I've just finished a bank of CB500 627B carbs for another member on here and oddly when they arrived I noticed the return spring wasn't standard. I asked where it had come from and was told that David Silver supplied it, if they did they no longer do as the original part number 16036-323-004 doesn't link to anything. I did note however how much lighter it was in operation.
After finishing the bank of carbs and oiling all the linkages I decided to test how much lighter it is in comparison to the standard spring. Results are quite interesting.
The non genuine spring is slightly shorter than the standard one and is quite stretched even with no throttle open, it takes 7lbs of pressure before the carbs begin to open.
In comparison, fitting a standard spring makes the throttle much harder, a hefty 14lbs of pressure required to get the carbs to start to open.
As an experiment I fitted a small 10mm stainless steel split ring and tested the standard spring again, just doing that reduced the pressure needed to 12lbs. Not a massive amount I'll admit but around a 14% reduction.
I tried the non genuine spring with the same split ring and that dropped it to 6lbs and the throttle felt extremely light, so light I was thinking maybe that's a little too light as it didn't exactly shut the bank down hard when it's let go.
Just goes to show how much pressure we are asked to provide to open the throttle on the 500/550, no wonder I had a bulging forearm but only on my right arm when I was younger. I do know it used to make my arm ache after a few hours.
A bigger split ring would reduce the pressure even further I reckon, there would be a limit though on how big, as you'd chance the spring coming off under fast contraction.
