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Author Topic:   qjet air valve
posted May 14, 2003 09:05 AM
I need some advice on this issue. Running a stock rebuilt qjet(minus choke)on a 350 sbo. When I get into the secondaries I get a backfire through the carb. I am thinking the air valve spring needs adjusting but which way, tighter or looser? Or am I way off and its something else?




posted May 14, 2003 09:25 AM
fwiw, "stock rebuilt" Qjets are far from race ready carbs. They are done for smooth idling around town and for stock street car driveabilitiy and not at all set up for race motors, no matter how mild the engine is that it's bolted on.
Your backfiring could be due to a sudden dumping of too much fuel, or that the secondary air flap spring tension is too tight (but that would feel much more like a huge hesitation than a backfire).
It also could be non-carb related, like a valve problem (burnt valves sometimes will do that).
The best advice I can give you is to set your fuel pressure to 6lbs at idle, adjust the secondary air flap spring tension to be near zero when the flaps are shut, then adjust it according to the vacuum demands of the motor when the car is running. Slam the throttle open and watch what the secondary flaps do. Are they opening at all? Are they flying right open and then bouncing back?
Also, make sure you are getting a shot of fuel from the accelerator pump circuit the instant the throttle begins to open and also that the carb isn't leaking fuel from the main wells while it's sitting (sometimes happens with rebuilds).

Hope that helps, but there's so much more to making it oval track ready, it would take a book to cover it all.


posted May 15, 2003 10:11 PM
I always set mine acording to how it ran. If they are to loose they will open to fast and the engine will hesitate for a second. I ran mine just tight enough that it wouldnt stumble. start loose and go tighter from there. If it hesitates thighten them up 1/4 turn at a time. just make sure that you dont over tighten the spring and bottom it out. some secondary flap springs are not strong enough and bottom out on the litle shaft they slide on. when that happens the flap wont open all the way. and if you force it it with your finger it will break the spring. Theres a way to make them stiffer but Im not even going to try to explain it. I have some in the garage, ill try to get pics tomorrow.

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