Carb sync - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 12 (permalink) Old 03-03-2012, 06:23 PM Thread Starter
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Carb sync

Beautiful day here, so I decided to sync carbs using my homemade manometer....very interesting excercise!
At first, the front cylinder/carb had significantly more vacuum than the rear...after a very slight change of the sync screw, I settled on the following:
At idle, the front cylinder/carb had slightly more vacuum but stabilized within a 3 inch difference.
At 4500+ rpm, they were both within a 1/4 of each other and rock solid.

Notes:
The manometer took longer to make than the actual sync.
You have plenty of time to sync the carbs WITHOUT using the T-fitting. Just attach each end of manometer to the vacuum ports, start it up and observe.
Very easy and satisfying task. Took approx. 15 minutes
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post #2 of 12 (permalink) Old 03-03-2012, 06:29 PM
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Actually Doc, I think you're better off syncing your carbs to equal vacuum at 1100 rpm. After you twist the throttle, you're opening the butterflies anyway.
I could be wrong, but Idle is where I sync them to exactly equal vacuum.

~~C8> Ratt
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post #3 of 12 (permalink) Old 03-03-2012, 06:40 PM Thread Starter
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Originally Posted by ZCraggRatt View Post
Actually Doc, I think you're better off syncing your carbs to equal vacuum at 1100 rpm. After you twist the throttle, you're opening the butterflies anyway.
I could be wrong, but Idle is where I sync them to exactly equal vacuum.
This is the kind of feedback I was hoping for.
Logic tells me to sync them at/near RPMs I'll be running. Why is idle better?
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post #4 of 12 (permalink) Old 03-03-2012, 06:53 PM
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Well, I'm no expert, but syncing the vacuum at idle provides the smoothest idle. Each jug pulls the same amount of fuel for matching joules created in each jug. This provides equal torque from each cylinder at idle.
When you twist the throttle, the air flow has less restriction and is less dependent on the exactness of balance between the two.
I am sure someone else can explain it much better and make more sense out of it.
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post #5 of 12 (permalink) Old 03-04-2012, 01:09 PM
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Bump.
I was really hoping someone with true understanding of properly synchronizing the carbs would provide input ...
Jerry? ... Knife? ... anyone? ...
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post #6 of 12 (permalink) Old 03-04-2012, 01:32 PM
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Well I really don't know why, but Ma Kaw says to sync them at idle on a warm engine. So that's good enough for me.
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post #7 of 12 (permalink) Old 03-04-2012, 02:50 PM Thread Starter
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Well I really don't know why, but Ma Kaw says to sync them at idle on a warm engine. So that's good enough for me.
Where does she say that??

Last edited by VoIPDoc; 03-04-2012 at 03:55 PM.
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post #8 of 12 (permalink) Old 03-04-2012, 03:26 PM
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Quote:
Originally Posted by VoIP Doc View Post
This is the kind of feedback I was hoping for.
Logic tells me to sync them at/near RPMs I'll be running. Why is idle better?
I have seen it done 2 ways. Most motorcycle shops will sync the carbs at idle with the engine fully warmed up. Racing tuners will sync the carbs near top rpms. They are more concerned about equal flow at max rpms than at idle. It usually takes 2 people to sync the carbs this way.

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post #9 of 12 (permalink) Old 03-04-2012, 04:04 PM
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Where does she say that??
That would be on page 2-10 of the repair manual. Bottom right of the page.
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post #10 of 12 (permalink) Old 03-04-2012, 07:38 PM
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Quote:
Originally Posted by CW-4 View Post
Well I really don't know why, but Ma Kaw says to sync them at idle on a warm engine. So that's good enough for me.
Quote:
Originally Posted by VoIP Doc View Post
Where does she say that??
Also in the Clymer manual at the end of chapter 3, page 102.
Actually calls for syncing the carbs at 1,000 rpm, a bit lower than recommended idle speed of 1,100 rpm.

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