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Carb idle mixture screws

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Guest

Guest
I have done alot of searching here and unfortunately it's still not crystal clear in my thick skull... 1981 gs1100e I'm getting close to done here.
A couple of months ago I pulled the carbs down, dipped in Berrymans and meticulously cleaned and installed all new orings then bagged for when the time comes, time.
I adjusted the valves, new to me solid well sealed airbox with a new oem filter, new carb boots between carb/intake manifold and carb/airbox as well. All new gaskets and clamps on exhaust and new spark plugs.
I dont remember where I set the idle mixture screws at but it was what was advised per this forum....mabey 2-1/2 out but I do know I pulled the brass plugs over them and stupid me didnt count the factory turns on them to mimic. I got to the point last night of hanging my auxiliary tank and trying to start it....it fired right off and purred away sounding remarkably smooth, mabey a little hesitation off idle but also mabey just not warmed up.
So, today I will attempt my 1st ever carb sync. I have a new in the box Morgan Carb Tune to get familiar with and have found pretty good advice on here to use it correctly. What I am somewhat confused about is the idle mixture screws. Do I 1st go through every one of them turning them to get the best idle? Highest idle? And then go to the sync process? Again I know this info has to be spelled out here someplace and apologies for asking but I'm a little grey on the subject and want this bike to run perfect.
 
Ok...I set the idle mixture screws at 2-3/4 turns out from seated and just did a carb sync at 1800rpm. All I did was adjust #1 to achieve what's pictured as it was the only one out of whack. Is this sufficient? I do have the exhaust crossover so this is my understanding as to how it should look? Thanks, again my 1st carb sync...and 2-3/4 turns out ok? Seems to be running very smoothly to me.
 

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All four should be as close to even as possible; remember though you aren't adjusting the idle mixture screws but the balance screws (has a lock nut on it) between the carbs (there are three of them).
 
The pattern looks good, according to the manual, but the difference is too big. :-k

Of course, there is no way to really tell, because the Suzuki gauge shown in the manual uses balls to show vacuum level and has no numbers. They show a difference of about half a ball, but nobody knew how much difference "half a ball" is. Until I did some experimenting.

By playing with a Suzuki ball gauge, my Carbtune, a Motion Pro (blue liquid) and my mercury sticks, I was able to show that "half a ball" is 2cm Hg. Your picture shows a difference of 4cm Hg, so I would adjust that. Your 2/3 sync looks good, you should only need to adjust 1/2 and 3/4.


Now, as to which order to do the job:
1. A valve adjustment should be first. You did that, great. That will ensure that each cylinder will mechanically try to pull the same amount of air on each intake stroke.
2. Pilot screws (mixture screws) should be pre-set (unless they were set previously). I like to run them rich the first time through, because a rich mixture will fire a LOT easier than a lean mixture. If it's just a bit too lean, you won't know if your running problem is ignition- or carb-related, so start rich to eliminate that option. Most bikes like to be in the 1 3/4 to about 2 1/2 range, so I start at 3.
3. Set the idle speed to about 1500. Yeah, the book says a bit higher, but that is mainly to smooth out the pulses in the gauge.
4. With the engine warm enough to not need any "choke", do the sync. The manual suggests doing the 2/3 sync first, then the 1/2 sync, finally the 3/4 sync. Not sure why that order, but as long as you do the 1/2 after the 2/3, it seems to work well.
5. With the gauges still connected, fine-tune your mixture screws. As you turn a screw in, you will eventually hear the engine slow down a bit. You should also notice the vacuum levels of all four cylinders dropping on the gauges. When that happens, back up about 1/4 turn, go to the next carb, repeat until all are done. Verify crisp throttle response.
6. Try turning the screws in again. You should notice a response within about 1/4 turn. When that happens, turn them back out 1/8 turn, repeat on the others. Verify crisp throttle response. If it's a bit sluggish, go back out that 1/8 turn that you just added.

If you try to do the mixture adjustment before the carb sync, you may find that one carb might be so out of sync that it is not drawing any air (really high vacuum). In that case, no amount of mixture screw twiddling will cause a change, so it's best to make sure all the carbs are open the same amount, THEN play with the amount of fuel being added.

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Excellent feed back Steve, I couldnt have asked for such thorough info. I am a little confused though when you say that 2/3 is good but I need to adjust 1/2 and 3/4. Dont I just fiddle with #1 and bring it down to within 2cm Hg of 2/3 then again with #4 to within 2cm Hg of 2/3? Am I saying the same thing your describing in simpler terms?
 
The "2/3" refers to the sync from carb 2 to carb 3. Those are the same on the gauge, so don't change them.

Adjust the screw between carb 1 and carb 2 to bring them to the same level, or at least have #1 no more than 2cm higher than #2.

Do the same with the 3/4 sync.

I never have understood why they called for different vacuum levels. Most of us adjust them all the same, just like we did before we saw that instruction in the service manual. The cylinders with a lower vacuum level have that lower level because the throttles are open just a little bit more. That means that they will be producing just a little bit more power. Since that is at the very low power of idle speeds, it's not going to twist the crank, but I still like even power production when possible.

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Gotcha! Thanks. I buttoned everything back up yesterday before seeing your post thinking I deserved a pat on the back. I'm headed out this morning to do this process all over again to do it correctly and get it spot on. I played with the idle mixture screws yesterday before the sync just like you state not too and again like you said could happen slowly turning the first one in never caused a change in idle so I set them all to 2-3/4 turns thinking that should be good!
Now that I understand this process better I cant wait to dive back in and get it right. I also have a 75' Cb400 SuperSport that will eventually want to stall at lights when its warm and the wait is to long so I'm gonna apply all this to that one with a good sync as well. Thanks again to all for the knowledge, I'm a vintage dirtbike nut who is slowly getting bit by the streetbike side so absolutely still learning.
 
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