idle mixture setup ? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

c10rookie · 03-30-2004, 05:21 PM

Hi guys, could someone advise me on idle mixture setting ? I have just rebuilt my 72 350 C10 and cant seem to get a good idle at all. I'm using the same 600 holley (single feed) which was OK on last engine, as motor idles for a couple of minutes it seems to load up and may even stall, fire it up and clear it, it's fine for another minute or so ?? what ' co ' reading should I be looking for with dual glass pack pipes??? A buddy has access to a sun diagnostic unit and we will hook up this weekend but he does not have any info on SBC's. Advice on ' co ' and any other tips would be great, motor is a little rough on idle but this may be new components being a bit lopey ? I have set ignition at 8' BTDC but engines idles best on 16 > 18 BTDC but engine wont turn to start set at that, back it off to 6 > 8' and it starts fine

rebuild specs
flat tops '0030 =9.6:1
edlebrock 70cc rpm heads
rpm inlet
600 holley vac 2nd's
1.6 roller tips
crane .467/.494 278/290 222/234 114'
hei & msd6
long headers

bpmcgee · 03-30-2004, 06:18 PM

C10,

I think the best way to set the idle mixture is with a vacuum gauge or a tach. In either case, adjust the mixture to maximize the RPMS or the vacuum.

Make sure that both sides are adjusted equally.

Brian

c10rookie · 03-31-2004, 03:14 AM

Thats the problem, when I set mixture for best idle it's so far advanced it wont spin over to start, spark is so early it pushes against starter ???

bpmcgee · 03-31-2004, 10:50 AM

Early spark is TIMING. Idle mixture doesn't have anything to do with timing.

Idle mixture is set by the two small screws in the front of your carb.

Brian

**COBALT** · 03-31-2004, 01:29 PM

You don't really need a vacuum guage or tach to set the idle mixture, but they sure help. You can do it by sound.

Basically, it's a process for achieving the highest RPM at idle. You're basically ballancing the idle mixture screws with the fast idle screw on your throttle linkage to get that optimal RPM at idle.

DISCLAIMER: Always keep track of the number of turns each screw makes for every adjustment so you don't loose track where you are. Count 1/2 or 1/4 turns to help be precise.

Start by counting the number of turns the screws are already set. Turn them all the way in, and count the number of rotations until they seat. Back them out the same amount to go back where you started. Note the number of turns for each screw. THEY WILL PROBABLY BE DIFFERENT.

If you're using a tachometer hook it up per instructions with the engine at normal operating temp. Check to see what the RPMs are. Typically it should be between 800-1000 rpm depending on your transmission. If your idle speed is out of whack adjust it using the fast idle screw.

Now start on the driver's side idle mixture screw. Start turning it clockwise (to close off the idle passage) until you hear the RPMs of your engine start to drop, or your tack shows a drop in RPMs. Note where that point is in number of turns. Now start backing it out (counter-clockwise). You will hear the RPMs pick back up, and as you continue to back it out the RPMs will start to drop again. Note where that point is based on number of screw turns.

What you should imagine is a bell curve, with both ends being the place in which the RPMs are dropping because of either too rich or too lean of an idle mixture. You want to be on the front part of that bell curve (let the idle screw out as far as it will go before the RPMs drop).

Once the first one is set perform the exact same steps with the passenger side. You may have to go back and forth until you've got both screws' max RPM idle-mixture discovered.

Finally, readjust the fast idle screw to the appropriate idle RPM.

You're done.

Timing and ignition adjustment is a totally different process...

**COBALT** · 03-31-2004, 01:31 PM

...um...if your starter isn't turning over you have another problem. Don't be messing with the idle screws AND turning your distributor. You'll have your fuel/air for idle, and your ignition timing all screwed up.

