based on common sense. How particular are you? Will you try/test several times or just remove some material once and call it good enough? You don't want to remove too much.
My GUESS would be based on this.....raising the jet needle about .045" is generally one full position/groove (if you had adjustable needles). This amount of adjustment is significant and is often too much to compensate for a free flow exhaust alone. Something closer to half that, .022", is usually a better choice though that may even be too much because free flow exhausts tend to only flow much freer at the higher rpm's that are associated with the main jet and not the jet needle. That's why many exhaust manufacturers don't suggest jet needle changes for an exhaust only mod but do say you need larger mains.
But I doubt your bikes jet needle circuit is as lean as a bike that has had a free flow exhaust installed. I don't think you even need a 1/2 position (.022") change to your spacers. Since you don't want to remove too much from the spacer, I'd try .015" at the most. Maybe .012 if you have the ability to be precise. If you remove too much then you'll be back out hunting for replacement washers.
That's my best guess. Remember, at approx' 1/4 throttle position and less, there is SOME pilot jet effect that overlaps the jet needle circuit. So those little bit richer screw adjustments you made will already have some effect on the jet needle circuit. With the spacers being modified to raise the jet needles approx' 1/4 position (about .011/.012), I would think that would be enough to compensate for the factory lean jetting. No way to know of course than testing.
Also, If you went out with new spacers and did a good 1/3 to 1/2 throttle chop test you might get acceptable color on your plugs. Unless you noticed obvious jet needle performance issues, leave the new spacers alone and be content with richening the mixture screws only.

[IanF]

If you know the carbs are reasonably clean and there are no leaks on either side of the carbs or airbox body, then my concern is the missing factory spacer for the jet needles.
Now, since you believe the bike comes a little bit lean with totally stock jetting then here's what I think would work for you.
You just want to richen it up a little. I doubt the main jet needs changing so that leaves the pilot circuit (to help cold-start/warm up period) and the jet needle.
Minor adjustments to the pilot circuit can be made at the mixture screws. The mixture screw assists the pilot jet and is intended to be the fine tuning for each cylinder. The mixture screws should be capable of giving you that little extra richness you want. Turning them out (counter-clockwise) richens that cylinder. How much to adjust them depends on how much on the lean side you actually are. I'd try an additional 1/2 turn out from where they are, provided they are currently very close to the factory setting (but I think you said you moved them). GENERALLY, the factory would set them about 3/4 to 1 turn out. Based on that, I'd try the additional 1/2 turn and then test. If your screws move freely and the carb body threads aren't gummed up with old factory sealant, etc, then I'd record the settings you have now so you can always return them to where you started should adjustments give poor results. Remember to only seat the screws lightly. I see no reason simple richer screw adjustments won't be enough to make your pilot circuit a little richer as you want it.
As for the jet needle adjustment, that's a little more difficult. Replacing the spacer is necessary but how much to remove is the question. Again, it depends on how much on the lean side you really are. You're only trying to richen it up a little to make up for the factory leaning things out to meet restrictions as you believe. You're not richening it to allow for significant intake or exhaust changes. There's no pre-determined amount to remove from the spacer because each bike is a little different. Without a Dyno or such equipment all you can do is guess based on common sense. How particular are you? Will you try/test several times or just remove some material once and call it good enough? You don't want to remove too much.
My GUESS would be based on this.....raising the jet needle about .045" is generally one full position/groove (if you had adjustable needles). This amount of adjustment is significant and is often too much to compensate for a free flow exhaust alone. Something closer to half that, .022", is usually a better choice though that may even be too much because free flow exhausts tend to only flow much freer at the higher rpm's that are associated with the main jet and not the jet needle. That's why many exhaust manufacturers don't suggest jet needle changes for an exhaust only mod but do say you need larger mains.
But I doubt your bikes jet needle circuit is as lean as a bike that has had a free flow exhaust installed. I don't think you even need a 1/2 position (.022") change to your spacers. Since you don't want to remove too much from the spacer, I'd try .015" at the most. Maybe .012 if you have the ability to be precise. If you remove too much then you'll be back out hunting for replacement washers.
That's my best guess. Remember, at approx' 1/4 throttle position and less, there is SOME pilot jet effect that overlaps the jet needle circuit. So those little bit richer screw adjustments you made will already have some effect on the jet needle circuit. With the spacers being modified to raise the jet needles approx' 1/4 position (about .011/.012), I would think that would be enough to compensate for the factory lean jetting. No way to know of course than testing.
Also, If you went out with new spacers and did a good 1/3 to 1/2 throttle chop test you might get acceptable color on your plugs. Unless you noticed obvious jet needle performance issues, leave the new spacers alone and be content with richening the mixture screws only.

Thank you very much Keith! I will follow this to the letter and leave the spacers at stock thickness first and try for for good plug chops then go with 0.012 t0 0.015in. of shaving if that does not get the job done. I was surprised to find the rubber velocity stacks completely missing from the airbox which would explain why I could not get it sealed. Rechecking float height to get it precisely at 22.4mm while I'm at it.

[IanF]

The stock setup works

Put the spacers in without shaving them. I was surprised to find they are thicker than folks thought they would be at exactly 0.100 in. or 2.54mm thick The original rubber velocity stacks on the airbox side were pushed inside the airbox and chewed up from my previous sessions of removing and replacing the carbs without removing the intake boots. I feel like a cack-handed amateur now but I guess that's what I am. It's really much easier to just remove the four additional bolts and be done with it. So with the new airbox parts and the original spacers back in and the pilot screws set to 1.5 turns open the bike is much more responsive to small throttle openings and runs at 6k or 6500 rpm on the freeway at 1/3 to 1/2 throttle openings without surging. There's still a slight "flat spot" at 5k but it can be compensated for with more or less throttle. Didn't do any plug chops yet.

Thanks again to all the contributors!