PotteryPal

Fellow members,

I noticed that when I am cruising around and I go full throttle, 90% of the time my C4 won't downshift. Because it doesnt downshift I have started manually downshifting one gear.

I know she is running a bit rich (based on the plugs) and I plan to check the accelartor pump adjustment and vacuum secondary springs but is this normal operating procedure for the C4 not to consistently downshift?

Its a 289cc, 4bb Holley 4160. It running ~13-14"mg on a vacuum gauge.

Gun Jam

I believe that system uses engine vac to operate a modulator on the trans to help control shift timing. Make sure that vac line is connected and not broken. If that does not help it could be a modulator issue but I dont know how to diagnose that.

Starfury

I think you need to hook the modulator up to one of those MightyVac hand vacuum bleeder tools with a gauge on it and drive around while varying the vacuum.

But if you can verify the integrity of the line going to the modulator, I'd just replace the modulator and be done with it. They're not all that expensive.

Gun Jam

did the c4s use any type of mechanical kickdown linkage from the carb or was it all vac?

imp

Vacuum modulated to control shift point speeds, kickdown via linkage which actuated at almost wide open throttle, was the most prevalent system. Look around the carb. for linkage other than gas pedal.


Year of car would help, but guess it's < 1968. imp

PotteryPal

good info. I'll do some more research.

PotteryPal

It is a 1967 Coupe with a 4bbl holley.