carb tuning confusion

[79-GS550-L]

I just dismantled and cleaned my carbs and when I counted the turns to seat the air & fuel screws I realized why my bike has been running so poorly (very rich). The air screws were all exactly 1 turn out and the fuel screws were all exactly 2 turns out! Next to the air screws were little grooves for the factory positions but the screws were NOT set to these grooves. I'd like to put them back to those positions, but don't know how many turns out before I get to those grooves.

Also, I've read just about every carb tuning guide I could find and am still a bit confused about what to do. I've read that you start by adjusting the air screws until you get maximum rpm, then fine tune the fuel screws to get the proper plug color, then adjust the air screws to get highest rpm again.

First question, since the air screws have default positions, wouldn't I want to leave the air screws where the factory wanted them (everything on the bike is stock afaik) and just use the fuel screws to adjust the mixture? And how many turns out before lining up with the groove (air screw)?

Second question, if I've just fine-tuned the mixture with the fuel screws, why do I want to re-adjust the air screws for highest RPM? won't that change the mixture again, requiring another fine tuning with the fuel screw? And assuming I had gotten the mixture correct with the fuel screw, why would I be able to raise the RPM with the air screw? I would assume all I could do is lower the rpm as I'd be making it either too rich or too lean.

Finally the VM carb tuning PDF on this site says to do the plug chop at 30mph in 4th gear... I would assume that's just a little off idle. Can I just raise the throttle to 1/8th position or so and do plug chops in the garage like that? Maybe with one or two plugs deadened to provide resistance (like somebody suggested to me in the "putting the bike under load in the garage" thead)?


Sorry for the very long post, I'm just very confused right now. Too much reading! Thanks guys,

Alex

 

[79-GS550-L]

bump? anyone?

 

[BassCliff]

Tired, dry, red eyes?

Tired, dry, red eyes?

Mr. 79-GS550-L,

I hope you don't mind. All I can offer you is more to read. 8-[

http://justkdx.dirtrider.net/printcarbtuning.html
http://www.dansmc.com/carbs2.htm
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html
http://www.mikuni.com/fs-manuals.html

Thank you for your indulgence,

BassCliff

 

[mark]

I wouldn't worry too much about those notches on the carbs (actually I just ignore them). Set them so your bike runs right.

Stock settings for your '79 are one turn out on the mixture screw and 2 turns out on the air screws. Sounds like your PO got it backwards. Start there and see how the bike runs. If you want to change things, start with your mixture screws and then fine tune with the air screws. The most important thing of course is smooth off-idle response and plug color. Especially with the mixture screws, small changes can make a big difference.

As far as plug chops go, do them at throttle settings, not rpm settings. 1/8 throttle is a great setting to check your air and fuel screw mixtures. But also do them at 1/4 and full throttle to make sure your good all across every throttle setting.

Others may be different, I haven't had much luck using high rpm method with my air screws on my 850. I go for good plug color and good throttle response, a good carb synch and consider it good.

 

[79-GS550-L]

Mr. 79-GS550-L,

I hope you don't mind. All I can offer you is more to read. 8-[

I don't mind one bit. That last one especially (official mikuni guide) looks quite interesting.

 

[79-GS550-L]

I wouldn't worry too much about those notches on the carbs (actually I just ignore them). Set them so your bike runs right.

Stock settings for your '79 are one turn out on the mixture screw and 2 turns out on the air screws. Sounds like your PO got it backwards.

Yeah that's what I assumed

Start there and see how the bike runs. If you want to change things, start with your mixture screws and then fine tune with the air screws. The most important thing of course is smooth off-idle response and plug color. Especially with the mixture screws, small changes can make a big difference.

"mixture" screw being fuel screw, right? I guess this is the thing that confuses me the most... I understand how the jets normally work, where the jet size really determines the mixture since the more air that flows, the more fuel the jet lets out. But the idle circuit seems to me like it's got 2 separate jets, one letting air through and one letting fuel through... aren't these independent? So if you turn out your air screw, you lean the mix, and if you turn out your fuel screw, you richen the mix?

