[A1Judge]

Well, now that I think I understand most about a Q-Jet, I was just wondering why the primary throttle plates have a small hole in them.

This would affect idle mixture, and maybe part throttle mixtures. Just wondering why some have the holes and some dont??

Thanks

[A1Judge]

Well, now that I think I understand most about a Q-Jet, I was just wondering why the primary throttle plates have a small hole in them.

This would affect idle mixture, and maybe part throttle mixtures. Just wondering why some have the holes and some dont??

Thanks

[Calibrator]

The drilled hole is to allow you to close the throttle plate back to a stock setting or near stock setting and still provide enough air to give you the idle speed you need. If you have a large displacement engine or a cam with long duration that requires large amounts of idle air you might need to open the throttle plates at idle so much that the transfer idle port is exposed more than it should be. Problem is, when you start to drive the car since the transfer idle is being used to maintain the idle speed, it isn't there for low speed driving. You'll get a bog. Drill the plates, set the throttle back closer to closed and the proper throttle plate/transfer idle port relationship is restored.

[WARPed]

Calibrator, I don't think I could have explaned this as well as you did [img]smile.gif[/img]

[A1Judge]

That was a grat explination. Thanks

[Tom Vaught]

Quote:
"The drilled hole is to allow you to close the throttle plate back to a stock setting or near stock setting and still provide enough air to give you the idle speed you need. If you have a large displacement engine or a cam with long duration that requires large amounts of idle air you might need to open the throttle plates at idle so much that the transfer idle port is exposed more than it should be. Problem is, when you start to drive the car since the transfer idle is being used to maintain the idle speed, it isn't there for low speed driving. You'll get a bog. Drill the plates, set the throttle back closer to closed and the proper throttle plate/transfer idle port relationship is restored."

I agree too. A very good explaination.
I would comment that I don't have my Rochester
4-barrel book in front of me but on a Holley 4 barrel the idle discharge port circuit is a
different circuit than the transfer discharge slot circuit. Exposing the transfer slot activates
that circuit and reduces control on the normal idle circuit mixture screws, therefore turning
the mixture screws at idle has little effect. The
transition circuit (transfer slot) is used a great deal of the time in around town/ off idle driving prior to main circuit activation. Not
having the right sized idle air hole (or lack of
hole) is one of the main tuning problems for most
people. Tom V.