Argh! Stumble off idle with new parts?! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Kixwy2 · 02-01-2008, 12:35 PM

So I have a brand new crate engine, a professionally rebuilt Rochester quadrajet carb, new fuel pump, new accelerator pump, and mostly all new parts in my car. Engine is timed at 10* BTDC and fast/slow idle set to recommended specs.

Engine runs like a dream, except for the stumble off idle. Damn. I have narrowed it down (I think) to my fuel mixture being too lean, but I need some opinions and advice to be sure. And I forget: is turning the screws in clockwise = more air or more fuel?

...This is where the Haynes manual fails - "adjust mixture as necessary." Er, ok, great directions there, Lou...

James McClure · 02-01-2008, 07:15 PM

Kix: Idle mixture screws meter only fuel. Unscrew, back out, counter clockwise rotation, whatever = richer. "Q" jet idle screw settings about 3.5 to 4 turns out from seated. jim

Slonaker · 02-02-2008, 05:14 AM

AFAIK, those screws only effect the mixture at idle.

Look here under "Air and Fuel Delivery."

http://www.73-87.com/7387garage/garage_drivetrain.htm

Slonaker

Kixwy2 · 02-04-2008, 04:42 AM

All right, I will try turning them out a 1/4 screw at a time and see if that helps any... could it also be a function of a new motor still settling in?

I don't want to jump the gun here so I'm checking all my options. It's hell being a new truck addict and a worry-wort

Ivan D. · 02-04-2008, 01:49 PM

I had similar symptoms with my engine in the summer, upped the initial timing to 14*BTDC and reset the idle mixture with a vacuum gauge and all was peachy.

Kixwy2 · 02-05-2008, 12:08 PM

Quote:

Originally Posted by Ivan D.

I had similar symptoms with my engine in the summer, upped the initial timing to 14*BTDC and reset the idle mixture with a vacuum gauge and all was peachy.

What kind of engine are you running? The recommendations that came with this engine was 10*. I think it's actually at 11* because I don't have a digital timing light but it's close.

Maybe it's my imagination, but it seems less pronounced now. I'm still going to richen the mixture slightly and see if my performance is any better...

Ivan D. · 02-05-2008, 03:49 PM

It was a '79 350 block with stock heads ported by me, Melling RV/towing cam, Offenhauser split-port intake and a spreadbore double-pumper Holley carb, stock HEI ignition. I've been told cam is the same as the Edelbrock Performer cam, for which Edelbrock recommends 14*BTDC when ran with stock HEI ignition, so that's what I did and it was all it needed to start running beautifully.

Also when playing with the idle fuel mixture tis best to read off a vacuum gauge, so hook one up to the manifold if you haven't done so already.

02-01-2008, 12:35 PM	#1
Kixwy2 Registered User Join Date: Dec 2007 Location: Central Coast, CA Posts: 177	Argh! Stumble off idle with new parts?! So I have a brand new crate engine, a professionally rebuilt Rochester quadrajet carb, new fuel pump, new accelerator pump, and mostly all new parts in my car. Engine is timed at 10* BTDC and fast/slow idle set to recommended specs. Engine runs like a dream, except for the stumble off idle. Damn. I have narrowed it down (I think) to my fuel mixture being too lean, but I need some opinions and advice to be sure. And I forget: is turning the screws in clockwise = more air or more fuel? ...This is where the Haynes manual fails - "adjust mixture as necessary." Er, ok, great directions there, Lou... __________________ 1974 GMC C-10; my new addiction 1998 4 Runner; daily driver

02-01-2008, 07:15 PM	#2
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: Argh! Stumble off idle with new parts?! Kix: Idle mixture screws meter only fuel. Unscrew, back out, counter clockwise rotation, whatever = richer. "Q" jet idle screw settings about 3.5 to 4 turns out from seated. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

02-02-2008, 05:14 AM	#3
Slonaker Insert Witty Text Here Join Date: Oct 2002 Location: San Antonio, TX Posts: 3,415	Re: Argh! Stumble off idle with new parts?! AFAIK, those screws only effect the mixture at idle. Look here under "Air and Fuel Delivery." http://www.73-87.com/7387garage/garage_drivetrain.htm Slonaker __________________ '86 Chevy C10 (Sold 04/19/13 ) Stock '01 Silverado

02-04-2008, 04:42 AM	#4
Kixwy2 Registered User Join Date: Dec 2007 Location: Central Coast, CA Posts: 177	Re: Argh! Stumble off idle with new parts?! All right, I will try turning them out a 1/4 screw at a time and see if that helps any... could it also be a function of a new motor still settling in? I don't want to jump the gun here so I'm checking all my options. It's hell being a new truck addict and a worry-wort __________________ 1974 GMC C-10; my new addiction 1998 4 Runner; daily driver

02-04-2008, 01:49 PM	#5
Ivan D. glamoros piece o' lowlife Join Date: Mar 2007 Location: Redford, MI Posts: 1,164	Re: Argh! Stumble off idle with new parts?! I had similar symptoms with my engine in the summer, upped the initial timing to 14*BTDC and reset the idle mixture with a vacuum gauge and all was peachy. __________________ The more I drink, the more I drink, the more I drink... diesel, reg cab, long bed, 4 across the rear, single stack, wooden stakes, and lotsa lights - the Hay Express

02-05-2008, 03:49 PM	#7
Ivan D. glamoros piece o' lowlife Join Date: Mar 2007 Location: Redford, MI Posts: 1,164	Re: Argh! Stumble off idle with new parts?! It was a '79 350 block with stock heads ported by me, Melling RV/towing cam, Offenhauser split-port intake and a spreadbore double-pumper Holley carb, stock HEI ignition. I've been told cam is the same as the Edelbrock Performer cam, for which Edelbrock recommends 14*BTDC when ran with stock HEI ignition, so that's what I did and it was all it needed to start running beautifully. Also when playing with the idle fuel mixture tis best to read off a vacuum gauge, so hook one up to the manifold if you haven't done so already. __________________ The more I drink, the more I drink, the more I drink... diesel, reg cab, long bed, 4 across the rear, single stack, wooden stakes, and lotsa lights - the Hay Express



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