I bought my CB400F in 2017, took forever to rebuild the carbs but got it on the road in 2019. When I bought it, it ran well but had a hanging idle. My carb rebuild and syncing did not fix it.
In all other respects the bike ran great. I lived with the hanging idle - around 3000 RPM, only after the bike warmed up but then relentlessly. I tried most of the easy diagnostics, vacuum leaks etc., with no success. I tried to see if one cylinder was causing an issue by reading the heat on the exhaust pipes, but that revealed nothing.
I suspected weak springs in the spark advance unit (as a contributor but not necessarily the whole issue), but did not want to cut loops off of my springs. So, I decided to measure spring tension of the old springs and try out some replacements, also measured.
I spent a good deal of time doing the measurements in January and February (non-bike season here in New Hampshire). I posted my research on it here in SOHC/4, but somehow managed to post in the wrong forum ("Open Forum" - for non-SOHC/4 bike topics; Doh). Here is the link, lots of detail there:
http://forums.sohc4.net/index.php/topic,206602.0.htmlSo, my New Hampshire dirt road has thawed and dried out, and I took the bike for its first ride of the Spring, long spin today. With new springs installed on the spark advance unit. OMG, for the first time ever I did not have a hanging idle. It idled down to 1300, consistently, only hanging one time but with a little slip of the clutch idle came right down. I got it good and warmed up, but the idle stayed docile. Never seen that before.
Not a perfect experiment, as I had adjusted the valves (they were hardly out of adjustment, however) and replaced spark plug boots, one of which was wildly out of spec. Redid the static timing on my nice set of NipponDenso points and ND OEM quality plate, necessary after removing the points plate to pull the advancer unit.
My replacement springs (both) were from Lee Springs, model LE 037C 0 S (.37" wire diameter, S=stainless steel). I picked these as they have an unloaded length (mfr. spec) of 0.75", a good fit for my 400F advancer, and other specs looked promising. Lee specifications for Lbs/inch for initial spring movement: 0.833; and for maximum lbs/inch: 24.24. Max extended length - 1.01 inches. See my linked post in the wrong forum for measurements comparing to my original advancer springs - the Lees are a good bit stronger at initial tension and somewhat stronger at the full extension (full advance) point.
As before this modification, the bike runs great from 4k RPM on up. Nothing really amiss below 4k, although I think I need to do a thorough adjustment to/syncing of the carbs and of course check the ignition timing with the motor running (only static timing set at this point).
I have read many many posts here regarding hanging idles - often filled with lament. If my initial impression is correct that one weak link - the advancer springs (a known weak part from the get-go, per HondaMan, with little prospect of good original function here five decades later) - could be replaced with new, quality springs, at low cost, that would be something of a boon.
Of course, the 400F springs are not the same as on other models - one can see that just looking at them, as most other advancer springs have more loops (a mere seven on my 400F vs it looks like ten loops on advancers for other SOHC/4s). So, we might need to check out other spring options (Lee Springs has a gazillion - I bought five to experiment with, to get to a $40 order so that shipping was free) for other bikes.
I bought a CB550F last week, so maybe I will do some measuring on that unit in the near future.
