Carburetor Synchronizing

[RodneyCB750K1979]

I'm getting a lot of cam chain noise at idle so I thought to check the carb sync.  It was off as shown in first  photo.  I adjusted #1 to match #2.  Then #3 to match #4. See second photo. When I try to bring #3 and #4 to match #1 and #2, the idle rose to 2,500 rpm.  I adjusted the large black knob underneath the carbs to lower the rpm but it won't go below 2,000 rpm. I made sure the choke was off.  Is there some other adjustment I need to make?  Could the choke idle lever need adjusting?  

Also, how the heck does one adjust the #2 and #3 idle mix screws underneath the carbs?  

Carb sync as-found


Left and Right Pairs synchronized

[shinyribs]

If I remember correctly, on the DOHC models,like yours, the #2 carburetor is not adjustable. Whatever #2 is doing you sync the other three to match. But I could be mistaken on this.

[TOOLS1]

I have never done a sync on a DOHC, but I also think you sync them to the No#2 carb. The syncing of #1, #2 then #3, #4 then both sets to each other is for Kawasaki carbs I know.
Also I have seen where the carbs have been adjust too far with the sync adjusters that they had been adjusted past the idle stop.
TOOLS

[RodneyCB750K1979]

In the second picture, I adjusted the #3 sync screw to bring the right bank vacuum down to the left bank. When I do that, the RPM rises.   I'll check the sync screws. They could be adjusted past the idle stop. Hadn't thought about that.

[rij1]

I had the same problem. My idle wouldn't lower. What I did was adjust the throttle linkage. it may be too tight. mine was. It might or might not be why its staying high.

[icerigger]

The easiest way to adjust the #2 & #3 idle mix is with a right angle screwdriver. Motion Pro makes a good one just for this, and Vintage CB750 have a Chinese made one with a bunch of interchangeable bits so you can use it for other things.

[Lucky 1]

To get rid of cam chain noise...Get a workshop manual and adjust the slack out of your cam chain FIRST.

Go to the grocery store and get this screwdriver in this package of batteries.
It perfect for your CB750 carbs.


Why buy the $90.00 Motion Pro 90º bevel screw driver?
That 90º Motion screw driver is very hard to use with a screwdriver bit.
You have to get the tip of the blade in the right orientation and
it can just slip off and then you have to find the right orientation again.
It works best with a socket  on the end of it.
But for a Honda V4 that Motion Pro screwdriver is a must have.


ALSO.... Make sure all of the mixture screws are set the same number of turns.
Do not move them when syncing the carbs.
Only adjust the idle adjustments on each carb to sync the carbs,NOT the mixture screws.

One more thing....
You won't like this. The dial gauges for syncing carbs is much much easier.
I know that because I bought both types.

[Lucky 1]

In that bottom photo you will notice how one and two are equal and 3 and 4 are equal but all 4 of them do not line up.

Part of the reason for this is the slop in the slip joint between the throttle shafts joining carbs 1&2 and carbs 3&4.
That is why I still like the 1969-70 CB750 carbs the best of all, because they do not have that problem.

[LukeM]

One thing to keep in mind: the closer your carbs are to sync with each other, the less throttle the engine needs to run.  Think of it as riding a bicycle where the cranks are not 180 degrees apart...it'll lope when you pedal.  When they are in sync, the engine works easier and smoother.

You will need to adjust the throttle cables once you get close to sync-ing all 4 carbs.  If memory serves, the idea is to get 3 and 4 in sync, and then 1 and 2 (with 2 as the reference), and then balancing pairs 1-2 and 3-4.  There should be one more screw that balances the pairs.  Go over the carb assembly in the manual a couple more times.  The write-up in the manual should help as well.

Luke M

[RodneyCB750K1979]

I tried again today to sync the carbs. I can get them to all agree but I can't reduce the idle below 2100 RPM.  

I checked the throttle cable and there is slack.
I checked the high speed idle knob and it is backed out
I checked the choke lever and it is not touching the carb throttle plate.
I pulled two carb covers and felt the throttle plates.
I inspected all three sync screws and verified correct washer/spring positions.

When I press down on the throttle lever between carb 1 and 2, the idle speed goes way down. Something is keeping the throttle plates from closing completely.  Next step is to remove the carbs for a closer inspection.  

Does anyone have advice on how to reach and adjust the #2 carb idle mixture screw? I bent two small screwdrivers to no avail. I'm gluing a screwdriver tip into a 5mm socket.

[TOOLS1]

I will bet that you have the sync screws way out, and they are holding the throttle open past the main idle adjuster. Try backing the sync screws out, and redoing the sync.
TOOLS

[icerigger]

Yeah, it sounds like the throttle plates are binding. When you get the bank off, try relaxing the screws on the top and front rails and lightly racking the carbs. Tighten the screws and check the action on the throttle plates. Repeat if they're binding even the slightest bit.

Heard of one guy on the DOHC forum that had them slightly out so that they were fine when cold but stuck when hot.

Good luck.

[RodneyCB750K1979]

Problem fixed.

I removed the carbs and found when the #2 carb throttle plate was closed, the #4 was not seated. This explains the inability to lower the idle rpm and why the rpm would drop when I pressed the throttle linkage.  I loosed each pair of throttle plate screws to allow the plates to adjust and seat. I then removed each screw one by one and applied Thread Lock Blue. After tightening all the screws, I punch marked each screw tip for added assurance they don't back out and get ingested.  I then proceeded to adjust the idle mixture using a Heathkit exhaust analyzer and then synchorized the carbs. I can now idle the engine as low as I want.  Thanks Icerigger for the advice. I tried loosening the carb rail screws but I couldn't access all of them with the carbs mounted. Since I had the carbs off, I chose to loosen the throttle plate screws.

 

[750K8]

You need to bring that exhaust analyzer over when you come down for a visit. We didn't do that last time on my 78...all we did was sync the carbs.