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fuel mixture questions

eil

eil

Forum Sage
Carburetor gurus, I have questions!

1980 CV carbs. All rebuilt as of a few months ago per the tutorials and advice here. The bike is almost running right, but still isn't quite perfect. Unless my tach is off, it doesn't like to idle much below 1500 without stalling. Judging from the spark plugs, I'm guessing that cylinders 1 and 2 are running rich, 3 is about right, and 4 is lean. This is leisurely riding around the neighborhood at relatively low RPMs (5k max).

Now my questions:

1. Which way do I turn the mixture screws to make the mixture leaner or richer? (I presume counterclockwise for richer, clockwise for leaner but am not 100% sure?)

2. What would be the factory setting for these? (I have a stock bike, so those settings should "just work", no?)

3. Do the mixture screws apply through all engine speeds or just around idle?

4. Is it permissible to have the four screws set at different amounts or would doing so indicate an imblance elsewhere, such as the float levels?

5. The head of one of my mixture screws is stripped and does not move easily. I fear that I'm eventually going to strip it completely. Z1 sells aftermarket screws, are these reasonable quality?

Thank you!
 
eil said:
1980 CV carbs. All rebuilt as of a few months ago per the tutorials and advice here. The bike is almost running right, but still isn't quite perfect. Unless my tach is off, it doesn't like to idle much below 1500 without stalling. Judging from the spark plugs, I'm guessing that cylinders 1 and 2 are running rich, 3 is about right, and 4 is lean. This is leisurely riding around the neighborhood at relatively low RPMs (5k max).

Now my questions:

1. Which way do I turn the mixture screws to make the mixture leaner or richer? (I presume counterclockwise for richer, clockwise for leaner but am not 100% sure?)

2. What would be the factory setting for these? (I have a stock bike, so those settings should "just work", no?)

3. Do the mixture screws apply through all engine speeds or just around idle?

4. Is it permissible to have the four screws set at different amounts or would doing so indicate an imblance elsewhere, such as the float levels?

5. The head of one of my mixture screws is stripped and does not move easily. I fear that I'm eventually going to strip it completely. Z1 sells aftermarket screws, are these reasonable quality?

Thank you!
Click to expand...

1) Out is richer (CCW)

2) Not a clue. Thinking 1-1/4~1-1/2 turns as a starting point

3) From idle to about 1/4 throttle.

4) Permissible as long a one isn't set at 1/2 turn and the others at 2 turns which would tend to indicate a problem.

5) I have spares

Adjust your valves, make sure there are no vacuum leaks, the air box is properly sealed and the float level are correct before you syncronize the carbs and set the idle fuel mixture screws.
 
I'll take a stabb

eil said:
Carburetor gurus, I have questions!

1980 CV carbs. All rebuilt as of a few months ago per the tutorials and advice here. The bike is almost running right, but still isn't quite perfect. Unless my tach is off, it doesn't like to idle much below 1500 without stalling. Judging from the spark plugs, I'm guessing that cylinders 1 and 2 are running rich, 3 is about right, and 4 is lean. This is leisurely riding around the neighborhood at relatively low RPMs (5k max).

Now my questions:

1. Which way do I turn the mixture screws to make the mixture leaner or richer? (I presume counterclockwise for richer, clockwise for leaner but am not 100% sure?)

Yes

2. What would be the factory setting for these? (I have a stock bike, so those settings should "just work", no?)

Usually 2 1/2 turns; It should be listed in the manual.

3. Do the mixture screws apply through all engine speeds or just around idle?

They are always there but there is so much more gas coming from the needle and mains at other throttle setting to make mixture screws only effective at idle with throttle shut.

4. Is it permissible to have the four screws set at different amounts or would doing so indicate an imbalance elsewhere, such as the float levels?

There is always some imbalance but should not be more than about 1/2 a turn. Anything could create this including low compression I would suspect. More of a drivability issue i would assume. Make it idle the best what ever it needs.

5. The head of one of my mixture screws is stripped and does not move easily. I fear that I'm eventually going to strip it completely. Z1 sells aftermarket screws, are these reasonable quality?


No idea, Post in parts wanted I'm sure someone will sen you an OEM one used.

Thank you!
Click to expand...


Some reading on the subject

http://www.iwt.com.au/mikunicarb.htm
 
Last edited:
Dang you guys are awesome. And so quick!

rustybronco: Valves are already adjusted. I don't think I have any air leaks, but I have it on my agenda to double-check that with the propane test. Float level should be good, but if I continue to have issues getting the mixture right, I'll pull the carbs and double-check.

posplayr: Far as I can tell, the factory settings for the mixture screws aren't in the manual because Suzuki didn't consider them a serviceable part. (The whole EPA regulations thing.)

I'll probably do some testing/adjusting over the weekend and report back.

Thanks guys!
 
eil said:
posplayr: Far as I can tell, the factory settings for the mixture screws aren't in the manual because Suzuki didn't consider them a serviceable part. (The whole EPA regulations thing.)
Click to expand...
They came pre-adjusted with a plug over them.
 
Click on the STICKY at the very top and see what others have posted for the bike. But a general rule ofmthumb..as a start point..is 2 to 2 1/2 out. Be well sure there arent any broken tips in there too.
 
eil said:
posplayr: Far as I can tell, the factory settings for the mixture screws aren't in the manual because Suzuki didn't consider them a serviceable part. (The whole EPA regulations thing.)
Click to expand...


Basscliff lists a bunch of setting in his website, and they do have a pretty big range. 2.5 turns just happens to be for an Canadian 83 GS1100ED :o

Pilot Screw = Preset (actually, these can be adjusted - start with about 2 turns out from lightly seated)
Click to expand...

http://members.dslextreme.com/users/bikecliff/images/carbspec_float_height.html
 
Last edited:
Hey BassCliff, I appreciate the link. I posed the questions above after reading everything I could find about these carbs on your site (including the adjustment procedures) and was just searching for a deeper understanding of a few key points. Thanks!
 
Hi,

eil said:
Hey BassCliff, I appreciate the link. I posed the questions above after reading everything I could find about these carbs on your site (including the adjustment procedures) and was just searching for a deeper understanding of a few key points. Thanks!
Click to expand...

Which key points? Have you read this?

Mikuni BS32SS Carb Manual

This particular document is GS850-specific as far and specs and jetting, but the principles are the same for all the CV carbs.

Thank you for your indulgence,

BassCliff
 
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