Keihen CV34 Sync

I am about ready to put my carbs back on the bike after running into an issue of the #3 cylinder not firing.

I have adjusted the float levels and they are good now. One bowl is leaking a drop of fuel every so often but I think that might be a bowl or drain screw issue.

My question is about bench syncing/vacuum syncing the CV34s. In other threads I have read that the adjustment involves the height of the slides. From what I have seen on the CV34s, the synchronization is actually changing the opening of the throttle butterfly valve. Am I correct on this?

If that is what I am trying to do, how should I bench sync the carbs? An old thread that came up with a search here is so just a sliver of light is visible through at the top and bottom of the butterfly valve. Is this right? I have heard of using a very small wire to get them all similar. Can someone comment on that?

My bike has not been operational in far too long. I need to get out on the road real bad!

I would not relay on the light. The wire will work fanstastic. You must make sure that you can see the idle mix holes (engine side)that they are all partially and equally exposed to the engine side. Back off the idle screw until all butterflys are closed. Turn the screw in until you see about a fourth of each of the holes inside the carbs throats on the engine side. If any are open more or less than the others, adjust the synch screws untill they all are equal. Double check your work with the wire. DONE Except! Maybe you want to put your mouth around each carb and suck. Do they all feel the same? No? What do u do? Ask Larry Caven if u can borrow some tools to straighten out some F L O W. Bet if you syched them that way they would be super accurrate, and if any vaccuum was uneven at further openings, It's the engines fault. If the opening are not equal, adjust the butterflys that are open farther by closing them to mathc the rest. If you open the ones most closed it can cause a high idle that will piss you off tho no end. :woohoo:

I have the same CV34 carbs but have never tried to bench synch. I've always gone straight to the vacuum gauges as I think bench synch is just to get them in the ball park before a full and proper synch.

You are correct regarding the opening of the throttle butterflies - no throttle slides in these carbs - but the tiniest change in the opening has a big effect. It's like that with the vacuum gauges. I would try to "tiny smidgen of light" approach or perhaps a very fine wire like a 1st guitar string or smaller if you can get one

With a tiny opening of the butterflies I would expect a high idle when you put the carbs on the bike as they are normally fully closed at idle.

Let us know how it goes ......and good luck

So I finished it up but I wanted to verify that I did it right. It seemed like there was no adjustment screw for Carb #2 like there was for the other 3. I ended up using the idle thumbscrew to set the throttle plate for #2 then used the adjusters between the other carbs to do the rest. Does this sound right?

Never got a chance to try it out as I set one of the floats way too high. Luckily it was one of the outside guys so I didn't have to take the rack off to get the bowl off.

Getting the rack on is SO much easier with new air boots and carb holders. Wow.

Sounds like you're on the right track.
Yep, thats it. No adjustment screw on the #2 carb. Everything is synched to #2.

Good luck

Got the bike on the road yesterday. Thanks for all your help!

New question related to float/fuel level check. I am getting a steady drip of fuel out of the overflow of carb #4. I think the brass tube is okay as I removed the bowl, filled it with water at the sink and checked and it didn't leak. After thorough drying I put it back on the bike.

When I checked the fuel level using a clear tube, the level was approximately 4mm below the bowl gasket, which is the spec. I am doubting this measurement though. When I unscrew the drain screw the fuel would start to come up but then there would be an air pocket right at the drain/overflow nipple. The fuel level in the tube would be very low (~2 cm). If I tapped the tube eventually the air bubble would work its way back into the carb and more fuel would come out.

Is the measurement off due to this bubble? It happened on all four carbs so initially I dismissed it and assumed the final reading was correct.

Needle seems to seat well because if I take the bowl off and hold the floats up and turn the petcock on, no fuel comes out.

A little confused as to what to check at this point.

Is it a vacuum petcock and do you have it set at on - not prime - when you are dong this ?

Bubbles can screw up reading the levels. When I have a bubble I either tap the tube to move the bubble or lower the tube with finger over the opening and move the bubble back up into the carb and then raise the tube again and let the level settle. Hopefully you just have the fuel level to high.

Constant drip could be the float valve and seat. I dont know if holding the floats up is a good test. I have taken the carbs off, taken the float bowls off held them upside down and blow into the fuel line and see if air escapes at the float valve. You could try cleaning the brass valve seat with a Q tip dipped in carb cleaner. Mine were so bad once that I had to dip the Q tip in Brasso - worked.

sorry but i nothing to add that hasnt been said.
i do have a question for another member though.
what is the float hight, and fuel level spec for the bs34, and cv34 carbs?
is the fuel level measured from the bowl gasket surface?

Mark

The Haynes manual for the 750s says:

Keihin CV 34 carbs fuel level 4 +/- 1mm.
No spec for float height

Mikuni BS 34 carbs fuel level 3 +/1 1mm
Float height 18.6mm

thanks JR.
i'll pass that info on.

I had the petcock set to prime when I was checking the level. Is this not correct? I kind of thought it would have to be set to prime otherwise the fuel level in the carb wouldn't be able to settle to its "final" level.

I did clean out the float valve seats with a Q tip and carb cleaner when I had them apart. I may try rechecking the level by covering the hole and moving the tube like you said you do.