General Category > Quadrajet Carb Talk and Tips
Idle Bypass Air ??
beertracker:
A few questions:
What is the purpose of adding idle bypass air?
Is it to get a better idle?
What happens if I add too much idle bypass air and how will I know when I have added too much?
Does idle bypass air affect part throttle or WOT operation?
Does adding idle bypass air lower the vacuum at idle?
thanks, bt :)
omaha:
The only thing that idle bypass air is used for is to prevent the primary throttle blades from having to be adjusted too far open (during idle). If you have to open the blades too far to get the engine to idle, then it affects the transition slot and also it lets too much of the signal get to the main circuit and that causes the dreaded nozzle drip (fuel being pulled form the main circuit at idle through the boosters). It does not lower the vacuum or affect part throttle or wot. The reasoning for it is that the Q-jets have a very small primary bores and if you have a large engine or one that "likes" a lot of air at idle, there is not enough adjustment with the idle screw without going too far. With bypass air you still have to adjust the prim. throttle plates but you have that extra air to help you out. To find the right size, just start drilling. When you can keep the blades open at a relatively small opening and have the correct idle AND no nozzle drip, you are getting pretty close. Basically you want the transition slot exposed at about .035-.040 ~. There are a lot of variables to consider especially with a big cammed engine.
beertracker:
good explanation, thanks, bt :)
beertracker:
I added .086" by pass air holes and the nozzle drip stopped but the idle too fast problem is back. Idle speed is 1350 rpm and idle mix or speed screws have no effect. Secondary throttle blades are fully closed. Primary throttle blades are fully closed against the primary barrel. Engine has 13-14" of vacuum at 1350 rpm. I finished installing a rebuild kit, jets, rods and primary bushings all from Cliff. I did not bushing the secondary shaft. I also upgraded my 17057274 to recipe 3 per Cliff.
Wondering if I went too big with .086" by pass air holes and should I start decreasing the hole size? If I do decrease the hole size should I use epoxy and re-drill to a smaller diameter or is there a better way to do this? Or should I continue to treat this as a vacuum leak problem and look for air leaks? I have been working on this problem on and off for months and am running out of ideas.
Setup From Previous Postings:
Pontiac 400 cid, 8.6:1 CR, slight head porting, headers, dual exhaust, auto trans, 17057274 q-jet, 2200 rpm stall, Summit 2802 cam: Cam Style: Hydraulic flat tappet Duration at 050 inch Lift: 224 int./234 exh. Valve Lift with Factory Rocker Arm Ratio: 0.466 int./0.488 exh. Lobe Separation (degrees): 114. Factory HEI dist. Ignition advance kit. I know the cam is too much for the engine.
Distributor is advanced 14 degrees BTDC and balancer hasn't slipped. Distributor vacuum advance is disconnected and plugged at carb source.
New PCV & grommit. No change in operation if I remove PCV and cap off carb vacuum source. I capped off the vacuum sources going to PB booster and AT. Choke has been disabled so it's fully open.
kenn:
Hello, I am sorry I can't help with the diagnosing, (I'v been working for months on my troubles also) but I have had to make smaller holes.
I have used aluminum welding rod. The size I have is .125 (1/8")
Drill out the hole and insert a small piece of the rod than drill the correct size hole into the piece of rod.
Make sure the hole for the rod is small enough that you have to tap the rod into it but not to small that you have to hammer it in.
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