tri-power carbs

[Blake W.]

Just put kits in my 67 tri-power holleys. Did not replace secondary dashpot diaphrams; they were good and held vacuum. When I started the car and made final adjustments I noticed that as the rpm's came up I never noticed the secondaries opening. Even blipping the throttle momentarily. Is this something that would occur with a load on the motor, ie driving? everything was hooked back as it came apart. I'm a little stumped. thanks Blake

[Mark #28455]

Re: tri-power carbs

secondary blades open by ported vacuum, so you'll only just see them crack open if you really "floor" it at idle.(I wouldn't do that!) They're meant to open when the air flow through the primary carb is high. You can easily tell if they're opening by the sound alone when you hit the gas in second gear.

[John H.]

Re: tri-power carbs

Blake -

The opening diaphragms are fed by venturi vacuum from the center carb, created by high-volume airflow past a tiny pickup tube in the venturi. You can't generate enough airflow by "revving it" in the garage to create a strong enough vacuum signal to open the end carbs - the engine has to be under load at WOT to generate the opening signal (driving it) - that's where the fun is anyway

[Blake W.]

Re: tri-power carbs

Thanks very much both Mark and John. Kinda figured that it had to be something simple like that. One last question if I may; Seems I read recently about adjusting the idle mixture screws on the center carb metering block with a vacuum guage attached. Is there another way to set them correctly or if I need to do it with the guage just which vaccuum line to I connect into? I've got the "tee'd" line for the primary and secondary carbs and there is also a short vacuum line on center carb to a small dashpot for the choke mechanism. If you can, be specific as I don't understand ported vrs non ported. thanks again for the help guys. Blake

[John H.]

Re: tri-power carbs

Blake -

You can either tee into the choke pull-off diaphragm hose, or use the fitting in the side of the intake below the center carb that has the PCV hose on it that connects to the PCV valve in the valve cover - both see full manifold vacuum.

[Blake W.]

Re: tri-power carbs

Thanks John, And then I just adjust both mixture screws until I get the highest vacuum reading?

[John H.]

Re: tri-power carbs

Yup, adjust back and forth between the two idle mixture screws for highest steady vacuum reading, and touch up the idle speed screw as you go.

[Blake W.]

Re: tri-power carbs Thanks John NM *NM*