I'm now on the home run with my bike carb installation - just the choke and throttle cables to go.

The choke is easy, but I have to modify the throttle cable position on the carbs to clear my nosecone/bonnet.

My question for today is - do I need to fit a spring to replace the throttle return cable? The return spring is quite strong on the CBR600 carbs - stronger that the one on my original Weber down-draft carb in fact - so I was wondering whether it was required.

I know that some people have done this, but my mods would be hugely easier (i.e. trivial) if I didn't.

I guess not essential on a car engine.

With the bike engine it spins up more quickly and with no flywheel effect it slows down more quickly too. The throttle return is spring is there to ensure that when the throttle is wound back in that it actually moves!. The last thing you would want on a bike is the throttle not backing off correctly/quickly.

Given that on a car engine things happen much more slowly in terms of revs picking up and dropping of in relation to throttle position i dont think a return spring is essential.

^^^ Correct, but, the butterfly return springs in the carbs may not be strong enough to return the accelerator pedal. If the pedal is sprung you might get away with it, if not the carbs may not close fully.

Other reason for the return cable on the bike, is to achieve a light throttle action.

Rgds.
Pete.

[Edited on 20/2/08 by r1_pete]

The carb itself has a fair sized return spring coiled around the shaft behind the cable connection - it's actually quite uncomfortable holding the carbs open for any length of time, due to the spring pressure (and the lack of anything significant to hold onto!).

When i first fitted the ZZR's on mine i had a slight teething problem whereby when i came off throttle and pressed the clutch fora gear change the engine would rev up a bit and slowly return to rest. I cured it by adding a second spring to the carbs. I think the butterflys werent closing down quick enough and sometimes werent being held shut tight enough at idle.

I found the throttle stuck slightly on my throttle bodys until i got the cable routed correctly, its now ok but if I had of thought about it before hand I would have kept the mounting for the return cable and attached the cable to a tension spring attached to the manifold or similiar.
Paul.

Sounds like I do need something - I'll have to do a bit of head-scratching now!

What sort of spring have people used? I was thinking of something around 5 cms long, 1cm diameter, about 1mm wire thickness. Length roughly depends on how much the throttle cable moves, but the rest is open to experimentation!

Luckily my loud pedal already had one. So maybe an option doing it at the other end.



Cheers

Rich

I'm not sure about putting a spring on the pedal - I think it would be better at the carb end of the cable.

Entirely my opinion you understand, based on no scientific facts whatsoever!

A local engineering factors had a box full of assorted springs for £15. Should last me a lifetime but proved to be useful.

... a spring was required.

When I took my foot off the throttle the mechanism didn't exactly snap back to closed, so an extra spring was added in place of the return cable. Much better now!