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I'll first state that I've never to this day have ever been able to properly tune a bike with 3-D carbs to get them to respond. They are the bane of my existance.

I'm currently trying to get my 69 CB350 to run correctly. It idles like it has a mind of its own. I can get it to idle at 1200 RPM, but as soon as I blip the throttle it will do one of several things: rev up properly, then take too long to fall back down to idle. Or, continue to rev to 4-5k RPM which requires a shut-off.

1) I've synced the carbs. Put a finger one one throttle arm and visually watch the other. Got them as close as possible. Feel left and right back-pressure and fine-tune the throttle screw stops.

2) I've pulled the carbs off twice and rebuilt/cleaned them. They have new diaphragms.

3) New points/condenser. I've sprayed and did a quick clean on the advancer. It operates smoothly by hand.

4) Spray carb cleaner around the carb mounting area. No change in RPM.

I've been at a stand-still for two years on it.

Idle mixture screw is about a turn out from all the way in.

I'm wondering if this is a timing related issue at this point. My next thought is to take a take my timing light and to continue to diagnose to see if this is a timing related issue. Advancer staying open/weak springs?

If it's a carb-related issue then I have no idea what I can do other than to buy new carbs.
 

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3D and the later 722a carbs are basically the same other than the idle mixture screw. You probably have done or are aware of several points I am making.
To set a baseline ... stock mufflers and air filters? OE brass parts or aftermarket?
Be sure the battery is fully charged and check the voltage while it is running at the coils, tank off is easiest. Voltage needs to be within a few 10ths of a volt of the battery. If it isn't then you need to check the key switch, kill switch and wiring in the headlight bucket.
The 350 engine tends to load up with fuel if you play with it for too long especially if you are running original coils etc. Throw in a fresh set of spark plugs before tuning up and be sure the caps are good and the plug wire is making proper contact.
The brass idle ciruit emulsion tube has a very small center hole that usually gets plugged and is difficult to open up. I use a single led bulb to check it. You must see a nice small circle of light. How are the o-rings and rubber plug for the jets?
I clean and polish the barrel where the slide moves with Liquid bar keepers friend. I use sparingly as you need to clean up any residue.
Honda setting for the float is 26mm. I set mine to 27mm. THe specific gravity of fuel has changed over the years is the reason.
Inspect the clamps on the manifolds. Out of 30 or 40 clamps I think I have found 3 that didn't need repair. After straightening I use a small washer on the screw. If the manifolds have been taken off then I use a copper gasket spray on the gaskets and if the engine is in the frame so using an impact is difficult on the screws then I use allen or socket head fasteners to obtain proper tightening.
Good luck.
 

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I have a ‘69 CL350 with 3D carbs, all original brass parts except for one idle jet. I ignore trying to adjust the idle speed screws by feeling for exhaust pressure. I just make sure they are adjusted exactly the same. Idle mixture screws adjusted by ear, and mine are more like 1-1/4 turns out. As Boomer343 said, my float height had to go a bit lower than spec, to 27.5 mm for the carbs to mix correctly. That said, mine has an extremely consistent idle, and when the throttle is blipped, the engine immediately returns to the proper idle. It took a lot of cleaning and fiddling to get it right, but when I finally did, it was worth it.
 

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It sounds to me like you have a vacuum leak. One place that these carbs will leak vacuum at that is hard to find is the throttle shaft seals. I replace them with O-rings.
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