Fast idle is slow to come on

[theastronaut]

Originally Posted by geezer#99

The fast idle cam is engaged as soon as you tap the throttle.

This. When cold, you press the pedal to the floor once before starting to trip the linkage to engage the cold start/fast idle throttle stop and close the choke flaps. Then you start it.


If the choke flap gap and fast idle speed are set correctly it should idle just a little faster than normal, and the flap being closed just the right amount will add the extra vacuum in the carb throat to enrich the mixture. Too far closed and it'll die from being overly rich.

Depending on what carb it is, some fully close the choke flap initially (engine off) then a vacuum dashpot pulls the flap open slightly once it's running (using manifold vacuum) so that it gets a lot of instant/initial vacuum/enrichment while cranking over to start faster, then the gap opens up as soon as it starts to keep it from running too rich at higher engine speed than cranking. Then the choke element warms up to slowly lean out the mixture as it warms up.

The idle mixture screws should be set once its fully warmed up with the choke fully opened. Then, you let it cool off and adjust the choke tension/opening gap to suit how much richer it wants to be at cold start- this will take some trial and error as it has to be fully cold with a fully cold choke element whenever you're doing the adjustment.