Adjusting Fuel Mixture - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Myride67c10 · 01-22-2003, 12:14 PM

I have a 67 with the original 283 and 2 barrel carb. I put a new exhaust on it a while back and the pipes are turning black. It also smells like it is running on the rich side. I know how to adjust the mixture on holly’s but I have no idea on this 2 barrel I’m not sure what brand it is but I’m sure its original. Any pics or info would be greatly appreciated.

jhow66 · 01-22-2003, 02:44 PM

There are 2 adjustment needles on base of carb. The 1st thing to do is run one all the way close and count the # of turns. set the other needle the same # of turns out. Now each needle is the same. Install vacumn gauge (be sure it is manifold vacumn) and adjust each needle to get the highest vacumn reading. (make sure timing is right before adjusting.) P.S. carb should be a 2 bar. Rochester).

**GMCPaul** · 01-22-2003, 06:15 PM

Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge.
GMCPaul

Myride67c10 · 01-23-2003, 10:45 AM

Wow I just thought you truned the mixture screws until it studders a little and then go in a half turn. Thank you guys for all the help and info. I think I might just take it to a shop and get it done seems how I do not have a timming light nore a vacuum gauge. Any idea of what this might runn $$$.

ChevLoRay · 01-23-2003, 10:51 AM

When you turn those idle are mixture screws in, be careful that you don't bottom them out with too much elbow grease. If you do, and you cause grooves to be cut in the needle end of the screw, you'll have a hard time getting the idle where you want it. I've seen it happen too many times, and it doesn't matter what kind of carburetor it is, or what it is installed on....lawnmower on up.

Tynee · 01-23-2003, 02:33 PM

I have the same carb on my 307. When I pull away from a light, my truck stumbles all over itself. I have to let out of the gas, then ease back into it sometimes 2 or 3 times to get away from a stop light or sign. Would the above adjustments fix my problem or is it time for a carb rebuild/ 4bbl upgrade?

jhow66 · 01-23-2003, 03:43 PM

Sounds like a carb rebuild--your booster pump in carb sounds like it is worn out. These are easy carbs to fix even if you have not done one before.Kit comes with instructions and exploded views. Try it.

**GMCPaul** · 01-23-2003, 06:42 PM

You can pick up a vacuum gauge for $10-$12 and a timing light for about $50, then you have the tools to do yours this time and any others later in life, taking it to a shop they will most likely charge a hour minimum even though it won't take that long and most shop rates run $40-$60 a hour.
Tynee you definately sound as though you could use a good timing and accelator pump with a carb rebuild, the accelator pump is most likely causing most of the lag. When rebuilding a quadrajet be sure to epoxy the plugs on the bottom base of carb under the fuel bowl. These begin to leak over a period of time and can reach the point that the bowl will run dry overnight and lead to extended cranking of the starter to get carb primed again to start truck.
GMCPaul

rusty ol' 69' · 04-03-2003, 12:48 PM

ok

Blue Beard · 04-03-2003, 02:29 PM

GMC Pauls

I think that's the best run down I've seen in a while on proper adjustment.

Mike Culpepper · 04-03-2003, 02:36 PM

Would you use the same method for 4 BBL Q-jets?
Mike

01-22-2003, 12:14 PM	#1
Myride67c10 Registered User Join Date: Oct 2002 Location: Asheville, NC Posts: 165	Adjusting Fuel Mixture I have a 67 with the original 283 and 2 barrel carb. I put a new exhaust on it a while back and the pipes are turning black. It also smells like it is running on the rich side. I know how to adjust the mixture on holly’s but I have no idea on this 2 barrel I’m not sure what brand it is but I’m sure its original. Any pics or info would be greatly appreciated. __________________ 1967 C-10 Use to be Riptide

01-22-2003, 02:44 PM	#2
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	There are 2 adjustment needles on base of carb. The 1st thing to do is run one all the way close and count the # of turns. set the other needle the same # of turns out. Now each needle is the same. Install vacumn gauge (be sure it is manifold vacumn) and adjust each needle to get the highest vacumn reading. (make sure timing is right before adjusting.) P.S. carb should be a 2 bar. Rochester). __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

