Idle speed vs. throttle blade angle

[Bailey28]

Question on throttle blade angle at idle.   I rebuilt and set the primary throttle blades so the transition slots were tiny squares.  Probably .020" of slot was showing.   

On my stock 8:1 350 with stock cam, dual plane intake and initial timing set at 8* BTDC, the engine idles smooth at 475 rpm.   Unless I add more timing by going to manifold vacuum or opening the throttle blades by cranking in the idle stop screw I cannot get the idle to come up.  I'd like to be around 650-700 in park. 

If I screw in the idle stop I can get to 550 right at the point the left side blade uncovers the ported vacuum slot.   Strangely, there is no nozzle drip with the blades this far open.  With the blades set here is it running on the transition slots?

Is there a spec or range as to how far the throttle blade is allowed to be open at idle?  17059216

[Cliff Ruggles]

Do you have full control with the idle mixture screws?

You may need to add some idle bypass air to get the throttle blade angle lower and idle speed where it needs to be. 

I'd pull a small manifold vacuum hose and re-adjust the idle speed and mixture screws and see if this helps.  If so add some bypass air to simulate the same scenario.......Cliff

[Bailey28]

When I first set up the carb I noticed it did have bypass air which was open at .070" The choke has been converted to electric however I had failed to plug the choke vacuum passage. 

The car ran like poop at idle.  It ran hot, had continuous misfires, and the mixture screws were at 6 out.  I could only get a shaky 16" of vacuum at idle which was only around 550-600. 

It was running like it had a vacuum leak(s).  So I took it apart and plugged the bypass air in the baseplate, and plugged the choke passage.  Nothing else changed.  The vacuum gauge is now dead steady at 18" at idle. 

I'll try adding in bypass air, or opening just the choke passage back up.  Is it possible to add bypass air in small amounts, say like inserting restrictions in the bypass channels in the baseplate then drilling them to the size i want to try? I read in your book something to the effect of either the bypass air or choke passage would work ok.

The APT is one turn up from bottomed out, 72 mains 40K rods.   The car drives well, and fuel mileage is already much better than the Demon 625 I had on it.   

[Cliff Ruggles]

I would not open up the vacuum supply to the choke housing, that air is not filtered once Electric choke conversion was done.

Add a little idle bypass air to simulate what you lost from blocking off the hot air choke vacuum supply.  This will lower the throttle angle some so you can turn the idle speed screw in a bit and stay off the ported vacuum port and less transfer slots exposed.

If it acts lean at this point, or you can't get acceptable idle quality without backing the mixture screws out more than about 4.5 to 6 turns I would add some idle fuel with larger idle tubes and/or DCR's.

72 jets and 40K rods are extremely lean even for a "stock" engine on this new fuel, but if it's doing OK I'd leave that alone for now.

Not sure if you used one of our rebuild kits or not but the size of the fuel inlet seat and float height will effect the level of fuel in the bowl and the calibration as well.......Cliff