Carb tuning help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Classycustom71,95 · 10-25-2014, 01:53 PM

Hey guys,
i got my rebuilt 350 with mild cam into my truck last weekend and now am having some troubles getting the carb tuned right. its a 1806 - 650cfm edelbrock.
I am in Calgary where the elevation is 3438 ft, I have changed the rods to the .065 x .042 rod. and that has helped it run a bit better but i will still get a backfire after getting it to about 3500 rpm and letting off the throttle, it still hesitates alot at anything less than a 3000 rpm gear change.

i could keep trying different jets and rods but am unclear if i should be going richer or leaner?

my timing is also currently at 6 degrees.

70LonghornCST · 10-29-2014, 02:01 AM

Hi there. I'm not a carb expert. I generally mess with the timing first then adjust the air/fuel mixture needles until it smooths out. But I think you need some of our resident experts to chime in with advice. I moved this post from projects and builds to the message board to get more views.

JGray · 10-29-2014, 04:09 AM

I just finished dialing in that same carb on 71 Blazer/ZZ4 combo - my elevation is similar to yours (a little below 4000'). I ran that carb for a while as is with no tuning and had a stumble on hard acceleration. I talked to an Edelbrock tech who concurred the 16 deg. base timing I ended up at was a good place to be at this altitude. He also seemed pretty adamant that my stumble could be addressed by adjusting the secondary air valve.

After numerous iterations, I ended up richer on the primary rods (65x42), staging rich 1 step sooner on the step up springs (pink - 7"?), moving the pump rod linkage to the outer hole and bending the linkage slightly to maximize the shot. All that helped, but it took adjusting the secondary air valve per the tech's recommendation to really make everything happy. I did end up taking the air valve adjustment past where the manual says to stop turning the screw (delaying opening of the air valve).

I'm very pleased with how it runs...

Edit to add - just noticed your 6 deg. initial timing and that is where I'd look first. That sounds low for your altitude - my ZZ4 spec is 10 deg. initial and 16 is better at similar altitudes. With a similar engine and altitude, mine responded well to a bit more fuel.

geezer#99 · 10-29-2014, 09:01 AM

X2 on the timing.
16 or more initial is what you need.
Last sbc I had I ran 22 initial, 12 in the dizzy with 10 from vac advance. I lived west of Red Deer at the time.

Classycustom71,95 · 10-29-2014, 04:26 PM

70LonghornCST thank you for moving my post I'm still new to the forum.
JGray how aggressive is your engine build?
I will defiantly try changing my timing to at least 16 and see how see runs.
Also I can only get my idle down to 1000rpm does that sound right for a noticeable idle cam?

geezer#99 · 10-29-2014, 04:58 PM

[QUOTE
Also I can only get my idle down to 1000rpm does that sound right for a noticeable idle cam?[/QUOTE]

Not at all!
Should have no problem getting the idle down to 700 rpm.
At 1000 rpm your mechanical timing is kicking in big time.

Classycustom71,95 · 10-29-2014, 06:44 PM

what can I try to lower it?

JGray · 10-29-2014, 06:50 PM

Quote:

Originally Posted by Classycustom71,95

70LonghornCST thank you for moving my post I'm still new to the forum.
JGray how aggressive is your engine build?
I will defiantly try changing my timing to at least 16 and see how see runs.
Also I can only get my idle down to 1000rpm does that sound right for a noticeable idle cam?

The ZZ4 is a GM Performance crate motor rated at 355 Hp and 405 ft-lbs torque. It has L98 aluminum heads with 58 cc combustion chambers and 10:1 compression, and a fairly mild roller cam:

Duration at .050" tappet lift (intake/exhaust) is 208/221
Maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 474/510
(copied off the web)

JGray · 10-29-2014, 06:53 PM

Quote:

Originally Posted by Classycustom71,95

what can I try to lower it?

Get your timing set and then see what your idle does...

Can you share any specifics on your engine - crate motor vs. local build, etc.? You initially said mild cam, and then later said noticeable idle cam which has me a little confused...

geezer#99 · 10-29-2014, 07:15 PM

Quote:

Originally Posted by Classycustom71,95

what can I try to lower it?

Adjust the curb idle screw and check the high idle screw too.
Have you proved tdc #1 is correct on your timing tab?

Classycustom71,95 · 11-02-2014, 04:14 PM

I did a local build, bored 40 over with 2 valve relief pistons new stock specs crank, vortec heads plained and 500 lift springs, comp cam 470 lift 270 dur.
performer intake.
I have double check my TDC and will be attempting to adjust the timing now.

geezer#99 · 11-02-2014, 06:03 PM

[QUOTE
I have double check my TDC and will be attempting to adjust the timing now.[/QUOTE]

How did you check tdc?