01-22-2003, 06:15 PM	#3
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,721	Disconnect vacuum advance at distributor and plug vacuum line, start motor with timing light and a RPM gauge attached and set idle and timing to correct settings. Connect vacuum gauge to full unported vacuum at base of carburetor, now shut motor off and turn both idle mixture screws all the way in but be sure that you only use light pressure as you can turn the idle mixture screw past being lightly seated by twisting hard and then you are damaging the idle mixture opening with the needle causing it to flare the opening trashing a good carburetor body so please only lightly seat the needles. now back each out 1 1/2 turns, restart motor and check idle and timing and adjust as necessary to correct settings, now back each needle out slowly a 1/4 turn each alternating between the 2 until you reach the highest vacuum reading, check and adjust idle speed and timing, now repeat step one of backing idle mixture screws out alternating between the 2 a 1/4 turn at a time until you have reached highest vacuum reading again. Continue the process adjusting timing/RPM/idle mixture screws until you have no changes occurring and have reached highest vacuum reading possible. You have now optimized your timing idle mixture adjustments. Reconnect your vacuum advance while disconnecting the RPM gauge, timing light, and vacuum gauge. GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

01-23-2003, 10:45 AM	#4
Myride67c10 Registered User Join Date: Oct 2002 Location: Asheville, NC Posts: 165	Wow I just thought you truned the mixture screws until it studders a little and then go in a half turn. Thank you guys for all the help and info. I think I might just take it to a shop and get it done seems how I do not have a timming light nore a vacuum gauge. Any idea of what this might runn $$$. __________________ 1967 C-10 Use to be Riptide

01-23-2003, 10:51 AM	#5
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	....be careful.... When you turn those idle are mixture screws in, be careful that you don't bottom them out with too much elbow grease. If you do, and you cause grooves to be cut in the needle end of the screw, you'll have a hard time getting the idle where you want it. I've seen it happen too many times, and it doesn't matter what kind of carburetor it is, or what it is installed on....lawnmower on up. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

01-23-2003, 02:33 PM	#6
Tynee I miss this truck. Join Date: Dec 2001 Location: Lexington, KY U.S.A. Posts: 2,863	I have the same carb on my 307. When I pull away from a light, my truck stumbles all over itself. I have to let out of the gas, then ease back into it sometimes 2 or 3 times to get away from a stop light or sign. Would the above adjustments fix my problem or is it time for a carb rebuild/ 4bbl upgrade? __________________ '07 GMC Acadia SLT. Sweet ride. '08 Crew Cab Z-71 short bed. Really like this truck A LOT.

01-23-2003, 03:43 PM	#7
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Sounds like a carb rebuild--your booster pump in carb sounds like it is worn out. These are easy carbs to fix even if you have not done one before.Kit comes with instructions and exploded views. Try it. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

01-23-2003, 06:42 PM	#8
GMCPaul Senior Member Join Date: Mar 2000 Location: Rockville,In. USA Posts: 3,721	You can pick up a vacuum gauge for $10-$12 and a timing light for about $50, then you have the tools to do yours this time and any others later in life, taking it to a shop they will most likely charge a hour minimum even though it won't take that long and most shop rates run $40-$60 a hour. Tynee you definately sound as though you could use a good timing and accelator pump with a carb rebuild, the accelator pump is most likely causing most of the lag. When rebuilding a quadrajet be sure to epoxy the plugs on the bottom base of carb under the fuel bowl. These begin to leak over a period of time and can reach the point that the bowl will run dry overnight and lead to extended cranking of the starter to get carb primed again to start truck. GMCPaul __________________ Secure Online Catalog for-GMCPauls Truck Parts Email- GMCPAULS@GMAIL.COM

04-03-2003, 12:48 PM	#9
rusty ol' 69' Grubbin' Join Date: Aug 2002 Location: Chico, CA Posts: 293	ok __________________ project rust attack 1969 Chevrolet LWB w/ 307. I got power steering now woohooooo!!!!!!!

04-03-2003, 02:29 PM	#10
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,834	GMC Pauls I think that's the best run down I've seen in a while on proper adjustment. __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

04-03-2003, 02:36 PM	#11
Mike Culpepper Red Join Date: Feb 2003 Location: Llano... Texas Hill Country Posts: 448	Would you use the same method for 4 BBL Q-jets? Mike __________________ '72 Cheyenne LWB 350/350 "The sun don't shine on the same dog's ass all the time." - Catfish Hunter (commenting on why he was not able to pitch another perfect game).



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