classicgm

Hi folks. I been lurking here for a while but never had much to say till today....

First I'm trying to master the qjet on my 350 and have made some pretty good combinations with older 70-74 vintage carb cores. Everything except the idle system is accepable to me. I have done what damon suggests drilling the idle discharge port in the venturi and while that helped I thinks there is more to be had.

So I getting brave and have some extra long drill bits to drill out the idle tubes in search of idle system nirvana. I've read all I can find on the web and in the archives and the accepted rule is the idle tube restrictions can be drilled from .035-.049 depending on the tune of the engine. The questions are what size drill for what cam? Do you choose the drill by the duration of the cam? Does anybody have a chart or can help in anyway?

I'm ready to drill but dont want to take a chance on ruining an otherwise good carb.

TIA!!

Damon

TIA forget the super long drill bits. You don't need to drill out the tip of that pickup tube way way way down there.

You can accomplish the same thing by drilling out the much more accessible Idle Channel Restriction. It sits right next to the idle feed tube and the restriction itself is up and the TOP of the main carb body so you can drill it out with standard length bits.

I just went through all this in great detail with another buddy on another board. I'll see if I can find the thread and post the link here........ so I don't have to retype. It's a good thread and covers everything.

Damon

Found it:

http://www.nastyz28.com/ubb/Forum5/HTML/008172.html

classicgm

Thanks damon. I read about this mod on a site somewhere out there (credited to you) but unless I'm hitting the wrong holes or have weird carbs the restriction is still down in there. Come to think of it, the junk bodies here that I am playing with have both been epoxied so maybe they have already been drilled and that is my confusion...

Back to the original question. Is there a drill size guideline if you want to bother with it? I know you say there is no need. I just want to collect as much info as possible... Roe does not go into it that deep in his book. Makes me wonder how Jet can build your carb to your engine specs...

The carb on my 350 is as far as I know a virgin so time to pull it and break it down and at least measure the restrictions

classicgm

damon, Ive been looking at the cores I have over here and what you say will work but only if your casting does NOT have and idle air bleed in the choke housing. If it does then the effect would be to draw more air into the idle passages and not more fuel. Ibn this case you would have to drill the idle restriction (the one 1/34 inch down).

Maybe I am all screwed up.I'll try to take some pics tonight to show what i'm talking about and see if you agree.

Damon

Sorry for the long reply time.

It doesn't stick straight up, usually, even though there's a hole cast into the airhorn that looks like it should have an idle air bleed bushing at the top of it. It's usually drilled at an odd angle, pointing slightly down into the venturi. Might be drilled in the airhorn or it might be a small hole drilled at an angle just above the idle channel restriction in the main body.

OR IT MIGHT NOT HAVE ONE. I've run into this a few times. Some of them are just like that. Beleive it or not, drilling out the ICR STILL WORKS. I have no real idea why it still works but it does.

As far as the size of the various origninal holes/restrictions they can be all over the place. The ICR is usually in the range of .054". The idle feed orifice is usually around .035" and the idle air bleed (if equipped) can be all over the map. There are often additional intermediate air bleeds further down in the venturi but still above the idle transition slots in the lower TB portion of the carb.

The ICR should definitely only be down in the main body of the carb about 1/4" below the gasket surface. Never seen one much deeper than that. If things have been epoxied up and redrilled it's certainly been messed with by a previous owner.

Working on these things for 20 years I would agree with Forrest Gump: Qjets are like a box of chocolates........