[C1] Fuel Mixture Adjustment
Thread Starter
Racer
Joined: Nov 2013
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From: Sudbury Ontario
Fuel Mixture Adjustment
I have a 58 corvette with a carter carb and the it is running rich. Can anyone offer a advice on how to best adjust the fuel and air mixture?
I was told to close them both off then back each screw 1 and half turns. Can anyone offer some advice. I should mention that I do not have vaccum guage.
Its a 58 corvette 283 4 single carb 4 barrel.
I was told to close them both off then back each screw 1 and half turns. Can anyone offer some advice. I should mention that I do not have vaccum guage.
Its a 58 corvette 283 4 single carb 4 barrel.
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From: Inverness FL
St. Jude Donor '07
I have a 58 corvette with a carter carb and the it is running rich. Can anyone offer a advice on how to best adjust the fuel and air mixture?
I was told to close them both off then back each screw 1 and half turns. Can anyone offer some advice. I should mention that I do not have vaccum guage.
Its a 58 corvette 283 4 single carb 4 barrel.
I was told to close them both off then back each screw 1 and half turns. Can anyone offer some advice. I should mention that I do not have vaccum guage.
Its a 58 corvette 283 4 single carb 4 barrel.
Bill
Thread Starter
Racer
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From: Sudbury Ontario
Race Director
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From: Inverness FL
St. Jude Donor '07
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Just want to add that it's important when finished to have each side approx. the same turns out. Also, make this adjustment after the engine is warmed very good and the plenum is hot because the fuel vaporizes better.
After you adjust the A/F emulsion screws if you have to re-adjust the engine idle speed it's important to recheck the A/F screws once the idle speed is set. Anytime you adjust the idle speed the A/F ratio changes slightly so a quick check of the side screws is a final tweak.
After you adjust the A/F emulsion screws if you have to re-adjust the engine idle speed it's important to recheck the A/F screws once the idle speed is set. Anytime you adjust the idle speed the A/F ratio changes slightly so a quick check of the side screws is a final tweak.
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From the initial adjustment of 1.5 turns out from the seat turn each screw clockwise in quarter-turn increments. When idle quality/speed drops go back out 1/4 turn. That will ensure that both screws are out the same number of turns. Then try 1/8 turn increments.
It should probably end up somewhere between 1 and 1.5 turns out, equal on both sides.
Duke
It should probably end up somewhere between 1 and 1.5 turns out, equal on both sides.
Duke
Le Mans Master
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From: Las Vegas - Just stop perpetuating myths please.
Are you trying to tune just the idle mixture? Or do you want to tune the entire power band including cruise and WOT? The idle mixture screws adjust the idle mixture only . For acceleration you have an accelerator pump and then primary jetting for cruise with secondary jetting for WOT - depending on which carb you have.
FYI modern method of tuning the entire carb uses an air to fuel monitor. Getting pretty cheap these days and they take the guess work out of rich or lean. BTW a vacuum gauge is cheap as pocket change and I don't see how you can do much old school diagnostics w/o on.
Let's us know if you need further help here. One more thing, myself I could never get a four barrel carb idle screws to adj using just RPM. I always seemed to chase the screws and RPM w/o consistent result.
Good luck.
FYI modern method of tuning the entire carb uses an air to fuel monitor. Getting pretty cheap these days and they take the guess work out of rich or lean. BTW a vacuum gauge is cheap as pocket change and I don't see how you can do much old school diagnostics w/o on.
Let's us know if you need further help here. One more thing, myself I could never get a four barrel carb idle screws to adj using just RPM. I always seemed to chase the screws and RPM w/o consistent result.
Good luck.
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Army
I stopped reading this at the first post when the OP stated he didn't have a vacuum gauge. Trying to do diagnostics and tuning on a car without this $20 item is just making your life harder for no reason. After the OP gets the proper instrumentation then I'll believe he really is interested in getting his car running properly.
From the WCFB Service Manual:
From the WCFB Service Manual:
Last edited by Frankie the Fink; Sep 10, 2016 at 05:19 AM.
Le Mans Master
Joined: Oct 2002
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From: Las Vegas - Just stop perpetuating myths please.
I stopped reading this at the first post when the OP stated he didn't have a vacuum gauge. Trying to do diagnostics and tuning on a car without this $20 item is just making your life harder for no reason. After the OP gets the proper instrumentation then I'll believe he really is interested in getting his car running properly.
From the WCFB Service Manual:
From the WCFB Service Manual:
I was thinking the same but went and replied anyways - maybe a little bored and couldn't get to sleep. But let's see if he participates. This is good topic and can be fun to learn more or better methods.Oh well.
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Army
I'm about as old school as it gets but you still need the instruments. I tune my carbs with tach/vacuum gauge and do a final tweak by walking to the rear and putting a palm up to the two exhaust pipes... If the idle is uneven, loping slightly, or, occasionally 'dropping shots' I adjust the mixture screws every so slightly to eliminate that. Sometimes its just a 1/16th of a turn on a mixture screw to get perfection.
That procedure gets lots of funny looks though...
That procedure gets lots of funny looks though...
Last edited by Frankie the Fink; Sep 10, 2016 at 07:33 AM.
Thread Starter
Racer
Joined: Nov 2013
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From: Sudbury Ontario
I'm about as old school as it gets but you still need the instruments. I tune my carbs with tach/vacuum gauge and do a final tweak by walking to the rear and putting a palm up to the two exhaust pipes... If the idle is uneven, loping slightly, or, occasionally 'dropping shots' I adjust the mixture screws every so slightly to eliminate that. Sometimes its just a 1/16th of a turn on a mixture screw to get perfection.
