Idle air bypass?

[72turbojet]

I installed a rebuild quadrajet on my 402 and am trying to solve a rich idle problem. The carb was built for a stock big block and I have been going through the steps to tune it for my motor which has a mild cam and intake. I've replaced the power piston spring and maxed out the timing. The float level looks good and I have a fuel pressure regulator set at 5 psi yet still a rich idle. It idles highest with the mixture screws turned all the way in. I've done some more research and I'm thinking I need to enlarge the idle air bypass ports so can someone please explain where these holes are located and how I'd go about drilling them out. Any other possible ideas are welcome since I'm new to these carbs and really want to get this thing to work for me.
 Thanks

[Cliff Ruggles]

What is the carburetor number.

What are the engine specifications?

What has been done to the carburetor (how is it set-up).....Cliff

[72turbojet]

The carb number is 17085292.

The engine I'm almost certain has stock 8.5:1 compression heads, headers, an edlebrock aluminum intake manifold, and an edlebrock cam 300/306, 240/246 @ .05, .560/.573, 112 LSA.

The carb was supposed to be built to stock specifications. The only thing I changed so far was the power piston spring because with my cam it was extremely rich with the stock spring.

[bry593]

I may be off base here, but if the idle mix screws are in all the way, I believe that closes off the idle circuit.  If it is still running, you must be pulling fuel from the primary nozzles.  That would tend to indicate the throttle blades are open too much causing drip.  What happens when you back the idle screw out so it is not touching?  If the engine won't run, you probably do need to add idle bypass so the engine will get some air, without pulling so much air thru the venturis causing idle drip.  That's what I just went through with my 413 sbc after I plugged off the idle bypass in a tuning experiment.

[72turbojet]

The engine wont run if I back off the idle screw. So it sounds like it needs more air

[Cliff Ruggles]

That is a HUGE cam for a low compression 402.  The idle system would need significant modifications to have enough idle fuel, so most likely you are up on the main system and why there is no control with the mixture screws. 

Bypass air alone will not help that situation.....Cliff

[72turbojet]

It really is too big of a cam. It was rebuilt by a previous owner and it's gutless down low with that setup. New heads are on the to-do list for sure.

But in the mean time would drilling holes in the primary blades help the situation? I'm starting to think I need to run a different carb until I get that compression ratio up where it should be

[Cliff Ruggles]

I would NOT drill the primary throttle plates, the q-jet has an idle bypass air system which is more precise, and effective.

The engine needs more fuel to the mixtures screws, which means larger idle tubes, and idle down channel restrictions.  Once you get enough fuel to the mixtures screws for the low vacuum ready and rough idle cam being used, idle bypass air will help to get the throttle plates low enough to keep the main system off line (nozzle drip).

The best course of action here would be to replace the camshaft vs upping the compression ratio...IMHO.

I'd look at something around 214/224 on a 112 LSA, or even out on a 114.  Lots of the factory big block cams were really wide LSA, and they needed to be as they had pretty long seat timing.  Those cams made great power in big block engines........Cliff