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.

[Cliff Ruggles]

Most likely a vacuum leak and/or the throttle plates aren't fully indexed and seated at idle speed.  .086' idle bypass air isn't enough to supply the engine at 1300rpms with all 4 throttle plates fully closed....Cliff

[beertracker]

Thanks Kenn & Cliff for the replies.   Cliff you already answered my question.  I removed a vacuum hose and the engine sped up.  I will increase the idle bypass air holes from .086 to .090 or as close as I can get with a numbered drill and re-test.  If you think this is too much of a increment let me know.  bt

[beertracker]

With idle bypass air holes at .086" I do not see fuel drips at the primary nozzles.   However I see a coating of fuel on the primary nozzles as the engine idles.   When I shut the engine off and touch the nozzles they are wet with fuel. 

Does this mean I am close to the correct idle bypass air hole size?  Should I add more bypass air?

[omaha]

I am agreeing with Cliff on this one for sure. That engine has to be getting the air from somewhere 'cause .086 holes are not that big at all. Even a lot of the factory carbs I have seen have at least .100 holes and some bigger. And that is with no big cam or anything like that. I don't think that your cam is excessively large (...did you degree the cam to see if it is indeed the one you bought...just a thought). And especially because it is ground on the 114 centerline it would seem that it should idle "fairly" decent. With that said, I would be looking for something major in the way of air getting past the primaries and secondaries. Maybe a warped baseplate or the plug missing for the big vacuum port. I've seen cracked baseplates and incorrect gaskets cause all kinds of problems. It could even be a cracked intake manifold. As far as testing for vacuum leaks, there seems to be several methods, unlit propane torch, cleaning solvent sprayed from a squirt bottle even wd-40 sprayed around suspected areas. My favorite was lighting up a cigar and blowing smoke through the top of the carb. Tape up the top of the carb except stick a long straw through the middle of the primary area. Open the throttle a bit and hold it open, let fuel dry first. Now take a puff and blow the smoke right through the straw. (engine NOT running ). the smoke should appear where you have your vacuum leak. Let me remind anyone trying any of these methods that none of them are 100 percent safe, use your best judgement and take the precautionary measures. BE safe FWIW.

[beertracker]

Well, I am totally confused now.  I thought idle bypass air and idle speed are two separate issues?  My idle speed now is down to 1000rpm & I haven't been adjusting the idle mix screws.  I have been adjusting the idle bypass air holes to make primary drips go away.   As I stated in post 7 the drips are gone but not I see the primary nozzles are wet with fuel.

So I assume I need to add more idle bypass air?

[77cruiser]

What's your fuel pressure & float level?

[omaha]

ok i think you are getting close cause 1000 rpm is a lot better than 1350. Just wondering if your primary plates are still closed all the way. That is where I was getting confused because usually they should be open a bit, exposing part of the transition slot. If they are closed all the way that means all the air is coming through the bypass holes or someplace else, (like a vac leak or incorrectly indexed plates {the plates only fit one way correctly to fit the bore} you should not be able to hardly see any light around the plates if you hold them up to a light with the plates indexed correctly). Not sure about the wet nozzles as it would seem if the plates are closed up that much that there would not be too much of a signal to affect the main circuit. Maybe the plates are not correctly indexed which could affect all of the problems that you seem to be having. It is possible if the plates were not indexed correctly, that might cause a "pull" on the top of the transition slot due to the gap around the plates, causing the nozzles to be wet. But I am just thinking out loud here because I have never seen that happen. The idea is to have the bypass holes and also be able to adjust the idle with the screw for the idle adjustment. the mixture adjustment plays a part here also as this can increase or decrease the idle rpms to a certain degree. Its best to use a vacuum gauge to set the idle mixture. the correct mixture will have the best vacuum reading. I guess keep plugging away at it. I am thinking that you could get it to idle at 850 to 900. It's frustrating I know especially after all this time but you are getting some valuable on the job training.

[beertracker]

I am still working on the q-jet but disaster struck today.   The threads gave way when I was installing a  2.94" x .184" bolt (screw driver slot) in the base plate.  I think the base plate is made of aluminum?  I didn't even tighten it down the threads just gave way.   So how do I repair this threaded hole?    Use the next larger diameter bolt keeping the thread pitch the same?

[beertracker]

Here are answers to earlier questions.

vacuum is 12 to 14" Hg.  it varies with idle speed.
float height is 1/4" at toe per cliff.
fuel pressure fluctuates 7 to 8 psi.  my fuel pump is a oem mechanical.  No pressure regulator.  already had a regulator conversation with cliff.  idle bypass holes are at .113".  Should I go bigger?

Some where during all my testing the primary drips re-appeared.  I doubt if they ever went away because of poor lighting in my garage resulting in I couldn't see.   Primary blades are fully closed and very little light is visible between blades and barrels.  I think the blades are ok.   APT orange spring is working.  I can see the primary rods move under spring and vacuum conditions.

I think I am approaching the end of this project.  One last thing I could try is to swap out the intake manifold.  Maybe that's the source of my air leaks. 

[429bbf]

bt --i don't know squat about a pontiac .ive got a 460 ford set up on the low end of recipe 2    8.5cr 262 iske cam  and with the compression i have i can't burn all the fuel. I'm fixing to change heads and get about 10.2 cr and then ill tune the carb as needed .it seems with your setup if you bumped the cr another point thing would run better. maybe I'm wrong . will some of you pontiac boys chime in and correct me because my buddy is fixing to work on his 455 and could use a little advice as to what setup works best for all around driving (no drag racing )fwiw 1976 grand prix