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Air screw adjustment.....again

Larry D

Larry D

Forum Sage
Past Site Supporter
First of all, I've searched but couldn't find an answer. Second, I've done all the recommended maintenance that is throughout these forums. Valve adjustment, carb dipping, o-rings, etc....Third, the bike is stock, with the exception of a Mac 4-2 exhaust. And fourth, I've found no air leaks.

So, the bike has been having a hard time settling to an idle and surging also. I've also had some very experienced folks from this site in my garage and they have said that the bike is running rich.

Tonight (it was a beautiful day in Indianapolis today!) I decided to try the highest rpm method. The air screws were at 2.5 turns out from lightly seated. I warmed the bike up by riding around the neighborhood for awhile and parked in my garage and setup my remote tank and a fan for cooling.

After getting idle somewhat stable and at around 1100 with the large carb knob, I set forth....

My goals were to stabilize the idle and lean it up a little. I turned #1 air screw in and it seemed to have no affect. The same with #2. However when I turned #3 in, the idle took off like a scalded dog !! Great !! I turned the large carb knob so the idle went down to 1100 again and turned some more. It still climbed, so repeated the process until it made no percepted effect. Moved to #4 and it had no affect, similiar to #1 and #2.

So, I ended up with #1, #2 and #4, 1.5 turns out and #3 only .5 to .75 turns out.

I'm sure this leaned out the bike and after "dropping" the throttle from 5000 rpm's it settles nicely at 1000-1100 rpm's.

Finally the question for all of you....Is it unheard of for the air screws to be set at different turns out ? Is this common ? I've always thought that they would be somewhat set the same.

The bike now sits in the garage with the remote tank still attached waiting for another nice evening tomorrow. I'll try firing it up and see how it acts then. I know plug chops are in order but it's difficult because, I'm not all legal yet.:oops: I'm going this week though.:)

Thanks Everyone and don't forget to vote tomorrow.
 
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Intake o-rings replaced? :-k If the #3 had that much of an effect...I'de suspect a leak somewhere...
 
Larry D said:
Perhaps, I have my nomenclature confused. I thought the intake o-rings were the rubber intake boots that are bolted to the engine side on some models.
Click to expand...

There is a large O-rings between the rubber boot and the cylinder head. These O-rings flatten out and leak with almost 100% certainty if they are the originals.

intakeb.jpg
 
Nessism said:
There is a large O-rings between the rubber boot and the cylinder head. These O-rings flatten out and leak with almost 100% certainty if they are the originals.

intakeb.jpg
Click to expand...

My bike is an '81 GS750L, 16valve. I do not have these. I have rubber angled cylinder shaped boots that fit over the engine intake ports.

Edit: And they're brand new.
 
Last edited:
Larry D said:
My bike is an '81 GS750L, 16valve. I do not have these. I have rubber angled cylinder shaped boots that fit over the engine intake ports.

Edit: And they're brand new.
Click to expand...

16V engine? Now that you mention it, I don't think you have those O-rings. I'm suffering a little brain fade tonight it seems.:oops:
 
No, you DONT have those orings, did you check to see that your vaccum gauge port screws are completely tight? And to answer your original question, NO its NOT uncommon for the mixture screws to be set a little differently from one another. I dunno how much differently yours are, ( i didnt read the whole post, sorry:oops:) But its quite COMMON for them to be set a little off, as you're supposed to set each for highest idle. And, some would go further to say that when dynamic or vacuum syncing the BS type carbs, its best to set the INNER two cylinders a little lower than the outer two as far as the amount of pull goes. Now, for instance, on my former 1100G, we found that highest idle settings and best running had the screws set completely different across the board. One might have been at 2.25 turns out, Two at 2 flat, Three at 2.50 out and four at maybe 2.75 out. I dont remember the actual number of turns, but they were all different. With the bike running, a temporary tank hooked up and the vac line that goes to the carbs from the petcock PLUGGED (important!) run the bike till at normal opperating temp. Its best to usually have a fan blowing over the motor, somtimes two if its really hot out. When adjusting the carbs, it may take some time, especially if you're syncing, and you dont want the motor overheating, as you have the revs a bit above idle, and no air moving across the aircooled head. So use a fan. Anyway, once the bike is warm, adjust each screw, one at a time, listening carefully for the idle to climb to the highest point. Go slowly, to allow the motor time to catch up to your adjustment, and when the idle reaches a point where it doesnt seem to speed up anymore, stop and turn the screw back a bit to find the exact point that it reached highest idle and stopped. Then move to the next. You may end up close, you may end up drasticly different. If your screws end up more than 3 turns out, you may have a jetting issue, as in they are too small for the intake and exhaust combo you're running. If its a stock set up, you may have a dirty pilot circuit, or someone has messed with the carb jetting prior to your purchase. Let us know what you find, and I hope this helps..

TCK

Oh forgot one tidbit. Once you have the screws set for highest idle, and are satisfied, be sure to turn your idle adjust KNOB back to normal idle of around 900-1200RPM.

TCK
 
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Quote; and the vac line that goes to the carbs from the petcock PLUGGED

...Damn i knew i had forgotten to do something when synching recently. This might explain why #3 cylinder is running rich. Thanks.
 
Jackbob said:
Quote; and the vac line that goes to the carbs from the petcock PLUGGED

...Damn i knew i had forgotten to do something when synching recently. This might explain why #3 cylinder is running rich. Thanks.
Click to expand...
No problem. Yes, on some of my bikes, it simply refused to idle at all without that vac line plugged. I just use a phillips head screw driver shoved in the line, works well. It likely is the source of your problem, as you had to turn it out so far just to keep it running that once you plugged it, its way rich. Give it another go and let us know if it worked!

