71' LS5 Q-Jet

[William P.]

I have a restored 71' LS5 with a correctly restored Q-Jet with a very interesting problem, please bear with me as the explanation is lengthy.
First the carb works fine with the exception of the secondary air valve. When you floor the carb sometimes the air valve opens and the secondaries work as designed, shift gears and floor it and the air valve does not open. The bypass vacuum does not work therefore not allowing the air valve to open. Drive the car several miles and try it again and all works well again until you lift off the throttle to shift and no secondaries.
Sometimes they never work and then all of a sudden they work as designed.
Any and all help is needed. If you need more detail call me at 303-652-3185

Bill

[Kevin G.]

Re: 71' LS5 Q-Jet

Recently I experienced a similar problem with my 72. It was the throttle cable bracket that mounts on the rear carb bolt. A small amout of movement by the bracket was the cause. Hope your fix is as simple.

Save The Wave,

Kevin

[Clem Z.]

Re: 71' LS5 Q-Jet

i have found on GM cars including corvettes with a cable throttle you need to take the extra slack out of the cable at the throttle arm under the dash by using some spilt shot lead fishing sinkers to shorten the cable.

[William P.]

Re: 71' LS5 Q-Jet

Let me be more clear about what appears to be the problem. First the primary and secondary throttle plates are opening fully. Second the air valve which is controlled by the vacuum bypass is opening only some times as the vacuum bypass is not getting, what appears to me, a vacuum signal consistantly. Third the air valve is where the secondaries get their air and fuel as the air valve pulls up the secondary metering rods.
I hope this additional information helps.

Bill

[Bill C.]

Re: 71' LS5 Q-Jet

I would guess that something is intermittently clogging the vacuum passage way, either inside the carb or between it and the gasket/intake.

The vacuum source for the choke pull-off is not ported (fulltime vacuum), this is why at WOT the vacuum signal drops to no-existent, allowing the butterfly plates to swing open.

Is there any chance the choke plate heating coil is not working (spring) and when you are flooring the car, the choke plate is closing?

Just a wild guess with this -

Best of luck troubleshooting this.

[Clem Z.]

Re: 71' LS5 Q-Jet

Let me be more clear about what appears to be the problem. First the primary and secondary throttle plates are opening fully. Second the air valve which is controlled by the vacuum bypass is opening only some times as the vacuum bypass is not getting, what appears to me, a vacuum signal consistantly. Third the air valve is where the secondaries get their air and fuel as the air valve pulls up the secondary metering rods.
I hope this additional information helps.

Bill

air rushing thru the mechanical secondaries is what causes the top air valve to open and if it does not open every time at WOT the vacuum break is not allowing the vacuum to bleed down or something is binding up the air valve. with the engine shut off can the air valve on top be moved to open freely,if not that is your problem. i have seen the carb top distorted from excess screw tightening causing the air valve to bind shut. remember there is a secondary throttle lock out contolled by the choke linkage that prevents the secondary mechanical butterflies from opening and if they do not open the air valve will not open when the engine is running.

[Kevin G.]

Re: 71' LS5 Q-Jet

A question, could the rod between the choke and the secondaries be getting caught up? I've seen that a small tweak or bend can resolve this problem.