That procedure gets lots of funny looks though...
That procedure gets lots of funny looks though...
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The vacuum gauge wants to measure manifold vacuum. You need to "T" it into the vacuum advance (or perhaps PCV line, or unused vac port).. The measurement itself is not important, you are just looking for 'maximum' vacuum.
But back to the main point... as Cardo said.. these screws only adjust the idle mixture. If the car is 'running rich', it's not going to effect it. You might have a problem deeper in the carb??
What makes you say it's running rich?
But back to the main point... as Cardo said.. these screws only adjust the idle mixture. If the car is 'running rich', it's not going to effect it. You might have a problem deeper in the carb??
What makes you say it's running rich?
Last edited by SDVette; Sep 10, 2016 at 01:15 PM.
Thread Starter
Racer
Joined: Nov 2013
Posts: 316
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From: Sudbury Ontario
The vacuum gauge wants to measure manifold vacuum. You need to "T" it into the vacuum advance (or perhaps PCV line, or unused vac port).. The measurement itself is not important, you are just looking for 'maximum' vacuum.
But back to the main point... as Cardo said.. these screws only adjust the idle mixture. If the car is 'running rich', it's not going to effect it. You might have a problem deeper in the carb??
What makes you say it's running rich?
But back to the main point... as Cardo said.. these screws only adjust the idle mixture. If the car is 'running rich', it's not going to effect it. You might have a problem deeper in the carb??
What makes you say it's running rich?
Thread Starter
Racer
Joined: Nov 2013
Posts: 316
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From: Sudbury Ontario
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Army
I wasn't belittling you, but, you now have one of the best diagnostic (and cheapest) tools in your hand for these old motors that you will ever own. Yes, "T" into the vacuum advance rubber line and you may want to spend some time with the scenarios at the bottom of this web page to determine what it normal and what is abnormal and the various causes. If you learn just those; you'll know more than at least 20% of the folks on here...
http://www.secondchancegarage.com/public/186.cfm
Once you've learned what normal is you can tune the mixture screws (alternating between the two) for the smoothest idle at the highest vacuum.
Your symptoms may, or may not, indicate a rich mixture. Many times these old cars smell of fumes when running and black, chalky deposits in the tailpipes are absolutely NORMAL...
Your spark plugs and general performance are your best indicators of air/fuel ratio..
http://www.secondchancegarage.com/public/186.cfm
Once you've learned what normal is you can tune the mixture screws (alternating between the two) for the smoothest idle at the highest vacuum.
Your symptoms may, or may not, indicate a rich mixture. Many times these old cars smell of fumes when running and black, chalky deposits in the tailpipes are absolutely NORMAL...
Your spark plugs and general performance are your best indicators of air/fuel ratio..
Last edited by Frankie the Fink; Sep 10, 2016 at 04:49 PM.
Thread Starter
Racer
Joined: Nov 2013
Posts: 316
Likes: 6
From: Sudbury Ontario
I wasn't belittling you, but, you now have one of the best diagnostic (and cheapest) tools in your hand for these old motors that you will ever own. Yes, "T" into the vacuum advance rubber line and you may want to spend some time with the scenarios at the bottom of this web page to determine what it normal and what is abnormal and the various causes. If you learn just those; you'll know more than at least 20% of the folks on here...
http://www.secondchancegarage.com/public/186.cfm
Once you've learned what normal is you can tune the mixture screws (alternating between the two) for the smoothest idle at the highest vacuum.
Your symptoms may, or may not, indicate a rich mixture. Many times these old cars smell of fumes when running and black, chalky deposits in the tailpipes are absolutely NORMAL...
Your spark plugs and general performance are your best indicators of air/fuel ratio..
http://www.secondchancegarage.com/public/186.cfm
Once you've learned what normal is you can tune the mixture screws (alternating between the two) for the smoothest idle at the highest vacuum.
Your symptoms may, or may not, indicate a rich mixture. Many times these old cars smell of fumes when running and black, chalky deposits in the tailpipes are absolutely NORMAL...
Your spark plugs and general performance are your best indicators of air/fuel ratio..
Well now that I have the gauge, I am asking myself where do I connect it? After reading the article provided by Frankie, I see that the hose and adapters with my gauge are much smaller than some of the vacuum hose I am looking at except for the one coming off the carb feeding the distributor. BUT, if I understand this correctly, using that vacuum connection is not the appropriate connection for idle. So my question is, which vacuum connection can I use that will provide the readings I need at idle. Can anyone provide me with that information or a picture showing the actual gauge connected?
I am trying to learn but want to learn and perform these tasks accurately. I know most here are seasoned mechanics and I hope to someday be able to share what I have learned with newcomers to the corvette world. I thank you all for your patience with my ignorance in this field.
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Army
The line coming off the base of the carb to the distributor (vac advance) will work fine if it is FULL vacuum....if you hook up your gauge there and get any reading from 10-21 its fine to use.
Last edited by Frankie the Fink; Sep 11, 2016 at 06:36 AM.
Thread Starter
Racer
Joined: Nov 2013
Posts: 316
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From: Sudbury Ontario
Now is this where I use that T type connector for the hose to the distributor and the hose to the gauge coming from the base of the carb?