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Old 07-28-2012, 12:08 PM
Shockman54 Shockman54 is offline
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%$#@!%^$#& carb sync

ok, i sprung for a vacuum gauge setup, calibrated the gauges, put the plastic pieces in to stop them from fluttering, then tried to sync the carbs. bike was running fine (except for my hanging idle) and when i tried to get all the gauges reading the same, nothing happened when i turned the idle mixture screws. i tried each gauge on each carb, and no change when i turned the screws. what is going on and how do i fix it?
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"If it runs bad, it's usually the carbs. If the tires go flat, I suspect the carbs. If the wind blows the bike over, the carbs were probably to blame. If my wife yells at me about something, I tell her to talk to the carbs" -Incubus
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Old 07-28-2012, 12:13 PM
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Diskman01 Diskman01 is offline
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The idle mixture screws are for setting the IDLE mixture and have zero to do with the syncing process until the end..

You sync the carbs with the setscrews located on the throttle linkages. Twist the throttle while looking at the carbs and you see the linkages moving.. There are set screws that tie all the carbs together. THOSE are the ones you adjust when syncing.

Here is a quick pic from the manual..



Sync carbs 1 and 2 then 3 and 4 then between 2 and 3.


Do the idle mix screws AFTER a sync. With no tool.
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Last edited by Diskman01; 07-28-2012 at 12:18 PM.
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  #3  
Old 07-28-2012, 12:30 PM
crazy steve crazy steve is offline
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I'll expand on this a bit....



These are the sync screws. This allows you to adjust throttle plate opening until you get the same vacuum reading on each carb. You adjust 1 to 2, 4 to 3, then 1/2 to 3/4. It's important to note that there's no adjustment screw per se for 3, you use the 'main' idle adjustment screw to set all idle speed on all four after syncing the other three to number 3. You want your idle speed to be about 1000 rpm, you may have to adjust the main idle speed screw in or out during the process to keep that rpm. Once you have the same vacuum reading at each carb and about the right idle speed, you can go back and adjust each idle mixture screw for the highest vacuum reading, this may throw your sync back off slightly, repeat the sync process one more time and you're done...
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Last edited by crazy steve; 07-28-2012 at 12:36 PM.
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  #4  
Old 07-28-2012, 12:39 PM
motoman motoman is offline
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Quote:
Originally Posted by Diskman01 View Post
The idle mixture screws are for setting the IDLE mixture and have zero to do with the syncing process until the end..

You sync the carbs with the setscrews located on the throttle linkages. Twist the throttle while looking at the carbs and you see the linkages moving.. There are set screws that tie all the carbs together. THOSE are the ones you adjust when syncing.

Do the idle mix screws AFTER a sync. With no tool.
Actually Shockman, the idle-mix screws have EVERYTHING to do with the sync process, whether they be the early style BS34 carbs or the later BS34 carbs. Idle mixture changes vacuum draw. Back out idle mix screws till just hits the highest inch vacuum, lower idle speed to 1000rpm or UNDER, sync #4 to #3(the 'fixed' carb), then #1 to#2, then center screw, match the pairs together. Kill it and blip the throttle several times to seat the screw threads, re-start and repeat process. Go back thru the idle mix adjustment again followed with the sync adjustment. Re-set idle back up to the 1100 rpm. Your hang idle can be a result of a number of things, besides being out of sync. It can be from cam chain tensioner adjustment(which BTW needs to be changed to an auto tensioner if not already done) a lean mixture in the idle circuit or just a restricted idle circuit from needing cleaned. Either way, a hang idle IS the result of a lean cond. caused somewhere. The cam chain tension is not a common cause of a hang idle, but it can be, and if too loose a chain, besides the extra noise will give you a hang idle of about 5sec. and 3-400rpm. This is all with the assumption that you have checked for vacuum leaks.
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Old 07-28-2012, 12:42 PM
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Yeah but the sync has to be done first Brant or the mix screws won't affect anything at all. The fine tuning is done with those.
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Old 07-28-2012, 12:47 PM
motoman motoman is offline
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Originally Posted by BA80 View Post
Yeah but the sync has to be done first Brant or the mix screws won't affect anything at all. The fine tuning is done with those.
Exactly, guess from what was said, I just assumed that was done, or at least tried. Good clarification.
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Old 07-28-2012, 03:06 PM
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jwhughes3 jwhughes3 is offline
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Brant what do you mean when you say kill it and blip throttle several times to seat threads?

