Topic: CV Carb Sync question

[Johnny340]

Which way to I turn the butterfly stop to decrease vacuum?  I know it should be obvious but they are way out and I need to adjust it with the engine stopped initially.  (I did do a mechanical bench sync first.)
Also, do I need to plug the vacuum line that I disconnected to hook up #2 vacuum tube to?
See pic.
Thanks.

[flybox1]

your #2 vac tube needs to connect to the vacuum sync inlet...just like all the others.
if #2 has no adjustment, you need to adjust all others to match #2
a decent bench sync would get those closer to the same vacuum from the get-go.

[Johnny340]

I have all 4 hoses from my manometer hooked up to the carbs but I had to disconnect the one in the pic because it was on the port that I needed.  It is for the fuel supply auto shutoff or something.  Should it run correctly with this hose simply disconnected or do I need to T it into my manometer or do I plug it?

#2 has no adjustment so I wanted to lower the vacuum on #1 to start with.  When I loosen the lock nut do I adjust the set screw IN or OUT to decrease vacuum?  Apparently my bench sync sucked even though they all looked absolutely identical.  Maybe I have an air leak too??

[flybox1]

that disconnected hose doesnt need to be connected to anything during a vac sync.  its an atmospheric vent hose, and should connect to a barb other than the barb used for your vac sync. 
vac sync holes need to be plugged w a screw after the sync is done.

rule out the air leak first. then proceed with the sync.
once its up and running, turn the sync screws on #1 first, counterclockwise


Clean looking bike BTW 

[Flyin900]

That's an interesting set up.

Not sure how that will work as usually you need to sync the gauges to a zero point of reference. The Motion Pro fluid gauges have a set up that syncs off your non adjustable in your case #2 CV carb for a initial set point that starts with the gauges all equal.
Even a mechanical gauge set needs to be zeroed out to begin the sync set up.

The bench sync is a good start, yet with the mechanical differences on a four cylinder older motor I find a vacuum sync is necessary to really fine tune the carbs.

[lucky]

What year is this bike?

[lucky]

1969-1978 CB 750's did not have vacuum operated carbs.
They were not "CV" carbs. CV=Constant velocity.
They compensated for altitude.

The 1969-1978 carbs did not.

[Flyin900]

That hose you disconnected is the AFV (automatic fuel valve) hose and can be left alone for the sync and not connected to anything. This is assuming you are running an aux gas tank for the sync. If you are still running off the bike tank and petcock it will need the vacuum and a T may be necessary. Make sure you hook it back up once the tank is in place as no fuel will flow if it doesn't see vacuum from the port your connected to presently.

You sync to the number 2 non adjustable carb that your AFV was connected to originally. There is no in or out initial adjustment as you propose. I honestly don't see how that set up will work. The gauges need to be calibrated initially for a base line to each other, which there is no way I can see to do that  with that set up.

Normally the sequence is you sync 4 to 3 and 1 to 2 and fine tune by getting the levels as close as possible to the base line#2 carb and each other. Since each two carbs are interconnected 1-2 and 3-4 they will follow along somewhat as you do the adjustment.

It is an investment for a proper sync gauge and it isn't normally used very often by the average owner, so I understand the attempt to McGiver a guage. Maybe someone here can offer a solution that worked for them or the trusty U Tube.

[flybox1]

1969-1978 CB 750's did not have vacuum operated carbs.
They were not "CV" carbs. CV=Constant velocity.
They compensated for altitude.

The 1969-1978 carbs did not.

This is not a CB750.  Its a CB650.  Check the picture at the top.

[Johnny340]

This is an '81 CB650.  I assume no fuel has been flowing because I haven't had that line hooked up.  I'll get a 'T' and try again.
The sync tool is a McGyver thing inspired from Youtube that should allow me to balance the vacuum between each carb (with #2 as the starting point).  Once I get fuel to it and it actually runs in this configuration, I assume it will be painfully obvious which way to turn the adjusters to affect the vacuum for each carb.  Thanks for the replies!

[Flyin900]

The AFV is visible on your last picture on the top of #4 carb. You should be able to run straight off the petcock on your gas tank with a longer gas line and bypass the AFV and run directly to the gas line that comes out of the AFV and leads down to the carb input.

The bike looks really clean and well looked after from the pics you have posted

[Johnny340]

You should be able to run straight off the petcock on your gas tank with a longer gas line and bypass the AFV and run directly to the gas line that comes out of the AFV and leads down to the carb input.

The bike looks really clean and well looked after from the pics you have posted

Thank you and thanks for the tip.  I'll try that tomorrow instead of getting a T fitting.

[RevDoc]

Johnny, those look to be the same VB42 carbs I have on my 750Custom. (Check on the side of #1 for the model #.)  When I first got it one of the other forum members was kind enough to share this link with me, http://cb750c.com. I found a wealth of info on their site on working with these carbs. You might find it very helpful as well.

One of the key items I discovered was that it is highly recommended to eliminate the AFV, which they refer to as "S.O.S" which stands for "Spawn of Satan"!  Seems those are a frequent source of fuel supply problems. I removed mine and haven't had any problems at all.

[Johnny340]

They say VB 44BASL on them.  I'll check out that link. Thanks!  Yes, that AFV seems like a Rube Goldburg design of overkill seeing as there is an 'Off' position on the petcock.  I think I'll bypass it.

That sync method worked perfect btw.  Runs way better but the mixture screws are over 3T out and it still doesn't like to rev up right off the bottom when I try to rev it up to downshift.  I have the stock pressed in pilot jets in these carbs with 120 mains and a 4 into 1 pipe on it.