Topic: CB750 K5, carbs hang open only when running

[Dunk]

I'm having an issue with hanging idle. I just replaced the throttle cables. and cleaned the throttle tube, cables lubed. Cables both routed underneath the ignition coils, gentle bends. Carbs were rebuilt over the winter. With the engine off the throttle operates smoothly and snaps closed even from just barely cracked open.

When the engine is running the idle hangs if letting go when just off idle to low amounts of throttle. A bit more throttle and it'll return slowly then hang. It returns better from higher throttle openings, but usually doesn't quite close all the way to idle unless turning the throttle tube the other way to use the return cable. Leave it hanging and turn the engine off, and the throttle closes completely to idle. I had the same issue with the old cables, which were old and routed slightly differently with tighter bends.

Weak return spring maybe? Throttle doesn't feel any harder to turn than my other CB750 which has never had a hanging idle issue. Thoughts?

[Dunk]

Replaced throttle return spring with one off a spare rack of carbs. The problem was obvious once I removed the spring. The tight spring had a rubber sleeve, it was somewhat somewhat hard and caught the spring so I removed it, cleaned the spring and sleeve, and soaked the sleeve in wintergreen oil and acetone until it softened a bit. Installed with some silicone spray lube between spring and sleeve. Throttle now returns to idle every time, though is slightly slow to return if just cracking it open. No noticeable difference in effort at the throttle tube.

I see David Silver has repop return springs. I'll probably order a few. I have several spare racks of carbs that I'd tired of collecting dust. Probably going to rebuild and sell them so I'll add a new return spring to my normal carb rebuild to-do list. Curious if the repop spring will close the throttle any faster when just off idle on this K5.

[ekpent]

Some good info for someone to try who may have similar issues.

[newday777]

Dunk
I wonder if someone stretched that longer spring to lessen the pull tension as has been posted elsewhere??

I just checked the part numbers for the springs on CMS and noticed the K6 carbs uses a different part number for the return spring. I have K6, K5 and K4 carb racks, I'll have to check the differences.