Hangin Idle and other idle issues

gatorforge

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I know this has been addressed before (I've read most of the threads trying to solve this problem) but having tried most of the methods listed, I still cant seem to pin this one down.

I rebuilt the top end on my 73 CB750K and have it up and running, except for a hanging idle and sometimes idling to a stall at stop lights. Idle is set around 1k, but under riding conditions, when I come to a stop sign or stop light it wants to sit at about 2k, but will eventually start to fall to the point that without a throttle blip it will stall out or nearly so. On the side stand, it did have a hanging idle issue, but I was able to work this out when I vacuum synced the carbs and re adjusted the idle screws, now it revs and drops to a nice idle once warm on the stand.

The bike is running pods and 4-1 exhaust, I'm not looking to change that set up, just need help finding the cause of this. Bike is running 115 mains and 40 pilots, needles are in the middle position. This set up was used because the bike was actually running to rich with the larger mains I had originally used to compensate for pods and was backfiring on deceleration.

Here is what I have done so far:
As mentioned, carbs were vacuum synced last week, all jets are brand new, float bowl level seems to be good (but not ruling them out) , and there is no gas leaking from overflow tubes.

I performed a WD 40 air leak test with the bike hanging a high idle on the stand before I adjusted that out, and had no results at carb boots or intake manifolds.

Valve tappets were adjusted to the specifications in the manual using the hondachopper.com instructions.

Timing has been set to as near perfect as it will go and tested with two different multimeters. Advance springs don't seem particularly heavy, but being able eliminate hanging idle reliably on the side stand would seem to indicate that the advance springs are still doing their job, as the hanging idle is only occurring while riding.

Plugs are fresh.

I've adjusted the air mix screws to 1-1/4 turn out, and this removed the hanging idle when the bike was on the side stand, but any more adjustment out, and the bike starts to stutter and falter.

I've tried everything I've seen in the other threads and still cant seem to eliminate this, everything else on the bike is running great, so I really just want to get out and ride more, but this has been holding me back for a while. Any help would be appreciated.
 

slartybartfarst

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I dont know if you have solved this problem but this is what I found on my 1970 K0. Potentially new fuel with higher Ethanol exacerbates the idle problem. If you can get an spare ignition advance unit then as Honda Man describes the springs are going to be tired. In my experience you need to hold the retarded state until a higher rpm then advance further later. Strip the parts and grind off approximately 0.3mm off the stop lands on the body so you can make the bob weights fall back to a lower point, bend in the spring ends to make them shorter, lubricate the parts and reassemble. Now set the timing as normal and ride the bike. I found the idle running now easy to set and more performance from the engine. I checked the timing with my timing light and compared to my old advance unit and found the bike did not started to advance till 2,500 rpm compared to 1,500 rpm before and advanced approximately 5 degrees more at higher rpm. You may have to experiment and maybe check with a timing light but the differance in idle and performance is worth it.
 
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