Lean idle and Exhaust popping

[TylerG]

Hey Guys, thanks in advance for any info I can get on this. I'm finishing up my first SBC engine rebuild including my Quadrajet with a great performance kit from Cliff. I'm in the final stages of tuning the QJet now and am a little stumped on getting my idle right. I'm getting a lot of obnoxious popping out the exhaust when I let off the gas, under load unless I turn the idle mixture screws out 5 turns. This seems excessive based on stock specs but understand I should be drilling the idle mix holes out and possibly the pick up tubes and down channels to richen the idle circuit to better match my build. When the truck is idling the exhaust is eye watering nasty. The idle screw on the side of the carb is turned all the way out and I cant get the idle below 900RPM. Any pointers on getting my idle mix corrected would be greatly appreciated.

Engine Build: 350 small block honed 20 over, 64cc Edelbrock heads, stock iron intake, Summit 214/224 cam, long tube headers, Cliff's HP Qjet rebuild kit (not including idle circuit parts) can get jet/rod numbers if needed. Timing is set at 14 idle and 38 all in. I think i'm getting about 9.5:1 compression.

Thanks
Tyler

[Cliff Ruggles]

What is the carburetor part number?

How is it set-up, idle tubes, DCR's, idle airbleeds, bypass air, etc?

[TylerG]

Hey Cliff, the carb is a #7041208. From what I can tell its the factory carb for my 1971 350. The carb is unmollested, so the idle tubes, DCR's, idle air bleeds and bypass air are untouched. The only thing that has been done to the carb is the new O power piston spring, 76 jets, 43 primary rods, AX secondary rods and SR Kit that I got from you.

Thanks
T

[77cruiser]

Possible vacuum leak, if you can't get the idle down.

[TylerG]

Thanks Jim, i'll double check for vacuum leaks. I'm getting steady 16" of manifold vacuum, for what it's worth.

T

[77cruiser]

Check to make sure the linkage isn't holding the secondaries open a bit.

[Cliff Ruggles]

It's simply wanting more fuel to the idle mixture screws and may need a bit more bypass air. 

Did you remove the idle tubes during the rebuild? 

[TylerG]

Yes Cliff, I removed the pick up tubes and cleaned them. Do you suggest drilling the idle mix holes .09? How should I give it more bypass? I’m learning as I go and appreciate all this info.

[TylerG]

I think figured it out, Cliff. Looks like the bypass is right next to the idle mix hole. I'll open this up a touch as well. Thanks again.

[77cruiser]

I'd try to figure out why the idle won't go below 900 with the idle screw all the way out.

[TylerG]

Busy weekend but got the bypass opened up to .092” and idle mix opened up to .091” late last night. Reinstalled this morning, mix screws turned out 2 turns. I only had time for a quick test drive to get it up to temp and it was def running smoother. I have full control of idle mix now and I was able to get the idle down to 750. I noticed last night when I had it apart that the primaries were cracked a bit far so wondering if that was causing the high rpm Jim!? Hoping to have a bit of time tomorrow night to get back into this and get it dialed in better now that I’ve got the lean idle under control.

[77cruiser]

Fast idle cam could have been hung up I suppose. Sounds like you're getting a handle on it.

[Too Fast]

Way back in the day I had a 1972 Chevy van G20 with a 350 and a manual transmission 3 on the tree.  I ran dual exhaust on it with some long glass packs.  It had a lot of over-run popping thru the exhaust.  Think like 4000 RPM, let off the gas in 2nd gear.  Sounded almost cool, but my memory may be a bit foggy.  Anyway, I added a balance pipe to the exhaust (H-pipe, I worked and an exhaust shop at the time) before the glass packs and that eliminated about 75% of the over-run popping.

IDK if that is part of what you are experiencing.  I had no headers, just 2" dual exhaust.