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luvnwunderwoman
Guest
Thanks for posting, this thread is a must read for all of us noobies.
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G
GSOH
Guest
Thanks for sharing this. I need to do this to my bike tonight. Sounds like it may be easier/more effective than each screw by itself.
My main debate is deciding whether to make my old temp fuel supply off the bike so I can adjust the screws with the tank off.
My main debate is deciding whether to make my old temp fuel supply off the bike so I can adjust the screws with the tank off.
A
AJ
Guest
I've posted links about carb tuning before. The chart below helps explain the relationship between the various systems and how they work in various throttle positions. Here's where I pulled the current pic from (http://justkdx.dirtrider.net/printcarbtuning.html). Looks like the entire article was copied from elsewhere.
If I understand the KK's approach correctly, the key thing is that:
1) overly lean low-speed mixture will make the idle hang a bit (or be slow to return to idle) when you blip the throttle
2) overly rich low-speed mixture will make the idle sag a bit when you blip the throttle.
Makes total sense, not that I have tried it.
Last thing to be mentioned: On a 4 cylinder engine, it's really best to adjust valves first, then tune carburetors, then synch carburetors. Especially be sure to do the valves before synching or you may be misadjusting your throttle plates to compensate for a poor valve adjustment.
Mikuni has a nice guide to carb adjustment here: http://www.mikuni.com/fs-tuning_guide.html
If I understand the KK's approach correctly, the key thing is that:
1) overly lean low-speed mixture will make the idle hang a bit (or be slow to return to idle) when you blip the throttle
2) overly rich low-speed mixture will make the idle sag a bit when you blip the throttle.
Makes total sense, not that I have tried it.
Last thing to be mentioned: On a 4 cylinder engine, it's really best to adjust valves first, then tune carburetors, then synch carburetors. Especially be sure to do the valves before synching or you may be misadjusting your throttle plates to compensate for a poor valve adjustment.
Mikuni has a nice guide to carb adjustment here: http://www.mikuni.com/fs-tuning_guide.html
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Steve
GS Whisperer
I think a carb sync should be done BEFORE a mixture tweak.
Look at it this way: if one carb is out of sync and happens to be closed, tweaking the mixture won't do much.
If you make sure that carb is flowing the same amount of air as the others (by matching the vacuum levels), you will know that the mixture tweak might have a chance.
EDIT: I just went up a bit farther and read AJ's post, Have to agree with some and dis-agree with some.
Yes, adjust the valves before synching the carbs.
No, sync the carbs before adjusting mixture. Reason already stated above.
By the way, some of the items listed in that chart might be a bit confusing, because it is for the VM carbs, not the BS-series CV-type carbs.
Other than that, it's a good chart. I will have to read the rest of the article when I have some time.
.
Look at it this way: if one carb is out of sync and happens to be closed, tweaking the mixture won't do much.
If you make sure that carb is flowing the same amount of air as the others (by matching the vacuum levels), you will know that the mixture tweak might have a chance.
EDIT: I just went up a bit farther and read AJ's post, Have to agree with some and dis-agree with some.
Yes, adjust the valves before synching the carbs.
No, sync the carbs before adjusting mixture. Reason already stated above.
By the way, some of the items listed in that chart might be a bit confusing, because it is for the VM carbs, not the BS-series CV-type carbs.
Other than that, it's a good chart. I will have to read the rest of the article when I have some time.
.
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K
Kjpeter
Guest
Sorry to bring up an old post.
I am following the directions and getting good results. One question I have is about the idle.
In the directions, it says to put the idle at 1300 during initial adjustments while on the stand. It doesnt specify if the idle should be set at 1300 during road tests and then dropped down to the factory 1050?100 after adjustments are completed. Or if it should be at the factory 1050?100 during adjustments.
Is it easier to tell if the adjustments are wrong with it set at 1300?
Kyle
Bone stock 1980 GS850G
I am following the directions and getting good results. One question I have is about the idle.
