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Cowl Induction 70 SS396 not working

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Bob Carroll

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Discussion starter · #1 ·
I have a restored 70 SS396 with after market Cowl Induction system that worked fine when I bought the car last April. A few weeks ago, I had my mechanics tune up the car and rebuild the carb (original Rochester Quadrajet) Now, the "flapper" pops up when the car shuts off and closes when the car is started. But on hard acceleration it does not open at all. The vacuum hose is connected to a port at the bottom front of the carb and I have checked the connections and they are they are all tight.

I do realize that the build manual shows the vacuum hose coming off of a fitting at the rear of the intake manifold....the same fitting services the power brakes, auto tranny, etc. But since the system worked fine when I bought the car, I'm wondering what has gone wrong.

As usual, I would appreciate any help/advice the membership can give.

BobC
 
#3 ·
The outer door is vacuum operated. If it's popping up with engine off (loss of vacuum) and closing on start up (max vacuum at idle). I'm going to guess there's a crimp in the hose that is keeping it from realizing the change in vacuum signal. Perhaps the hose got re-routed and pinched in the hood closing.
 
#5 ·
My understanding is the main vacuum line needs to connect directly ( or as close as possible) to manifold vacuum. Make sure it's not connected to some sort of "ported"/restrictive vacuum.
 
#6 ·
Connect to the manifold vacuum - the door should stay open when off and close when started - its only going to open when the vacuum completely drops off - who rebuilt the Qjet ? If your not getting the secondary's open you may not be losing enough vacuum to open the flapper. The elec. switch on the gas pedal ONLY operated the inside door - not the one on the top of the hood.
 
#7 ·
I have the same issue, I was running a hot cam that was only generating 10 pounds of vacuum. When I had a new cam installed which generates 17 pounds the flapper began to function the way yours is. I enjoyed the flapper responding more often, and now I am researching for a more restrictive in line check valve. Maybe someone can help us both.
 
#8 ·
Bob Carroll said:
I have a restored 70 SS396 with after market Cowl Induction system that worked fine when I bought the car last April. A few weeks ago, I had my mechanics tune up the car and rebuild the carb (original Rochester Quadrajet) Now, the "flapper" pops up when the car shuts off and closes when the car is started. But on hard acceleration it does not open at all. The vacuum hose is connected to a port at the bottom front of the carb and I have checked the connections and they are they are all tight.

I do realize that the build manual shows the vacuum hose coming off of a fitting at the rear of the intake manifold....the same fitting services the power brakes, auto tranny, etc. But since the system worked fine when I bought the car, I'm wondering what has gone wrong.

As usual, I would appreciate any help/advice the membership can give.

BobC
Click to expand...
Nothing else should share the vacuum fitting to the brake booster. it should be the only hose/fitting used for the brake booster.
 
#9 ·
LeoP said:
Nothing else should share the vacuum fitting to the brake booster. it should be the only hose/fitting used for the brake booster.
Click to expand...

Leo,

I think he's saying the factory vacuum source is "shared", which is correct, (the screw-in fitting had a different nipple for each option ordered) I don't think the OP ever had a "tap" off of the brake booster's vac line. At least, that's my take on the post. :)
 
#11 ·
If you have the gold/black check valve in place, take it out. I think Alan is on top of it, there's a notch in the passenger side of the hood where the hose and the wire to the inner door are supposed to run.
 
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#12 ·
The vacuum line to the flapper actuator should be connected directly to the manifold tree on the intake.
 
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Discussion starter · #14 ·
Thanks for the postings! Does anyone know who sells the "manifold trees" that the build manual shows to be the correct installation? Also, I cannot find a notch in the (after market) hood where the vacuum line should pass. I do note that the ZL2 vacuum line on my car is slightly compressed where the hood pinches it at the top of the fire wall ridge. I may file down that portion of the ridge where the line passes through and see if that helps. Thanks again! BobC
 
Discussion starter · #16 ·
Steve......thanks for the pic!! I located the notch under the hood and now need the retainer to hold up the vacuum hose at the notch.....could not find them at OPGI or Ecklers. Also called OPGI on the manifold trees...struck out. Even the "Complete Cowl Induction Kit" sold by OPGI does not show them as part of the kit. I guess they assume that the buyer will use the carb fitting. Maybe I should try eBay! Again, I appreciate the help! BobC
 
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