How do I adjust my Carb?

[Isaiah92260]

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]

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]

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.

[jdl71]

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.

[prostreetC-10]

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]

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]

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?

[crazy longhorn]

Originally Posted by Isaiah92260

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

I would say .017 to .018 on the points....plug gap around .032? If I missed it, no issue....the rest of the guys here, will fix it longhorn

[TJ's Chevy]

Crazy Longhorn nailed it right down the coffin.

.017-.018 on the points and about .030-.035 on the plugs.

[Isaiah92260]

Originally Posted by TJ's Chevy

Crazy Longhorn nailed it right down the coffin.

.017-.018 on the points and about .030-.035 on the plugs.

Thanks Long Horn and T.J. I did the carb, I believe I did it right. It's 118 outside in Palm Springs and my truck was idling at 190 when I set the carb, I set the idle at 650 and turn the F/A mixture screw in until the truck was barely turning, then all the way back to 650 RPM's. It seems good, I've noticed my truck vibrates a lot when at idle, my mirrors are blurry, that's how much it vibrates :/ is that because of my engine mounts need to be tightened? None are broken, the driver side mount was loose and I have it a good tighten a few weeks ago. Sorry for going off topic BTW, I have a million questions about my truck, I need a good Chevy guy to come look at my truck for free one day to answer all my questions hahah

[68gmsee]

Yeah, loose mounts can cause engine vibration but there are other causes. If you have a manual transmission, the flywheel or clutch/pressure plate may be out of balance. The crankshaft, front engine pulleys and the h-balancer could be the culprit and don't overlook failures in ignition. This can cause vibrtions also.

Did you check the exhaust to see if there was any popping or sputtering?

Finally, internal engine problems like difference in compression can also cause vibration.

[TJ's Chevy]

Originally Posted by Isaiah92260

Thanks Long Horn and T.J. I did the carb, I believe I did it right. It's 118 outside in Palm Springs and my truck was idling at 190 when I set the carb, I set the idle at 650 and turn the F/A mixture screw in until the truck was barely turning, then all the way back to 650 RPM's. It seems good, I've noticed my truck vibrates a lot when at idle, my mirrors are blurry, that's how much it vibrates :/ is that because of my engine mounts need to be tightened? None are broken, the driver side mount was loose and I have it a good tighten a few weeks ago. Sorry for going off topic BTW, I have a million questions about my truck, I need a good Chevy guy to come look at my truck for free one day to answer all my questions hahah

Drive that bad boy up here to my house and we'll tinker with it for a day!

[Isaiah92260]

Originally Posted by TJ's Chevy

Drive that bad boy up here to my house and we'll tinker with it for a day!

I would if you didn't live hundreds of miles away Hahah

[Isaiah92260]

Originally Posted by 68gmsee

Yeah, loose mounts can cause engine vibration but there are other causes. If you have a manual transmission, the flywheel or clutch/pressure plate may be out of balance. The crankshaft, front engine pulleys and the h-balancer could be the culprit and don't overlook failures in ignition. This can cause vibrtions also.

Did you check the exhaust to see if there was any popping or sputtering?

Finally, internal engine problems like difference in compression can also cause vibration.

My exhaust sounded very good, no sputtering or popping, very smooth and smells like a 47 year old motor lol. I did notice that my truck was running a little hotter than usually, maybe because it was 116 outside today haha, but a little hotter than the past few days, maybe I should tighten the F-A mixture screw a little?

[TJ's Chevy]

If its just idling around in 116 weather it will run a bit hotter than normal. I would leave it as is if its running that good. Might double check your water level. Leaning it out will make things hotter.

[Isaiah92260]

Originally Posted by TJ's Chevy

If its just idling around in 116 weather it will run a bit hotter than normal. I would leave it as is if its running that good. Might double check your water level. Leaning it out will make things hotter.

Your right, in the morning she stays so damn cool, I wish it was like that all the time... No my radiator coolant is pretty full.

[TJ's Chevy]

Its just the heat then. lol You were probably overheating more than the engine was! lol That's some killer weather!

[68gmsee]

Originally Posted by Isaiah92260

Your right, in the morning she stays so damn cool, I wish it was like that all the time... No my radiator coolant is pretty full.

My 68 had the same problem. It has the original radiator. I had it rodded out several years ago and it still did the same thing so I rigged up an overflow reserve tube and problems solved. You might want to try the same on yours. The only thing you have to make sure of is that you have a radiator cap that supports it.

I got my parts from local auto stores and Lowes. The total cost was less than $20 including the cap. Or you can do it professionally with parts from internet.

Here's some pics. First one is the cap needed. (the 2nd pic shows a temp cap I had for testing). Second is where I mounted it and last pic shows the parts I used.

[Isaiah92260]

Originally Posted by 68gmsee

My 68 had the same problem. It has the original radiator. I had it rodded out several years ago and it still did the same thing so I rigged up an overflow reserve tube and problems solved. You might want to try the same on yours. The only thing you have to make sure of is that you have a radiator cap that supports it.

I got my parts from local auto stores and Lowes. The total cost was less than $20 including the cap. Or you can do it professionally with parts from internet.

Here's some pics. First one is the cap needed. (the 2nd pic shows a temp cap I had for testing). Second is where I mounted it and last pic shows the parts I used.

That's awesome! What does a over flow system do for the cooling? Do I add cola t it others as well or does it need to be empty?

[Isaiah92260]

And what do you mean I need the right radiator cap? What does that do for the cooling? I thought a cap was just a cap, mine says 14lb-16lb on it btw, I still not sure what that means.

[TJ's Chevy]

Radiator caps are rated at how many pounds of pressure they'll take before they allow "overflow" water to come out. All the caps have springs on them...that spring allows water to come out as "overflow." Usually if the engine overheats that will happen. You want a tight rad cap because if could be letting water out for no reason which will create less water flow. Caps get old and weak through the years.

[68gmsee]

Originally Posted by Isaiah92260

And what do you mean I need the right radiator cap? What does that do for the cooling? I thought a cap was just a cap, mine says 14lb-16lb on it btw, I still not sure what that means.

Okay. There are several psi rated pressure caps for our trucks. I don't remember the psi on my 68 with 250 but it is different than for the v8's. An auto parts store will be able to look that up for you.

The cap has to be able to not only let fluid out to the reservoir tank when hot but also allow it to be sucked back into the radiator as it cools back off. This is where the reservoir tank comes in. This keeps the radiator full and minimizes fluid loss which helps the cooling. Again, a good parts store will know which cap you need.

Check here for some info on this.