What specifically have you done up to this point?

c10rookie · 03-31-2004, 04:29 PM

Thanks Colbalt, that idle set-up routine has settled engine, my other problem was that when setting timing the motor ran best at 14 > 18 btdc but would not turn to start, back it down to 6 or 8 btdc starts fine, just idle a little lopey what should a sbc be set at

**COBALT** · 03-31-2004, 05:29 PM

Do you have vacuum advance? Are you disconnecting and blocking it when you check your timing? Are you sure your vacuum advance is working?

It may be just as simple as bumping up your fast idle setting...what RPM are you at idle right now?

**COBALT** · 03-31-2004, 05:59 PM

I responded to your PM.

tino0217 · 02-28-2019, 05:29 AM

Great advice. Thanks brother

GASoline71 · 03-05-2019, 10:15 PM

With that cam you're gonna want about 14 to 16 initial timing and then 35ish total.

Gary

**The Rocknrod** · 03-06-2019, 09:03 AM

If you have a couple of minutes you can read a bit about it here - http://67-72chevytrucks.com/vboard/s...318544&page=29

geezer#99 · 03-06-2019, 09:46 AM

And if you two guys take a second or two you might notice this is a 15 year old thread!

tdangle · 03-06-2019, 01:22 PM

GASoline71 · 03-06-2019, 10:02 PM

Quote:

Originally Posted by geezer#99

And if you two guys take a second or two you might notice this is a 15 year old thread!

Ya got me!

Gary

03-30-2004, 05:21 PM	#1
c10rookie Registered User Join Date: Jun 2003 Location: south east england Posts: 113	idle mixture setup ? Hi guys, could someone advise me on idle mixture setting ? I have just rebuilt my 72 350 C10 and cant seem to get a good idle at all. I'm using the same 600 holley (single feed) which was OK on last engine, as motor idles for a couple of minutes it seems to load up and may even stall, fire it up and clear it, it's fine for another minute or so ?? what ' co ' reading should I be looking for with dual glass pack pipes??? A buddy has access to a sun diagnostic unit and we will hook up this weekend but he does not have any info on SBC's. Advice on ' co ' and any other tips would be great, motor is a little rough on idle but this may be new components being a bit lopey ? I have set ignition at 8' BTDC but engines idles best on 16 > 18 BTDC but engine wont turn to start set at that, back it off to 6 > 8' and it starts fine rebuild specs flat tops '0030 =9.6:1 edlebrock 70cc rpm heads rpm inlet 600 holley vac 2nd's 1.6 roller tips crane .467/.494 278/290 222/234 114' hei & msd6 long headers __________________ if it aint broke , fix it till it is 355- flat tops - 70cc RPM heads 9.5:1 RPM inlet MSD6 & HEI headmans & glasspacks Crane .467/.494 278/290 222/234 114 1.6 roller's 2400 tci 650 Holley cold air induction electric w/pump,fans,f/pump

03-31-2004, 03:14 AM	#3
c10rookie Registered User Join Date: Jun 2003 Location: south east england Posts: 113	Thats the problem, when I set mixture for best idle it's so far advanced it wont spin over to start, spark is so early it pushes against starter ??? __________________ if it aint broke , fix it till it is 355- flat tops - 70cc RPM heads 9.5:1 RPM inlet MSD6 & HEI headmans & glasspacks Crane .467/.494 278/290 222/234 114 1.6 roller's 2400 tci 650 Holley cold air induction electric w/pump,fans,f/pump