In the directions, it says to put the idle at 1300 during initial adjustments while on the stand. It doesnt specify if the idle should be set at 1300 during road tests and then dropped down to the factory 1050?100 after adjustments are completed. Or if it should be at the factory 1050?100 during adjustments.
Is it easier to tell if the adjustments are wrong with it set at 1300?
Kyle
Bone stock 1980 GS850G
M
MechMessiah
Guest
1300 for adjustments and factory for riding. Yes it's generally easier with the higher idle and less likely to stall/stumble the engine while making the adjustments.
Steve
GS Whisperer
Actually, if you read the manual a little more carefully, a carb sync is done between 1500 and 2000.
One of the reasons for that is to minimize the bouncing of the balls in the Suzuki carb sync gauge.
For those of us that have other gauges at our disposal (I have several, including the Suzuki gauge), the fluctuations are not all that bad, even at 1100 RPM.
Will tuning differences be seen better? Not really.
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One of the reasons for that is to minimize the bouncing of the balls in the Suzuki carb sync gauge.
For those of us that have other gauges at our disposal (I have several, including the Suzuki gauge), the fluctuations are not all that bad, even at 1100 RPM.
Will tuning differences be seen better? Not really.
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K
Kjpeter
Guest
Steve, I was inquiring about the idle for adjusting the idle mixture screws not syncing. I did the carb sync at 1500rpms before and after adjusting the mixture screws.
V
vuendox
Guest
koolaid_kid! Thank you so so much for this thread and the write up. I have an 1981 GS650G (Shaft driven) bobber that I'm trying to get running reliably. I noticed some people advising to adjust valves before syncing carb and tuning. I wish that wasn't the case as I'm an inexperienced self mechanic who is anxious to get riding, but... I think I will go through it all. A big cheers, super helpful,
vuendox
vuendox
G
Guest
Guest
Is it normal for it to run fine on 2 rotations out from bottom snug? Did that and it responded just fine, good throttle response and nice steady fallback to idle rpm when rolling off the throttle. Didn't bother experimenting more, as it was running a lot better with the carburettors synchronized and pilot fuel/air mixture back to stock (they were all different for some reason...). Thanks for your work though!
M
MechMessiah
Guest
Yes, 2 turns from bottom is usually the factory setting if memory serves.
Steve
GS Whisperer
The "factory setting" is wherever their multi-thousand dollar tool told them that the mixture was "perfect", then they sealed it.
When I rebuild a set of carbs, I always check the setting and record it before I take anything apart. If the screws were still sealed, I make note of that, just for reference. I have found sealed screws to be anywhere from 1 3/4 to 2 1/4 turns out, so 2 turns is a good average, but that does not mean it's proper for your carbs. You are likely very close, but a little experimenting won't hurt.
It is easier for an engine to start and run when it is a bit rich. After all, that is the function of a "choke" on a carb, to provide a richer mixture. Because of that, when I rebuild carbs, I start with the mixture screws out three full turns, then adjust them in as necessary when the engine warms up. For example, if one particular carb required between 2 1/4 and 2 1/2 turns out for proper running, starting it at 2 turns out would be too lean. You would have some difficulty even getting it to run, but you won't know exactly what the problem is. Simply starting with a richer mixture at least lets it start.
.
When I rebuild a set of carbs, I always check the setting and record it before I take anything apart. If the screws were still sealed, I make note of that, just for reference. I have found sealed screws to be anywhere from 1 3/4 to 2 1/4 turns out, so 2 turns is a good average, but that does not mean it's proper for your carbs. You are likely very close, but a little experimenting won't hurt.
It is easier for an engine to start and run when it is a bit rich. After all, that is the function of a "choke" on a carb, to provide a richer mixture. Because of that, when I rebuild carbs, I start with the mixture screws out three full turns, then adjust them in as necessary when the engine warms up. For example, if one particular carb required between 2 1/4 and 2 1/2 turns out for proper running, starting it at 2 turns out would be too lean. You would have some difficulty even getting it to run, but you won't know exactly what the problem is. Simply starting with a richer mixture at least lets it start.
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M
MechMessiah
Guest
I stand corrected. Thanks for the knowledge, Steve.
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