TCK
 
Thanks All,

No problem, Nessism, my brain shuts down sometimes too.

The air screws are now set to: #1, #2 and #4 are 1.5 turns out and #3 is .75 turns out. The Mac exhaust, although it is a 4-2, and mimicks the the stock exhaust, of course, it is not.
When it was "searching" for idle and I removed the air filter box lid, it seemed to smooth out some. So, it needed more air. The only carb that had any effect was #3. So, I set #1, #2 and #4 as Mr Suzuki did. If I turn #3 any more than 10 degrees either way the bike starts to bog down.
After I get home tonight, I'm taking it for another little ride and see how she acts.

I curious as to why the vacuum line should be plugged while performing the highest rpm method ??

And yes, the carb sync bolts are tight. I'm very confident that there are no air leaks.
 
Larry D said:
Thanks All,

No problem, Nessism, my brain shuts down sometimes too.

The air screws are now set to: #1, #2 and #4 are 1.5 turns out and #3 is .75 turns out. The Mac exhaust, although it is a 4-2, and mimicks the the stock exhaust, of course, it is not.
When it was "searching" for idle and I removed the air filter box lid, it seemed to smooth out some. So, it needed more air. The only carb that had any effect was #3. So, I set #1, #2 and #4 as Mr Suzuki did. If I turn #3 any more than 10 degrees either way the bike starts to bog down.
After I get home tonight, I'm taking it for another little ride and see how she acts.

I curious as to why the vacuum line should be plugged while performing the highest rpm method ??

And yes, the carb sync bolts are tight. I'm very confident that there are no air leaks.
Click to expand...

I guess I should be calling these screws, the idle mixture adjust screws.:)
 
As far as the difference in settings with #'s 1,2, and 4 being exactly the same...I'de now suspect an air restriction of some measure on the #3. Could even be a casting issue from the factory. Possibly a different sized jet? :-k Nothing is perfect.

Oh and as I only think in 8-valve language...I must apologize for mis- directing the choir and needing the Kid to slap sense back into the masses...:oops:
 
Dave8338 said:
As far as the difference in settings with #'s 1,2, and 4 being exactly the same...I'de now suspect an air restriction of some measure on the #3. Could even be a casting issue from the factory. Possibly a different sized jet? :-k Nothing is perfect.

Oh and as I only think in 8-valve language...I must apologize for mis- directing the choir and needing the Kid to slap sense back into the masses...:oops:
Click to expand...


No problem at all Dave. My brain only thinks in 16 valve language.:)

I've been completely through the carbs when I cleaned and dipped them, they were all intact and stock.

The bike has always run rich. At least in the time I've had it anyway. probably because when I got it, it had a 4-1 exhaust and the PO tried to compensate with the mixture.

I'm thinking that because of the "almost" stock Mac 4-2 exhaust, that it requires a little more air than normal. I've read that these bikes came somewhat lean from the factory and were set 1.5 turns out, give or take.
I believe that this exhaust flows more easily and therefore requires slightly more air coming into the system. More air out, needs more air in.

It was just strange to me that the only carb that made any difference in the idle was #3. And I just thought that they would all be set approximately the same turns out.

It also seems like a very lean setting, but time and plug chops will tell....
 
It sounds to me like you have a low speed circuit issue on 3 of your carbs. Because it is running rich I would expect a problem with the air part of that circuit. But, it could be both air and fuel that's a problem.

A test for whether you are getting good air movement (and fuel movement) in the low speed circuit is to use compressed carb cleaner to blow through the system. (Just dipping carbs is not enough. You have to make sure all the passages are free and some form of this check is the only way to do it).

Pull the carbs and locate the low speed circuit air jet located at the aircleaner side of the carb throat. Pull the float bowls and plug the pilot jet with your finger or your friend's finger. Make sure you hold the carb cleaner can tip tightly to the low speed circuit air jet and blow through. You should see a solid hard spray (not just a dribble) come out of the idle passage in the engine side of the carb throat. Then cover that idle passage in the carb throat keeping the fuel jet covered too and blow through again. You should see a solid spray come from another passage in carb throat right where the butterfly valve is. This "off idle" passage is often neglected or even known about and is what gives you extra fuel when you pop the butterfly valves open from idle.

Give it another spray while holding your finger over the two idle passages in the throat of the carb and watch for a solid spray out the pilot fuel jet.

If you don't get a good solid spray on any of these then there is a clogged passage that needs to be addressed.

Chris
 
No problem. Yes, on some of my bikes, it simply refused to idle at all without that vac line plugged. I just use a phillips head screw driver shoved in the line, works well. It likely is the source of your problem, as you had to turn it out so far just to keep it running that once you plugged it, its way rich. Give it another go and let us know if it worked!

TCK

Thanks Cafe Kid - yes i will do so this weekend and report back.
Thanks
Jackbob.
 
TCK Feedback

TCK Feedback

Yea tried re syching with the vac line pluggged - still carb issues and took another GS for a spin on the weekend (Psy Guys) - much smoother/better than mine and good to compare.

Anyway have decided to install a spare GS1000ST head and larger 30mm
(I think) VM slide carbs + pods as these are in much better nick. Also my inlet rubbers were dodgy and the spare head has good ones so worth the effort cost wise.

Waiting for gaskets etc. Yes i know about the cam timing differences.

Plus more top end power no doubt! Yee hah.

Thanks.
 
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