10-29-2014, 02:01 AM	#2
70LonghornCST Moderator Join Date: Jun 2008 Location: Modesto, CA Posts: 5,775	Re: Carb tuning help Hi there. I'm not a carb expert. I generally mess with the timing first then adjust the air/fuel mixture needles until it smooths out. But I think you need some of our resident experts to chime in with advice. I moved this post from projects and builds to the message board to get more views. __________________ 2018 Chevy Crew Cab 2006 Impala SS 1965 Plymouth Sport Fury 1970 Chevy Longhorn Bucket Seats Shoulder Belts Front Tow Hooks Factory Air Factory Speedwarning, Tach & Vaccum Factory Tilt CST Package AM/FM Radio Bumper Guards Richard 70 Longhorn CST Build Adding an AM/FM Decal

10-29-2014, 04:09 AM	#3
JGray Registered User Join Date: Jun 2010 Location: Helena, MT Posts: 21	Re: Carb tuning help I just finished dialing in that same carb on 71 Blazer/ZZ4 combo - my elevation is similar to yours (a little below 4000'). I ran that carb for a while as is with no tuning and had a stumble on hard acceleration. I talked to an Edelbrock tech who concurred the 16 deg. base timing I ended up at was a good place to be at this altitude. He also seemed pretty adamant that my stumble could be addressed by adjusting the secondary air valve. After numerous iterations, I ended up richer on the primary rods (65x42), staging rich 1 step sooner on the step up springs (pink - 7"?), moving the pump rod linkage to the outer hole and bending the linkage slightly to maximize the shot. All that helped, but it took adjusting the secondary air valve per the tech's recommendation to really make everything happy. I did end up taking the air valve adjustment past where the manual says to stop turning the screw (delaying opening of the air valve). I'm very pleased with how it runs... Edit to add - just noticed your 6 deg. initial timing and that is where I'd look first. That sounds low for your altitude - my ZZ4 spec is 10 deg. initial and 16 is better at similar altitudes. With a similar engine and altitude, mine responded well to a bit more fuel. Last edited by JGray; 10-29-2014 at 04:18 AM. Reason: addition...

10-29-2014, 09:01 AM	#4
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,542	Re: Carb tuning help X2 on the timing. 16 or more initial is what you need. Last sbc I had I ran 22 initial, 12 in the dizzy with 10 from vac advance. I lived west of Red Deer at the time.

10-29-2014, 04:26 PM	#5
Classycustom71,95 Registered User Join Date: Aug 2011 Location: Calgary, AB Posts: 57	Re: Carb tuning help 70LonghornCST thank you for moving my post I'm still new to the forum. JGray how aggressive is your engine build? I will defiantly try changing my timing to at least 16 and see how see runs. Also I can only get my idle down to 1000rpm does that sound right for a noticeable idle cam?

10-29-2014, 04:58 PM	#6
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,542	Re: Carb tuning help [QUOTE Also I can only get my idle down to 1000rpm does that sound right for a noticeable idle cam?[/QUOTE] Not at all! Should have no problem getting the idle down to 700 rpm. At 1000 rpm your mechanical timing is kicking in big time.

10-25-2014, 01:53 PM	#1
Classycustom71,95 Registered User Join Date: Aug 2011 Location: Calgary, AB Posts: 57	Carb tuning help Hey guys, i got my rebuilt 350 with mild cam into my truck last weekend and now am having some troubles getting the carb tuned right. its a 1806 - 650cfm edelbrock. I am in Calgary where the elevation is 3438 ft, I have changed the rods to the .065 x .042 rod. and that has helped it run a bit better but i will still get a backfire after getting it to about 3500 rpm and letting off the throttle, it still hesitates alot at anything less than a 3000 rpm gear change. i could keep trying different jets and rods but am unclear if i should be going richer or leaner? my timing is also currently at 6 degrees.

10-29-2014, 06:44 PM	#7
Classycustom71,95 Registered User Join Date: Aug 2011 Location: Calgary, AB Posts: 57	Re: Carb tuning help what can I try to lower it?

11-02-2014, 04:14 PM	#11
Classycustom71,95 Registered User Join Date: Aug 2011 Location: Calgary, AB Posts: 57	Re: Carb tuning help I did a local build, bored 40 over with 2 valve relief pistons new stock specs crank, vortec heads plained and 500 lift springs, comp cam 470 lift 270 dur. performer intake. I have double check my TDC and will be attempting to adjust the timing now.

11-02-2014, 06:03 PM	#12
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,542	Re: Carb tuning help [QUOTE I have double check my TDC and will be attempting to adjust the timing now.[/QUOTE] How did you check tdc?



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