Or do I have it backwards, and the idle circuit works like the normal jetting, where the "fuel" screw adjusts the mixture and the "air" screw is like a throttle valve, when you let more air through more fuel also goes through?

As far as plug chops go, do them at throttle settings, not rpm settings. 1/8 throttle is a great setting to check your air and fuel screw mixtures. But also do them at 1/4 and full throttle to make sure your good all across every throttle setting.

Others may be different, I haven't had much luck using high rpm method with my air screws on my 850. I go for good plug color and good throttle response, a good carb synch and consider it good.

If I had good plug color, good throttle response, and a good carb synch I would be ecstatic! really I'd be happy with just any one of those

 

[mark]

I'm a long way from a carb expert, all I can tell you is that the pilot circuit is one circuit with one jet, the mixture (pilot screw) and air screws adjust the amount of gas and air that this circuit vents into the manifolds at speeds too slow for your needles and main jets to effectively manage. You're right, as you turn out the mixture screw you enrichen, as you turn out your air screw you lean things out. Frankly, that's all I know.

If your bike was set to one turn out on the air screw and 2 out on the pilot mixture screw, you'd be too rich.

You've tried the stock settings right? It still runs bad? Are you running stock air box and exhaust?

 

[79-GS550-L]

You're right, as you turn out the mixture screw you enrichen, as you turn out your air screw you lean things out. Frankly, that's all I know.

If your bike was set to one turn out on the air screw and 2 out on the pilot mixture screw, you'd be too rich.

You've tried the stock settings right? It still runs bad? Are you running stock air box and exhaust?

Good to know I'm not too far off then. So I guess adjusting the air screw and fuel screws together is kind of like doing an idle-circuit carb sync? And then using the linkage adjustment screws for the slide does an off-idle sync (at 3k rpm)?

I'm about to go try the stock settings right now. Hopefully that will get it running well enough to start playing around with (right now it will run with new plugs but fouls them after about 10 mins!)

 

[mark]

Good to know I'm not too far off then. So I guess adjusting the air screw and fuel screws together is kind of like doing an idle-circuit carb sync? And then using the linkage adjustment screws for the slide does an off-idle sync (at 3k rpm)?

Yes, though the recommendation from Suzuki is to do your vacuum synch at between 1200 and 1500 rpm I think. A lot of folks like to synch their carbs at much higher rpm (3 - 4 thousand rpm) because getting into that range is where they want their bike to run the smoothest. I usually go that route, although if its good at 1300 rpm the synch is usually good at 3,000 as well in my experience.

Hope it runs good.

 

[English Dave]

fuel jets

fuel jets

I'm no expert but this is my take on the fuel jet and air screw thing: The fuel jet would seem to regulate the amount of gas getting into the air throttle, the air screw can then be used to alter the amount of air being drawn in with that amount of fuel. I would imagine that the primary jet needs to be able to supply enough gas to handle the change over to throttle cutaway/needle fuel flow without the engine being starved. So - you could in fact get a correctly mixed primary jet without it actually being the correct air/fuel flow. I've looked around for clear setting instructions for setting the fuel jet but so far I have not seen any. I have yet to check out the links in this thread - so who knows...If this is bollocks - please feel free to say so but its how I see it working.Regards

 

[TheCafeKid]

Yes, though the recommendation from Suzuki is to do your vacuum synch at between 1200 and 1500 rpm I think. A lot of folks like to synch their carbs at much higher rpm (3 - 4 thousand rpm) because getting into that range is where they want their bike to run the smoothest. I usually go that route, although if its good at 1300 rpm the synch is usually good at 3,000 as well in my experience.

Hope it runs good.

This would be true..provided youve done a good bench sync. The slides should be synched together at the fully closed position, and also at WOT possition prior to vacuum sync. If that is proper your mechanicals should be smooth all the way thru the throttle range once youve vacuum synced. This is for mechanical type carbs, i dunno if you can bench CVs at WOT since they're vacuum operated.