John
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  #8  
Old 07-28-2012, 03:46 PM
motoman motoman is offline
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Originally Posted by jwhughes3 View Post
Brant what do you mean when you say kill it and blip throttle several times to seat threads?

John
Having the long Mo-Pro mercury carb stixs, I don't have to worry bout sucking mercury in blipping the throttle while they are hooked up and running. I forget some just use a vacuum set-up to sync. Either way, decades ago, a pro showed me while syncing the change that happens when the throttle shafts are opened and let snapped shut. That sudden stop allows the threads to seat in themselves changing the setting ever so little. It's something that SHOULD be done every time a sync screw is tweaked a little. Not to get anal with doing it because if you have synced your carbs and know they are dead-on even, I guarentee after you take it around the block for a test ride I can hook the gauges back up and they will be off. Blipping the trottle several times after a change is made reduces that change and the amount. Not to get off topic, but my Honda ST 1100 from the factory has a brown looking thread sealant on the sync thread screws to take up any gap the mating threads may have. I had it synced some 7yrs. ago.....and just got done removing that downdraft Keihn carb bank to replace collant elbows and O-rings. After back together, checked sync with my merc. stixs and they were still all even across..........not too shabby for seven yrs. running and a carb removal inbetween. And yes.......a little blue Loc-Tite dropped on the threads of the sync screws of THESE ole' carbs WILL net you similar results.....notice to the SIMILAR, as in IMO, I don't think the Mikuni carb bank is secured together quite as well as the downdraft Keihn set since these carbs have a rectangle cast filter housing base that incorperate four recessed large screws with a loc tab for each carb.....bout as solid of a mount I've seen.....and a PITA to remove, but that's for the 'other bike section'.
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  #9  
Old 07-28-2012, 11:32 PM
Shacknasty Shacknasty is offline
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...and put a fan on that motor while yer synching yer carbs. Synch and idle mixture adjustments should be done on a warm motor, BTW. Good luck.
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  #10  
Old 07-29-2012, 03:06 AM
Shockman54 Shockman54 is offline
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thanks so much guys. tomorrow i'll take the laptop out with me and follow step by step what you've told me. i knew nothing about motorcycles when i joined and since then i've learned so much from this site. i feel i can now say i now almost nothing thanks to you guys drilling it in my head. i'll post an update tomorrow night.
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1979 xs special, just finished the top end rebuild and then i still have lots of work.
progress thus far...
http://s1268.photobucket.com/albums/...t=IMAG0202.jpg

"If it runs bad, it's usually the carbs. If the tires go flat, I suspect the carbs. If the wind blows the bike over, the carbs were probably to blame. If my wife yells at me about something, I tell her to talk to the carbs" -Incubus
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  #11  
Old 07-29-2012, 05:58 AM
jimbilton jimbilton is offline
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%$#@!%^$#& carb sync

Is it true to state that the main purpose of carb synchronising is to achieve a smooth idle? I ask this because, when I had an XS Eleven, I used a mercury column type device for balancing, and though I could get a fairly even vacuum reading on all four carbs at idle, if I opened the throttle gently to - for example - 1500 rpm, the mercury levels at that speed would vary greatly between each carb. If I then allowed revs to drop back to idle, the columns would level up again. I wonder if this is caused by slight variations in fit of the butterflies and throttle shafts etc..
Is it also correct that vacuum columns are more sensitive than gauges?
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Old 07-29-2012, 10:50 AM
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