Author Topic: Drill Throttle Plates?  (Read 5051 times)

Offline von

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Drill Throttle Plates?
« on: January 18, 2010, 05:38:58 AM »
  I've set up my Q jet ( '73 Chevy 350, 7043202) exactly per the 2nd recipe specs. I even plugged the large .120 main air bleeds and redrilled to .070. The carb is on a Chevy 396 with mild (221/233 @.050) perf cam and stock intake, 12.5 to 13" of vacuum at idle in neutral. The dist is recurved and has 14 deg initial advance with additional 12 deg vacuum advance on full manifold vacuum. The carb specs are as follows: both sets of main air bleeds .070, mixture screw holes .098, idle bypass air .109, idle tubes .038, idle channels .055, upper idle air bleeds .070, lower idle air bleeds .070, primary jets 73, primary rods 44B. Primary rods stay down at idle. I've verified idle bypass air is open and working.
  Problem is no control with idle mixture screws. No appreciable difference in idle speed or vacuum from seated to 4 turns out. The pass side one will slow the idle down a hair and 1/2" of vacuum if seated. If I pull a small manifold vacuum line off the idle the idle speed increases a lot. The only thing left I can think of is to drill holes in the primary throttle plates. Anything else I should try first?

Offline 77cruiser

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Re: Drill Throttle Plates?
« Reply #1 on: January 18, 2010, 08:34:28 AM »
Before I drilled the plates I'd go bigger on the idle bypass. Mine are .14 I think.
Check your gaskets, especially the top one, sometimes it partially blocks the idle air.
Jim

Offline Cliff Ruggles

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Re: Drill Throttle Plates?
« Reply #2 on: January 29, 2010, 03:18:06 AM »
Verify that the mixture screws are fully seating and shutting off all the idle fuel.  I suspect with the large holes under them, they may not be....Cliff

Offline von

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Re: Drill Throttle Plates?
« Reply #3 on: January 29, 2010, 03:53:37 AM »
Hmm. Didn't think to check that. I've cut the springs so they're not binding and causing the screws not to seat. I did disassemble the carb again and verifed the idle air bypass is clear. Also drilled the idle air bypass openings in the base plate to .135. The main reason I did that was because I could pull a vacuum hose off and idle speed would increase dramatically. I haven't started it yet, waiting on a little warmer weather (Indiana).

Offline von

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Re: Drill Throttle Plates?
« Reply #4 on: February 07, 2010, 12:37:28 PM »
It's a '73 carb and the mixture screw holes are drilled to .098. Would '65-'71 mixture screws help?

Offline Cliff Ruggles

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Re: Drill Throttle Plates?
« Reply #5 on: February 09, 2010, 06:16:32 PM »
They should work as  they have larger tips on them with less taper....Cliff