1982 CB750C dies with sudden throttle

adi105

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Hello everyone, I have been a lurker of this forum ever since I got my CB750. I have an annoying issue which I have been dealing with, but I hope to find the source of the issue. The bike was in pretty rough shape mechanically when I picked it up just from lack of use. It has 12,000 original miles.

What I have done:
checked compression, all cylinders good
adjusted valve clearance as per amc49's advice on other forum posts (btw, if you read this, your posts have been crucial to me getting this bike to run, thank you.)
New spark plugs
full carb clean and rebuild. Ultrasonic cleaner, new gaskets everywhere, the whole 9 yards. Replaced rubber boots on both sides of the carb. Took it to a local shop for a vacuum sync.

After this process, the bike runs great. Has power throughout the powerband, idles great, almost no complaints. EXCEPT, when it idles, if I blip the throttle too quickly, the RPMs will drop at first, then rise. This was insanely annoying and took very much getting used to. To counteract this, I raised the idle to about 1400 or so, such that when this happens, the RPM drop usually doesn't stall my motor. Sometimes, it still stalls the motor if I am not careful and conscious of how abruptly I am twisting my throttle and how fast I am letting my clutch out. This issue only happens when I blip the throttle at idle, such as when taking off. It does not matter if the motor is cold or warm. It does it like clockwork, every time. My thoughts are that it is an issue with the accelerator pump, but I rebuilt the pump with a genuine Honda kit, the o-ring that many people forget is new and placed correctly, and I took special care with cleaning the accelerator pump. I tested the accel pump before installing the carbs, and it shoots gas a good distance away when it is compressed. As such, the accel pump not working is not an issue. Could this symptom be from the accel pump causing a rich condition at the moment that the throttle is blipped, causing the motor to bog? I am not sure if this is relevant, but I chose to delete my air bypass valve rather than replace the torn-up gaskets. I get more popping on decel, but I have not noticed any negative behavior other than that.

Has anyone else had this issue or can give insight to what I can check further? I can post a video to demonstrate the behavior if needed. Any help is greatly appreciated.
 
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