Is in synch out of sync?
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Senior Member
SuperSport
Joined: Nov 2007
Posts: 807
From: Hamilton, Ontario, Canada
Is in synch out of sync?
I was reading the UK Firestorm site and came across a discussion about carburettor balancing. One interpretation of the factory shop manual at p. 3-12 is that the vacuum of the rear cylinder should be 20 mm. Hg greater than the front cylinder at idle.
I've balanced the carbs on my VTR a number of times since I got it in 2002 and, like my NT650 before it, I always adjusted the carburettors so that the vacuum for both cylinders was equal. I thought that 20 mm. Hg was the maximum out-of-balance tolerance. Balanced means equal, right? Maybe not for our bikes.
When I adjusted the carburettors so that the vacuum at the rear cylinder was 20 mm. Hg greater than the front, the columns of mercury in the manometer I was using became equal in height a few revs above idle and up. The heights of the columns varied throughout the rev range when I adjusted the carburettors so that there was no difference in vacuum at idle.
The factory manual speaks of checking "the difference between the front and rear carburetors", specifies a carburettor vacuum difference, identifies the front carburettor as the base carburettor and states "synchronize to specification". It does not say make the vacuum for the front and rear carburettors the same.
For what it's worth, the Haynes manual says make them equal.
Any thoughts?
I've balanced the carbs on my VTR a number of times since I got it in 2002 and, like my NT650 before it, I always adjusted the carburettors so that the vacuum for both cylinders was equal. I thought that 20 mm. Hg was the maximum out-of-balance tolerance. Balanced means equal, right? Maybe not for our bikes.
When I adjusted the carburettors so that the vacuum at the rear cylinder was 20 mm. Hg greater than the front, the columns of mercury in the manometer I was using became equal in height a few revs above idle and up. The heights of the columns varied throughout the rev range when I adjusted the carburettors so that there was no difference in vacuum at idle.
The factory manual speaks of checking "the difference between the front and rear carburetors", specifies a carburettor vacuum difference, identifies the front carburettor as the base carburettor and states "synchronize to specification". It does not say make the vacuum for the front and rear carburettors the same.
For what it's worth, the Haynes manual says make them equal.
Any thoughts?
Last edited by comedo; Apr 21, 2012 at 07:49 PM.
Former Superchicken Owner
SuperBike
Joined: Sep 2008
Posts: 1,607
From: Ft. Worth, TX
I'm not sure how you would interpret that there SHOULD be a 0.8 (20mm) Hg difference. The way I read it is that there should not be any more than 0.8 in (20mm) Hg difference between the front and rear carbs.
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Superstock
Superstock
Joined: Jun 2005
Posts: 489
From: San Antonio, TX
Yep, the specified range of difference is 20mm, that would be plus or minus. Anything beyond 20mm is considered out of balance and requires adjustment. The specifications list at the beginning of Chapter 3 makes no mention of one required to be greater than the other.
Where do those UK guys get this stuff? Methinks wifi pubs could play a role.
Where do those UK guys get this stuff? Methinks wifi pubs could play a role.
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Squid
Joined: Feb 2012
Posts: 70
I just did mine, I cant remember what my gauge reads in its a Morgan carb tune, but I them almost perfectly balanced, and its running great, before they were way off
Last edited by masterhacker1989; Apr 21, 2012 at 11:46 PM.
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MotoGP
Joined: Aug 2007
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From: Lake View Terrace, CA
Yep, the specified range of difference is 20mm, that would be plus or minus. Anything beyond 20mm is considered out of balance and requires adjustment. The specifications list at the beginning of Chapter 3 makes no mention of one required to be greater than the other.
Where do those UK guys get this stuff? Methinks wifi pubs could play a role.
Where do those UK guys get this stuff? Methinks wifi pubs could play a role.
Senior Member
SuperBike
Joined: Sep 2008
Posts: 1,707
From: Essex, UK
I was reading the UK Firestorm site and came across a discussion about carburettor balancing.
How to balance your carbs
http://www.vtr1000.org/phpBB3/viewto...p?f=31&t=18430
Last edited by Wicky; Nov 19, 2012 at 07:20 AM.
Thread Starter
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SuperSport
Joined: Nov 2007
Posts: 807
From: Hamilton, Ontario, Canada
It's not discussed in that thread. I found it under the heading "Carb Balancing" by tomekrvr which was begun on August 24, 2009. I'm sorry that I cannot post the direct link.
It's not so much an error in procedure as a different approach.
The stock setup includes different velocity stacks, needles, main jets and emulsion tubes. Is a vacuum difference part of the stock setup too?
I know its counter-intuitive to have a vacuum difference at idle. It's just that when I set mine up that way, the columns of mercury were at the same height and remained matched as the revs increased from a few hundred rpms above idle to about 4 to 5k which is as high as I wanted to take it without load.
It's not so much an error in procedure as a different approach.
The stock setup includes different velocity stacks, needles, main jets and emulsion tubes. Is a vacuum difference part of the stock setup too?
