Author Topic: Why you must sync the carbs.  (Read 6169 times)

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Offline lucky

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Why you must sync the carbs.
« on: April 24, 2012, 03:54:26 pm »
I previously posted a story about getting good results on the carbs slides by using a vernier calipers when the carbs are off of the bike.

Today I found out the reason it is still needed to  sync them.

After you set all of the slides with a calipers to get all of the slides to match #2 carb slide when it is bottomed out, by measuring the top edge of the slide to the top edge of the carb casting, look into the carb openings on the air filter side and you will see that when the carb slides on #3 and #4 are all the way to the top that the #2 and #1 carb is 1/8 inch from the top!

The reason for this is the way the linkages between the carbs are put together.
So since #2 carb is the one that cannot be adjusted, and #1 carb already matches #2 within .001 thousandths. all you have to do is adjust carbs #3 and #4. to match.
You will end up dropping those two slides down to match #1 and #2.

When I sync'd the carbs with the dial gauges that is exactly the way it went. #3 and#4 were way off and almost the same amount off.

 
« Last Edit: May 01, 2012, 11:02:31 pm by lucky »

Offline DWS

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Re: Why you must sync the carbs.
« Reply #1 on: October 24, 2012, 11:33:51 am »
THANK YOU Lucky for clearing that up, I respect all you guys in-put on these carb's, and instead of you getting really mad at all, you went out and found more ways to help us/me fine tune my carb's.
Thanks Duane
« Last Edit: October 24, 2012, 03:46:26 pm by DWS »
Always looking for friends.

78CB750F3
PD 42A with original breadbox
4 into 1
valves adjusted to .003 (intake and exhaust) ( as was suggested for cooling )
slow jets#35 Main #110
 turned out to 1.5 some paper work says 1.75
4800 feet altitude

78 Honda CB 750 F3 Restoring now(Almost done)
BSA MX 50 Next
BSA b50T Parts

Offline scunny

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Re: Why you must sync the carbs.
« Reply #2 on: October 25, 2012, 01:45:06 am »
close then do a vacuum sync to get them closer.
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Offline mono

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Re: Why you must sync the carbs.
« Reply #3 on: October 25, 2012, 05:52:13 am »
I previously posted a story about getting good results on the carbs slides by using a vernier calipers when the carbs are off of the bike.

Today I found out the reason it is still needed to  sync them.

After you set all of the slides with a calipers to get all of the slides to match #2 carb slide when it is bottomed out, by measuring the top edge of the slide to the top edge of the carb casting, look into the carb openings on the air filter side and you will see that when the carb slides on #3 and #4 are all the way to the top that the #2 and #1 carb is 1/8 inch from the top!

The reason for this is the way the linkages between the carbs are put together.
So since #2 carb is the one that cannot be adjusted, and #1 carb already matches #2 within .001 thousandths. all you have to do is adjust carbs #3 and #4. to match.
You will end up dropping those two slides down to match #1 and #2.

When I sync'd the carbs with the dial gauges that is exactly the way it went. #3 and#4 were way off and almost the same amount off.

nice work!  thanks for taking the time to be so scientific. 

Offline BobbyR

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Re: Why you must sync the carbs.
« Reply #4 on: April 17, 2013, 06:12:49 pm »
THANK YOU Lucky for clearing that up, I respect all you guys in-put on these carb's, and instead of you getting really mad at all, you went out and found more ways to help us/me fine tune my carb's.
Thanks Duane
Oh we all get mad at them. We just stay at it until we get it right like Lucky. Then we find something else to get pissed at. Part of the charm of owning vintage bikes.  ;D ;D ;D ;D ;D
Dedicated to Sgt. Howard Bruckner 1950 - 1969. KIA LONG KHANH.

But we were boys, and boys will be boys, and so they will. To us, everything was dangerous, but what of that? Had we not been made to live forever?

Offline DWS

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Re: Why you must sync the carbs.
« Reply #5 on: August 20, 2014, 11:29:28 am »
https://www.youtube.com/watch?v=tTOx2LUmy-k&list=FLdjM_Zs-jiPdAGQXZc6YlAQ&index=1

Don't forget to calibrate each of the vacuum gauges before syncing the carbs or you might not get a true sync job.
it starts at 4:00 min mark.
« Last Edit: August 20, 2014, 09:12:11 pm by DWS »
Always looking for friends.

78CB750F3
PD 42A with original breadbox
4 into 1
valves adjusted to .003 (intake and exhaust) ( as was suggested for cooling )
slow jets#35 Main #110
 turned out to 1.5 some paper work says 1.75
4800 feet altitude

78 Honda CB 750 F3 Restoring now(Almost done)
BSA MX 50 Next
BSA b50T Parts

Offline lucky

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Re: Why you must sync the carbs.
« Reply #6 on: June 13, 2015, 10:01:55 am »
DWS is correct.
Make sure to sync the gauges before you start your final adjustment of the carbs.
BTW...The dial gauges are much better and much easier to use than the manometer type.
If you do not believe that- then try both types.
Do not adjust the pilot air screws while syncing the carbs.
Those pilot air screws should already be set to the same number of turns.
 Only the slide height should be adjusted.

 

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