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Old 09-16-2010, 06:14 PM
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how much vacuum for secondary's

I have a stage 2 q-jet from jet performance, i recently put new heads and a cam in. The cam is kinda big, 107 lobe separation adv dur 287/297 about (thumper comp cam). edelbrock performer rpm heads. Its all on a chevy 350 w/ a performer intake. Im looking to get a different carb seems to be running lean and im not sure if the vacuum secondary's are opening. The engine pulls about 10-12 manifold vacuum. how much is required before your forced to have mechanical secondary's? im looking into a holley or a demon carb. any recommendations or ideas?

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Old 09-16-2010, 06:45 PM
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Quote:
Originally Posted by PiperDavid14
I have a stage 2 q-jet from jet performance, i recently put new heads and a cam in. The cam is kinda big, 107 lobe separation adv dur 287/297 about (thumper comp cam). edelbrock performer rpm heads. Its all on a chevy 350 w/ a performer intake. Im looking to get a different carb seems to be running lean and im not sure if the vacuum secondary's are opening. The engine pulls about 10-12 manifold vacuum. how much is required before your forced to have mechanical secondary's? im looking into a holley or a demon carb. any recommendations or ideas?
You have a common misconception about vacuum secondaries. Vacuum secondary carbs don't run the secondaries off engine vacuum, the secondaries are opened by venturi vacuum, which is created by the speeding airflow through the carb barrels.

You don't EVER want to "force" the secondaries open on a vacuum secondary carb, all this does is create a bad bog. Thats the kind of thing "hacks" do, and think it is right.

A Q-jet is going to need a lot of playing with the power piston springs and metering rods to work well with one of those Thumpr cams, along with secondary air valve adjustments.

Holley or Demon vacuum secondaries are adjustable by changing the spring in the secondary vacuum pod, they sell a spring kit of different tension springs for these to tailor the opening point and rate.

I'd stay with the Q-jet and tune it, it will get better mileage and still make good power. Since it is a Jet carb, call them and get a starting point for jets, power piston spring, rod combos to match your engine. They should know, it is what they specialize in.
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Old 09-16-2010, 07:00 PM
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You can check for secondary opening by putting a paperclip on the secondary arm (rear passenger side), drive the car, as the secondaries open the arm moves and the paperclip moves. Search YouTube for the holley video showing how it's done. Secondaries won't open in park.
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Old 09-17-2010, 03:57 PM
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You can tell if the secondary open. It create a loud roaring sound. Q-jets are known for that. There is a spring tension adjustment on the side of secondary to control the opening rate.
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