03-31-2004, 01:29 PM	#5
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	You don't really need a vacuum guage or tach to set the idle mixture, but they sure help. You can do it by sound. Basically, it's a process for achieving the highest RPM at idle. You're basically ballancing the idle mixture screws with the fast idle screw on your throttle linkage to get that optimal RPM at idle. DISCLAIMER: Always keep track of the number of turns each screw makes for every adjustment so you don't loose track where you are. Count 1/2 or 1/4 turns to help be precise. Start by counting the number of turns the screws are already set. Turn them all the way in, and count the number of rotations until they seat. Back them out the same amount to go back where you started. Note the number of turns for each screw. THEY WILL PROBABLY BE DIFFERENT. If you're using a tachometer hook it up per instructions with the engine at normal operating temp. Check to see what the RPMs are. Typically it should be between 800-1000 rpm depending on your transmission. If your idle speed is out of whack adjust it using the fast idle screw. Now start on the driver's side idle mixture screw. Start turning it clockwise (to close off the idle passage) until you hear the RPMs of your engine start to drop, or your tack shows a drop in RPMs. Note where that point is in number of turns. Now start backing it out (counter-clockwise). You will hear the RPMs pick back up, and as you continue to back it out the RPMs will start to drop again. Note where that point is based on number of screw turns. What you should imagine is a bell curve, with both ends being the place in which the RPMs are dropping because of either too rich or too lean of an idle mixture. You want to be on the front part of that bell curve (let the idle screw out as far as it will go before the RPMs drop). Once the first one is set perform the exact same steps with the passenger side. You may have to go back and forth until you've got both screws' max RPM idle-mixture discovered. Finally, readjust the fast idle screw to the appropriate idle RPM. You're done. Timing and ignition adjustment is a totally different process... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

03-31-2004, 01:31 PM	#6
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	...um...if your starter isn't turning over you have another problem. Don't be messing with the idle screws AND turning your distributor. You'll have your fuel/air for idle, and your ignition timing all screwed up. What specifically have you done up to this point? __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

03-31-2004, 04:29 PM	#7
c10rookie Registered User Join Date: Jun 2003 Location: south east england Posts: 113	Thanks Colbalt, that idle set-up routine has settled engine, my other problem was that when setting timing the motor ran best at 14 > 18 btdc but would not turn to start, back it down to 6 or 8 btdc starts fine, just idle a little lopey what should a sbc be set at __________________ if it aint broke , fix it till it is 355- flat tops - 70cc RPM heads 9.5:1 RPM inlet MSD6 & HEI headmans & glasspacks Crane .467/.494 278/290 222/234 114 1.6 roller's 2400 tci 650 Holley cold air induction electric w/pump,fans,f/pump

03-30-2004, 06:18 PM	#2
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	C10, I think the best way to set the idle mixture is with a vacuum gauge or a tach. In either case, adjust the mixture to maximize the RPMS or the vacuum. Make sure that both sides are adjusted equally. Brian

03-31-2004, 10:50 AM	#4
bpmcgee Registered User Join Date: Mar 2003 Location: Northern Illinois Posts: 2,181	Early spark is TIMING. Idle mixture doesn't have anything to do with timing. Idle mixture is set by the two small screws in the front of your carb. Brian

03-31-2004, 05:29 PM	#8
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Do you have vacuum advance? Are you disconnecting and blocking it when you check your timing? Are you sure your vacuum advance is working? It may be just as simple as bumping up your fast idle setting...what RPM are you at idle right now? __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

03-31-2004, 05:59 PM	#9
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	I responded to your PM. __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

02-28-2019, 05:29 AM	#10
tino0217 Registered User Join Date: May 2017 Location: Seatac, wa Posts: 172	Re: idle mixture setup ? Great advice. Thanks brother

03-06-2019, 09:03 AM	#12
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: idle mixture setup ? If you have a couple of minutes you can read a bit about it here - http://67-72chevytrucks.com/vboard/s...318544&page=29 __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

03-06-2019, 09:46 AM	#13
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,785	Re: idle mixture setup ? And if you two guys take a second or two you might notice this is a 15 year old thread!

03-06-2019, 01:22 PM	#14
tdangle Registered User Join Date: Mar 2005 Location: Jurupa Valley, Ca Posts: 1,211	Re: idle mixture setup ? __________________ Terry 1970 Custom Camper/C20 , GM Crate 350/7004R, Dana 60, factory AC



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