I know its counter-intuitive to have a vacuum difference at idle. It's just that when I set mine up that way, the columns of mercury were at the same height and remained matched as the revs increased from a few hundred rpms above idle to about 4 to 5k which is as high as I wanted to take it without load.
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MotoGP
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From: Lake View Terrace, CA
I can see how someone might get the idea they should be a 20mm difference by reading the manual.
Though IMHO there are a couple of pieces of info missing if that is what they really wanted.
First they don't state which carb should be set higher, just a 20mm difference.
Second it doesn't say 20mm +\- XXmm and every Honda service manual I have seen always gives a acceptable tolerance range
Though IMHO there are a couple of pieces of info missing if that is what they really wanted.
First they don't state which carb should be set higher, just a 20mm difference.
Second it doesn't say 20mm +\- XXmm and every Honda service manual I have seen always gives a acceptable tolerance range
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SuperBike
Joined: Dec 2005
Posts: 1,689
From: South Lake Tahoe, CA, USA
I would definitely make them equal. There is not enough air flowing in to the carbs at idle to make them out of balance.
When you are setting the carb synch, you are just setting the throttle plates to have equal amounts of air flowing past them. At closed throttle, it is a very fine adjustment. As soon as you crack the throttle, the opening for airflow gets much larger and the the vacuum difference become insignificant. Carb sync primarily affects off-idle throttle response.
When you are setting the carb synch, you are just setting the throttle plates to have equal amounts of air flowing past them. At closed throttle, it is a very fine adjustment. As soon as you crack the throttle, the opening for airflow gets much larger and the the vacuum difference become insignificant. Carb sync primarily affects off-idle throttle response.
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Squid
Joined: May 2009
Posts: 40
From: Belmont, New Hampshire
I have found that my carbs get out of synch about a month after having set them. Is this normal? Should I just get a carb sync tool of my own and just keep doing it every month or do I need new springs for the set screws? Any thoughts?
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MotoGP
Joined: Apr 2006
Posts: 4,402
From: Kempner, TX
Yes, get a sync tool, add an OEM joint booster to the front cylinder, and a few pieces of vac line and fittings, plus two caps, and the job can then be done in minutes.
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SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
I check mine at every oil change, and every oil change they are off a bit.. most times in tolerance, but off.
Yes, get a sync tool, add an OEM joint booster to the front cylinder, and a few pieces of vac line and fittings, plus two caps, and the job can then be done in minutes.
Yes, get a sync tool, add an OEM joint booster to the front cylinder, and a few pieces of vac line and fittings, plus two caps, and the job can then be done in minutes.
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SuperBike
Joined: Sep 2008
Posts: 1,707
From: Essex, UK
A good job to do while in the innards installing MCCts >Brass screw-in vacuum hose connector for the front cylinder / Boost joint part no. 16214-MB0-000
Small hands required to fit the fiddly thing or just whip the tank off & airbox and use the right tools with extenders to reach by the carbs to get to it. install it by pushing it onto some hose, loctite the threads and locating it into the plug hole and tightening it up.
Don't forget to have a length of hose that crosses to the left of the bike and blank-it off with a golf-tee or bolt to ensure it doesn't leak.
Small hands required to fit the fiddly thing or just whip the tank off & airbox and use the right tools with extenders to reach by the carbs to get to it. install it by pushing it onto some hose, loctite the threads and locating it into the plug hole and tightening it up.
Don't forget to have a length of hose that crosses to the left of the bike and blank-it off with a golf-tee or bolt to ensure it doesn't leak.
Last edited by Wicky; Nov 19, 2012 at 07:28 AM.
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
This bike came from Tennessee so I don't think it has the same CA evap. equipment. It doesn't have the canister in front under the cylinder. So, I need 2 of the booster joints?
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MotoGP
Joined: Jul 2010
Posts: 3,871
From: Phoenix, AZ
Just need one. You already have on on the rear cylinder (it connects a vacuum line to your petcock... this is the hose that you need the "T" splitter for), so you just need one for the front cylinder.
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SuperSport
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From: Santa Fe, New Mexico
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Superstock
Joined: Oct 2011
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From: Pompano Beach, Fl.
20mm hg
I tried to tolerate the lumpy idle with the 20mm hg difference set one way and then the other per the Honda spec and gave up on it. It's even steven for me. Maybe nath should weigh in on this as Hawk did and it may spice this tread up smig. 
Add a generous dollup of Tweety 
expertise and and it's a wrap.
Add a generous dollup of Tweety expertise and and it's a wrap.
Senior Member
SuperSport
Joined: Jul 2012
Posts: 582
From: Santa Fe, New Mexico
I tried to tolerate the lumpy idle with the 20mm hg difference set one way and then the other per the Honda spec and gave up on it. It's even steven for me. Maybe nath should weigh in on this as Hawk did and it may spice this tread up smig. Add a generous dollup of Tweety expertise and and it's a wrap.
Add a generous dollup of Tweety expertise and and it's a wrap.
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