Trouble with rebuild

[Landshark]

I just finished a carb rebuild to go on a Chevy BigBlock. I used 71 jets, 43b primary rods, AX secondary rods, set the float at 1/4 inch, new accelerator pump and a red power piston spring from Cliff. There are all recommended parts/settings for a 427. This is running a divorced choke. Bad news is it idles like crap. Fuel mixture is so rich that I can be in the garage for more that a little while since my eyes are burning so bad. and it smokes really bad, not oil, just unburnt fuel The idle mixture screws have no effect.  I am not an experienced rebuilder as some of you are, but I have been going through Cliff's book to get at least a good working knowledge of a Quadrajet. Any help/advise would be appreciated.

Thanks

[Landshark]

I just swapped out a different seat .135 slotted. I had a solid .134 in. A minor change. Exhaust is still black and leave soot on floor and still now effect from the idle mixture screws.

[carmantx]

What is the carb number?
What are your engine specs?  What cam?
Did you make changes to the idle circuits? 

[Landshark]

The carb is a 7029201 which is a BigBlock carb, just not the number for my car. (709215)
I have a 69 Vette 427/390HP 4SP. I had the engine rebuilt a couple of years ago with the following info

CompCam Roller
.510/.510 lift
270/276 duration
112 lobe separation
They had to bore the block another .010 for a total of .040.
Power Forged Pistons
CompCams Magnum roller tip rockers
Resurfaced the heads, balanced the crank and flywheel, all new valves, guides and seats. New oil pump. 

[Cliff Ruggles]

Did you get the rebuild kit from us, or just a few parts?.....Cliff

[Landshark]

I sent this carb to you a couple of weeks ago to Helicoil the fuel inlet line, great work on that. And we talked and you and I decided that I purchase the the SR kit, 71 jets, 43B rods, gaskets and the red PP spring. I have installed all those parts. I think I just need to back over things and see if an obvious issue is present. I thinking a smaller orifice seat my be worth a try.

[Landshark]

I found this procedure out on the web and gave it a shot...Same results....running very rich, black smoke.

So now I'm not sure which way to go with this...

How can you tell if your power piston spring is too stiff and not allowing the power piston to “seat” at idle?
If your engine does not produce enough manifold vacuum at idle and/or cruise (due to a lumpy cam or other engine parameters), it is possible that the power piston is not being pulled all the way down to its seated position due to the power piston spring being too stiff. The result is that the car will run very rich at idle, and the idle mixture screws will have little effect or response.  Idle speed may also “float,” with idle speed starting high and gradually decreasing until the engine stalls due to the engine getting “loaded up.”  There will typically be a puff of black smoke out the tailpipes when you “flick” the throttle.

To test for this, pop the top off the carb, remove the power piston/rod assembly, and remove the power piston spring from its bore.  Re-install the rod/piston assembly without the spring and put the carb back together.  The carb will now run in the full-lean condition all the time.  If this clears up the idle, improves idle mixture screw response, and eliminates the black smoke when you flick the throttle, you need to install a softer spring. 

[77cruiser]

You can just push it down through the vent & try it.

[73ss]

older model carbs like that are prone to the bottom plugs leaking. Check that, It's outlined in the book. If you have a mightyvac you can test the n/s assy for leakage. I like to install the n/s and attach the mightyvac to the fuel inlet. This checks the n/s gasket as well. You have to remove the float or it will hold it open. If everything is good it should hold vacuum for a good 15min or so. You'll have to tear it all down to do this. I didn't have the seat tight enough in mine. Didn't pass the vacuum test. It leaked just enough fuel past the gasket to piss things off.

[carmantx]

With that cam, if you didn't modify the idle tubes and down channel restriction, and possibly bypass air, you are going to have idle issues.

[Cliff Ruggles]

What size are the idle tubes, idle DCR's, and how much idle bypass air?

Different size fuel inlet seats and messing with the power piston and spring are not the answer.  That isn't a very big cam, assuming your 427 has over 10 to 1 compression, it should be relatively smooth idle and not need a ton of fuel from the idle system right to start with......Cliff

[Landshark]

I believe the idle tubes are the stock .035, I don't know what the idle DCR's are and no idea about the idle bypass.


I have another question about the attached pictures. Can someone tell me why the difference between these 2 fast idle adjusting plates. The one on the left is P/N 30677 and the other is P/N 35806. I'm curious about why the top tang on the left on is bent and the other is straight.

[Ethan1]

I believe the idle tubes are the stock .035, I don't know what the idle DCR's are and no idea about the idle bypass.


I have another question about the attached pictures. Can someone tell me why the difference between these 2 fast idle adjusting plates. The one on the left is P/N 30677 and the other is P/N 35806. I'm curious about why the top tang on the left on is bent and the other is straight.

 What carb numbers are we talking about here? I see you reference 7029201. What is the other carb number? Different carbs used slightly different parts.

 EDIT: 7029201 uses the 35806 (or 7035806) fast idle lever. From my manual, it looks like the 30677 (or 7030677) was used on the B-O-P QuadraJets.