Did I go too far?

[cal1320]

Info: 350 cu in GM 10067353 crate engine (stock), 14 degrees initial timing, vacuum can on manifold source.  17084502 Q jet, 76 jets, 48 pri rod.  Problem was 6-7 turns on idle screws to get idle rich enough. Idle quality was very touchy at that many turns out. Would change from 12.5 AFR to 16 and into lean misfire randomly.  What I did: Idle tubes were cleaned out (not removed) with .023 wire and cleaner (could not determine actual size), Idle down channel at .046 (stock), Upper air bleed at .070 (was .068), Lower idle air bleed at .070(was .058), Idle bypass not drilled. Mixture screw holes at .090(was .060). No holes in throttle plates. Problem now is a 900 rpm idle with the idle adj screw not touching the lever. Will lower to 800 by unplugging vac advance can. I would run it on the ported side, but it has vacuum at idle too. Plugging in there gets me 900 rpm too. Throttle blades are completely closed. Did I go too far with the lower air bleed? It seems to be getting too much air. No external vac leaks that I can find. this engine idled at 750rpm before I screwed with it. 

[Frank400]

Are you positive that the throttle blades are completely closed ?  I'm asking because if you're getting vacuum at the ported vacuum port, that is a clear indicator that the throttle plates are NOT closed.  They could be held open by the fast idle/choke linkage on the passenger side of the carburetor. 

   If it won't idle below 900 rpm, it's got to be getting air from somewhere, no way around this.  What you did by slightly enlarging the idle air bleeds is leaning the idle circuit.  I would not have done it in that case.

    I suspect your AFR variation comes from the fact that your throttle blades are opened too far, causing some nozzle drip.  Nozzle drip is usually caused by too much air passing in the venturi (throttle blades opened too far) and too lean idle circuit (leaned by enlarging the upper and lower air bleeds in your case).

   Hope that helps.

   

[cal1320]

Just pulled the carb. The "ported" vac fitting is indeed manifold vacuum. It gets it signal from two holes below the throttle blades. The lower idle bleed I closed up and redrilled to .058 as it was stock. The idle adjustment screw is not touching the linkage at all. There is no nozzle drip at idle. I can close the mixture screws and the engine still runs, although at a very high AFR by the wideband. The only mod I made in the baseplate was opening up the idle mixture holes to .090 to reduce the # of turns to get a rich enough idle. Didn't touch the blades on the shafts. Fast idle linkage is not holding the linkage.

[cal1320]

Well, I fixed it. Apparently something in the base plate is FUBARed. Maybe that's why it was free.  Replaced the base plate with another one just like it except the mixture screw holes are .075 instead of .060 and the throttle bore is drilled for ported vacuum like it should be. 1 1/2 turns out now for the idle mixture at highest vac reading. Around 13.2 on wideband.

[Cliff Ruggles]

Good new, the fix was most likely ported vacuum to the advance.  With 14 degrees initial timing adding more timing will often cause exactly the issues that you were having........Cliff