How do I adjust my Carb? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Isaiah92260 · 08-13-2015, 01:33 PM

How do I correctly adjust my single barrel carb? I have a 67' straight 6, I'm assuming that I have a Rochester carb. My timing is at 12 BTDC and I have my idle at 650rpm's

Wrenchbender Ret · 08-13-2015, 04:39 PM

The only external adjustment is the idle mixture & speed. Turn the idle mixture jet out a turn or 2. Then slowly turn it in with the eng. idleing & warm. Turn it in till it slows down. Then turn it back out till it speeds up to normal speed which is usually about 1/2 turn. Your speed at 650 is about right if it is a manual trans. An automatic may want about 750 in neutral

**TJ's Chevy** · 08-13-2015, 05:44 PM

Easy way to adjust would be to run the engine until it is up to operating temp. Turn the screw in until the engine struggles to run them back it out until max engine RPM is reached. Then reset your idle speed to 500-700.

prostreetC-10 · 08-14-2015, 02:57 PM

Quote:

Originally Posted by TJ's Chevy

run the engine until it is up to operating temp.

Just make darn sure and do this first. A cold engine loves a really rich mix and you will be way too fat at temp if you set the A/R when cold.

68gmsee · 08-14-2015, 07:15 PM

Besides the above recommendations, another thing I do is listen to the exhaust while adjusting the carb. Should be smooth with no sputtering or popping.

Be aware, though, that ignition problems, bad gas, exhaust and vacuum leaks can make carb adjustments tricky or to be off so be sure you have fresh gas if the truck has been sitting for a while. Make sure spark plugs have the correct gap, points (if you have them) are gapped and set properly and all ignition parts are in good shape including the vacuum advance.

Isaiah92260 · 08-14-2015, 07:24 PM

Quote:

Originally Posted by 68gmsee

Besides the above recommendations, another thing I do is listen to the exhaust while adjusting the carb. Should be smooth with no sputtering or popping.

Be aware, though, that ignition problems, bad gas, exhaust and vacuum leaks can make carb adjustments tricky or to be off so be sure you have fresh gas if the truck has been sitting for a while. Make sure spark plugs have the correct gap, points (if you have them) are gapped and set properly and all ignition parts are in good shape including the vacuum advance.

Thank you for your reply and advice, what does the gap need to be for the spark plugs and points?

jdl71 · 08-13-2015, 09:10 PM

If you are looking for the "proper" way, the factory service manual says to set your idle speed then disconnect the PCV line and put a vacuum gauge in its place. The adjust the mixture screw until maximum vacuum is achieved. I would have to go look, but I believe 650 or so rpm is about right for idle with a manual trans. Personally I would just do what the other posters have said.

08-13-2015, 01:33 PM	#1
Isaiah92260 Registered User Join Date: Jul 2015 Location: Whittier, CA Posts: 851	How do I adjust my Carb? How do I correctly adjust my single barrel carb? I have a 67' straight 6, I'm assuming that I have a Rochester carb. My timing is at 12 BTDC and I have my idle at 650rpm's

08-13-2015, 04:39 PM	#2
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,215	Re: How do I adjust my Carb? The only external adjustment is the idle mixture & speed. Turn the idle mixture jet out a turn or 2. Then slowly turn it in with the eng. idleing & warm. Turn it in till it slows down. Then turn it back out till it speeds up to normal speed which is usually about 1/2 turn. Your speed at 650 is about right if it is a manual trans. An automatic may want about 750 in neutral

08-13-2015, 05:44 PM	#3
TJ's Chevy Post Whore Join Date: Mar 2013 Location: Southern Oregon Posts: 10,384	Re: How do I adjust my Carb? Easy way to adjust would be to run the engine until it is up to operating temp. Turn the screw in until the engine struggles to run them back it out until max engine RPM is reached. Then reset your idle speed to 500-700. __________________ 1966 Chevy C10 "Project Two Tone" http://67-72chevytrucks.com/vboard/s...d.php?t=596643 1964 GMC "Crustine" semi-build:http://67-72chevytrucks.com/vboard/s...d.php?t=665056 My youtube channel. Username "Military Chevy": https://www.youtube.com/channel/UC_h...fzpcUXyK_5-uiw

08-14-2015, 07:15 PM	#5
68gmsee Active Member Join Date: Jul 2007 Location: Centrally located between Houston, Austin and Waco. BCS area. Posts: 7,947	Re: How do I adjust my Carb? Besides the above recommendations, another thing I do is listen to the exhaust while adjusting the carb. Should be smooth with no sputtering or popping. Be aware, though, that ignition problems, bad gas, exhaust and vacuum leaks can make carb adjustments tricky or to be off so be sure you have fresh gas if the truck has been sitting for a while. Make sure spark plugs have the correct gap, points (if you have them) are gapped and set properly and all ignition parts are in good shape including the vacuum advance. __________________ 68 GMC 250/3 speed Saginaw p/b p/s 69 Chevy 350/350 currently in pieces still lookin for a cab 06 Trailblazer I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...

08-13-2015, 09:10 PM	#7
jdl71 Registered User Join Date: Oct 2012 Location: Eldon, MO Posts: 1,753	Re: How do I adjust my Carb? If you are looking for the "proper" way, the factory service manual says to set your idle speed then disconnect the PCV line and put a vacuum gauge in its place. The adjust the mixture screw until maximum vacuum is achieved. I would have to go look, but I believe 650 or so rpm is about right for idle with a manual trans. Personally I would just do what the other posters have said. __________________ Psalm 23 64 C10 66 K20

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