Author Topic: carb work after vacuum sync?  (Read 1018 times)

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Offline evinrude7

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carb work after vacuum sync?
« on: September 23, 2017, 06:00:26 AM »
i got my 750k6 running pretty well to a point where all that was left was a carb sync.  took the bike down to nashville motorcycle repair and frank sync'd them up and had it idling nicely at 1000 rpm.  i have some new insulator boots on order and would like to change out the old hard ones for new soft ones.  if i carefully remove the carb rack to change these out is there any reason i would need to have the carbs sync'd again?  i can't see why if i remove and install them carefully.
cb750 k6 - ugly

AirCanuck

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Re: carb work after vacuum sync?
« Reply #1 on: September 23, 2017, 06:22:29 AM »
What did the sync run you? You can get a carb sync kit for like 100 or less on Amazon and it looks pretty easy

Offline evinrude7

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Re: carb work after vacuum sync?
« Reply #2 on: September 23, 2017, 06:51:47 AM »
an hour labor.  think it was less than $65.  yeah i could have purchased a vacuum sync set up (which i will in the future) but i needed a front tire changed out and i wanted an experienced mechanic to check the bike over after my noob hands have been all over it for 9 months.   plus i want to support this particular local shop and become acquainted with them.  the owner also a solid mechanic goes out of his way for the local vintage bike community.  good folks. 
cb750 k6 - ugly

Offline Scott S

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Re: carb work after vacuum sync?
« Reply #3 on: September 23, 2017, 07:42:15 AM »
 Unless the sync somehow compensated for leaks on the old boots, I see no reason why you would have to do it again.
 Take some pics of the lock nuts and cable adjuster nuts so you can get them back as close as possible.
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AirCanuck

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Re: carb work after vacuum sync?
« Reply #4 on: September 23, 2017, 07:44:57 AM »
an hour labor.  think it was less than $65.  yeah i could have purchased a vacuum sync set up (which i will in the future) but i needed a front tire changed out and i wanted an experienced mechanic to check the bike over after my noob hands have been all over it for 9 months.   plus i want to support this particular local shop and become acquainted with them.  the owner also a solid mechanic goes out of his way for the local vintage bike community.  good folks.

Very cool! I'm in a very rural area.. we sorta have a couple older fellas who specialize in old bikes. That saiid... my bike has been with him a month today and he finally started in yesterday. Oh the regret.

Offline evinrude7

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Re: carb work after vacuum sync?
« Reply #5 on: September 23, 2017, 12:08:54 PM »
Unless the sync somehow compensated for leaks on the old boots, I see no reason why you would have to do it again.
 Take some pics of the lock nuts and cable adjuster nuts so you can get them back as close as possible.

agreed.  thanks scott.

an hour labor.  think it was less than $65.  yeah i could have purchased a vacuum sync set up (which i will in the future) but i needed a front tire changed out and i wanted an experienced mechanic to check the bike over after my noob hands have been all over it for 9 months.   plus i want to support this particular local shop and become acquainted with them.  the owner also a solid mechanic goes out of his way for the local vintage bike community.  good folks.

Very cool! I'm in a very rural area.. we sorta have a couple older fellas who specialize in old bikes. That saiid... my bike has been with him a month today and he finally started in yesterday. Oh the regret.

go down there, watch and learn. 

cb750 k6 - ugly

AirCanuck

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Re: carb work after vacuum sync?
« Reply #6 on: September 23, 2017, 06:40:29 PM »
Unless the sync somehow compensated for leaks on the old boots, I see no reason why you would have to do it again.
 Take some pics of the lock nuts and cable adjuster nuts so you can get them back as close as possible.

agreed.  thanks scott.

an hour labor.  think it was less than $65.  yeah i could have purchased a vacuum sync set up (which i will in the future) but i needed a front tire changed out and i wanted an experienced mechanic to check the bike over after my noob hands have been all over it for 9 months.   plus i want to support this particular local shop and become acquainted with them.  the owner also a solid mechanic goes out of his way for the local vintage bike community.  good folks.

Very cool! I'm in a very rural area.. we sorta have a couple older fellas who specialize in old bikes. That saiid... my bike has been with him a month today and he finally started in yesterday. Oh the regret.

go down there, watch and learn.

Absolutely plan to if I'm able!!

Offline ilikepinkcrayons

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Re: carb work after vacuum sync?
« Reply #7 on: September 24, 2017, 01:36:08 PM »
Yeah, if your old boots were leaking, you may have to redo the sync. I'm a big fat newb too, and I really recommend trying the carb sync yourself. Theres a set of gauges on Amazon that I wish I bought for $60, but instead, I made my own for about $40 (you can go cheaper if you try, but I just don't recommend it). It includes adjustable restrictor valves that allow you to dampen out the pulsations at idle, and includes the vacuum adapters that you screw into the vacuum ports on the intake manifold.

It's super easy, and very gratifying learn to do it on your own. You can find directions and videos pretty easily, but here's a brief run-down:
1. Loosen the jam nuts on 3 of the cylinders' slide adjusters. You need one of the cylinders to be a "reference" that is not touched, otherwise you might end up chasing yourself around in circles. I picked cylinder #2 to keep as reference.
2. Place a big fan in front of the engine to keep it cool while you're doing this. If you don't have a maintenance auxiliary fuel tank (I don't), remove your tank, set it aside on a bench or something at a higher level than your carbs, and attach long fuel lines from the petcock to the carb feed lines.
3. With the gauge attached to the bike and the engine warmed up, set the idle at your preferred idle speed (mine's at 1,050rpm).
4. Identify the cylinder with the highest vacuum, and lower it slightly. The adjustment is very sensitive, and just putting screwdriver pressure on the adjuster tends to move things around a lot.
5. Give the throttle a blip to settle things, and let it go down back down to idle. If the idle has drifted from your target, readjust it before proceeding.
6. Identify the cylinder with the lowest vacuum, and raise it slightly. Again, the adjustment screw is very sensitive!
7. Blip the throttle, readjust it as necessary.
8. Repeat steps 4 through 7 until all cylinders are as close to the same as possible.
9. Carefully tighten the jam nuts, making sure not to allow the adjuster screw to twist with it.
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Offline BomberMann650

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Re: carb work after vacuum sync?
« Reply #8 on: September 24, 2017, 05:04:50 PM »
A note for the unitiated - Carb sync is on the routine maintenance list.  Every 3k should at least check it. 

Offline ilikepinkcrayons

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Re: carb work after vacuum sync?
« Reply #9 on: September 24, 2017, 05:30:52 PM »
A note for the unitiated - Carb sync is on the routine maintenance list.  Every 3k should at least check it. 

Great point. An excellent reason to buy the right tools and learn how to do it yourself!
1968 MGB
1975 CB550K
1981 Volvo 244 GLT