Vacuum or Mechanical secondaries
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Scottyp, and/or Lars, I have a 70 350/300 with the Qjet, I do not have any issues with the carb but, I question if the secondaries open. Should I notice a huge kick at say 70MPH when I stomp on it, or hear a big sucking noise or surge in speed. What is the simplest way to test if the secondaries open at WOT?
What I like to do, is wire the choke lock-out to disable the secondaries, and tune the primaries. Then un-wire the secondaries, and adjust the secondary airvalve opening sooner and sooner, until it bogs, then go back a little bit. Learned it from Lars.
When you really put the spurs too her with a load, you should be hearing what I like to call the "Quadrajet Moan". You'll know it when you hear it.
Keep the shiny side up!
Scott
Last edited by scottyp99; Aug 25, 2012 at 04:26 PM.
Racer
Joined: Nov 2011
Posts: 261
Likes: 1
From: Montgomery TX
Actually, the Q-Jet is a mechanical secondary carb with a secondary airvalve. For a complete description of the Q-Jet secondary system, you can e-mail me for a copy of my "Q-Jet Setup" and the "Engine Vacuum Explained" papers - these papers cover the Q-Jet mechanical system and the vacuum secondary systems (only used on the Holley-based designs) in detail. Also - the original statement by the OP is incorrect: The Edelbrock 650 is not a vacuum secondary carb - it's a mechanical carb with a counterweighted airvalve secondary.
...and you're right - I've shown several times through dyno testing numbers that it's tough to beat the performance of a Q-Jet on a street-driven car:
http://forums.corvetteforum.com/c3-t...-question.html
Lars
V8FastCars@msn.com
...and you're right - I've shown several times through dyno testing numbers that it's tough to beat the performance of a Q-Jet on a street-driven car:
http://forums.corvetteforum.com/c3-t...-question.html
Lars
V8FastCars@msn.com
Let's all thank Lars for bringing some sanity to this thread!
THANKS LARS!
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I've have had two high HP motors in my Vette a 550HP 406ci and 600HP 427ci and contrary to the boost button in the movies, when I floor it the secondaries open up but you hardly feel anything, it's not like wow sit me back in my seat and flames come out of the exhaust pipes as the secondaries open, I don't really even feel it however with the mechanical secondary carbs I know they are opening and giving the engine the fuel it needs 
Le Mans Master
Joined: Dec 2003
Posts: 5,727
Likes: 38
From: Fremont CA
AFB carb had counter weighted secondary air valve but, if it is a AVS (Air Valve Secondary) the secondary air valve should be spring dampened and that spring should be adjustable.
Here is a link explaining the difference between the AFB and the AVS and how to adjust the secondary air door. The AVS is a MUCH better carb than an AFB and you should try to make it work.
Link: http://www.moparmusclemagazine.com/t...s/viewall.html
Here is a link explaining the difference between the AFB and the AVS and how to adjust the secondary air door. The AVS is a MUCH better carb than an AFB and you should try to make it work.
Link: http://www.moparmusclemagazine.com/t...s/viewall.html
Pro
Joined: Apr 2008
Posts: 596
Likes: 6
From: Irving Texas
Yes, I have run a Holley DP and a Holley Vac Sec.......and still drag race with my Holley Vac Sec.
That said, I found out what the seat of the pants feel comes from.........and it's not the carb secondaries. It turned out to be a properly set-up automatic transmission downshift linkage.
At a certain part-throttle rpm, if I stomp on the gas, the linkage will downshift to the next lower gear (I run a TH350) and WHOA....seat of the pants Zoomy Zoom Zoom as the rpms climb and the car lurches forward.
That said, I found out what the seat of the pants feel comes from.........and it's not the carb secondaries. It turned out to be a properly set-up automatic transmission downshift linkage.
At a certain part-throttle rpm, if I stomp on the gas, the linkage will downshift to the next lower gear (I run a TH350) and WHOA....seat of the pants Zoomy Zoom Zoom as the rpms climb and the car lurches forward.
