By Pass Air Needed?

[gtobob67]

I have a fresh .30 over Pontiac 455 with a hydraulic roller stump puller cam, 230/236 @ .050. I had an NOS draw through turbo quadrajet #17080243 that has been modified to supply vacuum to the power piston. Bought a kit from Cliff and after speaking with him  have the idle circuit set up as follows;
Idle tubes     .037
Idle down channel   .055
Lower idle air bleed  .070
Upper idle air bleed  .059
By pass air at   .102
Mixture screw holes at  .089
 
Problem is I can't get it to idle without nozzle drip and about 6" vacuum on the ported source. If I open a manifold source vacuum fitting RPM increases. I can then back off the throttle screw to get the RPM down and the nozzle drip goes away and the slight vacuum at the ported source goes away also. I can then stall the engine by leaning the mixture screws. This all leads me to believe I have to increase the by pass air. I thought it was large enough but thinking of opening them up to .120. Am I on the right track here??

[Frank400]

You are absolutely on the right track. 

  Hopefully some experts will chime in, but your situation does suggest increasing idle air bypass.

[gtobob67]

Been away from this carb for a while but I just opened up the by-pass air to .127 and the mixture screw holes to .097. Probably should have left the mixture screws alone. Anyway, fired it up and get it to a hot idle at 850RPM and it's drawing fuel from the transition slots. No nozzle drip but I can fully seat the mixture screws without stalling the engine and it still has some vacuum at the ported source. I can open a large vacuum fitting that will take a .184 drill bit and I can adjust the idle down and have control of the mixture screws. Fully seated the engine stalls. Idles nice, 16" of vacuum and the exhaust has just the slightest odor. Slowly block of the the open vacuum fitting and the idle decreases. I can't believe I have to add more bypass air but with the little knowledge I have I'm leaning that way. Suggestions please......

[77cruiser]

Go for I've ran as big as .140. How much timing doe you have at idle? Can you add more timing?

[gtobob67]

Timing is 15 BTDC, max 35 BTDC. Vacuum advance set at 10 degrees all in by 3000 RPM. Using ported vacuum. With the previous 7042240 divorced choke carb with similar carb settings (Recipe 2 Cliff's book) it idles fine and runs like a scalded dog. Bypass air at .106. Heads are ported and the heated crossover filled with aluminum. No choke with the filled crossover. I swapped to this carb mainly for the electric choke and APT. Bonus was it was NOS when I picked it up. If I switch to manifold vacuum for the distributor I'm sure I can get the throttle plates closed more but can't figure the difference between these 2 carbs at idle when set up so close together. I'm tryin to stay with ported vacuum to the distributor because I had some slight surging or bucking at low speed cruise and coast. 4 spd car. After talking with Cliff he suggested switching to ported vacuum to remove the advance. Sure enough it worked. So trying real hard here to use the ported source.

[Cliff Ruggles]

If you drill the holes under the mixture screws way out on the later carbs with the metric threads they may not seat as the threads don't go deep enough. 

I've had to tap some of them to 10-32 and use the early type mixture screws with more reach to solve that issue.

For bypass air just remove a small manifold vacuum hose and reset the idle speed and mixture screws.  If it works then add some more bypass air until you duplicate that scenario.....Cliff

[gtobob67]

Yesterday I drilled the idle bypass holes to .144. Got the engine up to normal running temp and adjusted the idle to 850RPM. If I seat either mixture screw the idle drops 75RPM. Can't quite stall the engine with both seated but it's close to stalling, running rough. So I do have some control. I only need to turn both screws out 2 turns for the highest idle and best vacuum. So it's still pulling fuel from the transition slots. Minimal odor from the exhaust.  Engine idles at 15" of vacuum now and I have 4"of vacuum at the ported source. The advance unit I have starts at 5" of vacuum so I don't see any advance at all at idle. Took it out for a ride and it is perfect. Idles well, off idle transition is spot on. No surging or bucking at coast or slow crawl. No hesitation at all. Jump on it in 1st gear from a slow roll and it will tear your head off. Time now to put the tools away and go have some fun....

[Cliff Ruggles]

Good news!