Question about adjusting idle fuel mixture
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Question about adjusting idle fuel mixture
I've read that the best way to adjust your idle fuel mixture is by using a vacuum gauge and setting the screws to the point where the vacuum is maximized. I've also read that if you don't have a vacuum gauge you can get fairly close by adjusting each to the point where the engine revs sound the highest (and then readjusting the idle speed afterwards). I imagine if you have a timing light with a readout of RPM (or a helper that can watch the tach), you can be actually be very precise with the second method.
My question is, does the timing light with readout / using the tach work just as well as the vacuum method? Maybe this obvious to most, but I do not know for sure if max vacuum occurs at max RPM... but I would naively think that is the case.
Thanks.
My question is, does the timing light with readout / using the tach work just as well as the vacuum method? Maybe this obvious to most, but I do not know for sure if max vacuum occurs at max RPM... but I would naively think that is the case.
Thanks.
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I've read that the best way to adjust your idle fuel mixture is by using a vacuum gauge and setting the screws to the point where the vacuum is maximized. I've also read that if you don't have a vacuum gauge you can get fairly close by adjusting each to the point where the engine revs sound the highest (and then readjusting the idle speed afterwards).
Thanks.
Thanks.
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I guess my main question is if max vacuum occurs at max RPM when adjusting these screws.
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Idle screws
Dr. Detroit
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GM called it the "idle drop method", and it works.....but you need to back them out just a tad from best RPM. The vacuum method works better as once you reach highest vacuum.....you are done.
Jebby
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You need to understand that curb idle speed (with choke off and engine warmed), timing settings, and idle jet adjustments are INTERACTIVE. You first need to set ignition timing/distributor/vacuum can as needed. Then you need to set curb idle speed. Lastly, you adjust the idle mixture screws.
If the curb idle changes when mixture screws are adjusted, you need to repeat the sequence until there is no change in idle speed.
The most important part of this process is adjusting the mixture screws to where 1/4 turn of a mixture screw will decrease the idle speed somewhat, no matter which direction you turn it. So you are searching for an 'optimum' position of those mixture screws. IMPORTANT: If you cannot find an optimum setting for the idle screws, you most likely have a vacuum leak somewhere or an intake gasket that is sucking excess air into the engine.
You can use the vacuum gauge method as an approximation of where it should be set. But that won't help you find if you have 'other' issues with the air/fuel intake mixture.
If the curb idle changes when mixture screws are adjusted, you need to repeat the sequence until there is no change in idle speed.
The most important part of this process is adjusting the mixture screws to where 1/4 turn of a mixture screw will decrease the idle speed somewhat, no matter which direction you turn it. So you are searching for an 'optimum' position of those mixture screws. IMPORTANT: If you cannot find an optimum setting for the idle screws, you most likely have a vacuum leak somewhere or an intake gasket that is sucking excess air into the engine.
You can use the vacuum gauge method as an approximation of where it should be set. But that won't help you find if you have 'other' issues with the air/fuel intake mixture.
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You need to understand that curb idle speed (with choke off and engine warmed), timing settings, and idle jet adjustments are INTERACTIVE. You first need to set ignition timing/distributor/vacuum can as needed. Then you need to set curb idle speed. Lastly, you adjust the idle mixture screws.
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When you rev your engine the vacuum will drop so you need to synchronise the idle mixture screws with the idle setting for your engine ,so the vacuum gauge is the best method 
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Sorry if I'm causing confusion... I'm not talking about revving the engine... I'm talking about monitoring the engine's RPMs as you adjust the idle mixture screws at a fixed idle screw setting. As you adjust the idle fuel mixture screws the engine RPMs change... I believe, and leadfoot backed it up, that the point when the engine RPMs are maximized, a vacuum gauge would also show a maximum. Thus, I believe both methods yield the same result. If that is not correct, someone please let me know why. Thanks!
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The vacuum gauge method does not give you any indication of a possible vacuum leak in your system. And that is one of the most common ailments of internal combustion engines. Finding an optimum setting for the idle screws WILL prove to you that there is no significant leakage of air into the intake charge (vacuum leak).
As I stated previously, the "maximum vacuum" method is fine, IF YOU HAVE NO VACUUM LEAK. But, this choice is yours. It is easy to do either method, but I much prefer getting the idle screws to demonstrate that idle air can be optimized using them.
As I stated previously, the "maximum vacuum" method is fine, IF YOU HAVE NO VACUUM LEAK. But, this choice is yours. It is easy to do either method, but I much prefer getting the idle screws to demonstrate that idle air can be optimized using them.
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And your point about a vacuum leak is a good one... hadn't really considered that. By the way, what should the vacuum gauge read if there are no leaks? I'm going to get a gauge and check this.
Dr. Detroit
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From: New Braunfels Texas
A vacuum gauge most certainly will detect a vacuum leak.....if the needle bounces and the engine leaks down ok (Healthy)....then there is a leak.
The RPM method and vacuum method are not the same in the fact that the highest vacuum is not achieved at the first indication of highest RPM......if you do adjust it that way, you have to turn the screws out just a tad.....that will be close to stoichiometric....The vacuum method is best because once the highest vacuum is achieved.....that is it....that will be peak efficiency.
And as stated.....timing has to be set first. Do not set to factory card.....set to achieve 36 BTDC @ 3000 rpm.....wherever the initial falls....it falls.....
Also check for proper vacuum can operation....you want to pull 12 degrees or so with the can.....at idle....so check the difference between timing with the hose connected and disconnected.
Jebby
The RPM method and vacuum method are not the same in the fact that the highest vacuum is not achieved at the first indication of highest RPM......if you do adjust it that way, you have to turn the screws out just a tad.....that will be close to stoichiometric....The vacuum method is best because once the highest vacuum is achieved.....that is it....that will be peak efficiency.
And as stated.....timing has to be set first. Do not set to factory card.....set to achieve 36 BTDC @ 3000 rpm.....wherever the initial falls....it falls.....
Also check for proper vacuum can operation....you want to pull 12 degrees or so with the can.....at idle....so check the difference between timing with the hose connected and disconnected.
Jebby
Last edited by Jebbysan; Dec 21, 2019 at 10:44 AM.
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Confusion
Sorry if I'm causing confusion... I'm not talking about revving the engine... I'm talking about monitoring the engine's RPMs as you adjust the idle mixture screws at a fixed idle screw setting. As you adjust the idle fuel mixture screws the engine RPMs change... I believe, and leadfoot backed it up, that the point when the engine RPMs are maximized, a vacuum gauge would also show a maximum. Thus, I believe both methods yield the same result. If that is not correct, someone please let me know why. Thanks!
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Your carb setup could be different than mine but for my Holley....
Ignition timing is set first. I would also advise verifying the float levels first before any carb adjustments. Then you can set the mixture screws 1.5 turns out from lightly seated. Adjust the mixture screw adjustments at that point, then setting the RPM where it belongs, then recheck the mixture screws. Make sure the accelerator pump linkage has zero slack at idle and some available additional travel when at wide open throttle to prevent damage. Done.
If you are still out of adjustment, the carb can come off to verify the transition slots are not over or under exposed. You can play with the secondary butterfly stop screw a little if you need more or less primary airflow to get the primary transition slot just right.
Then leave it alone.
Ignition timing is set first. I would also advise verifying the float levels first before any carb adjustments. Then you can set the mixture screws 1.5 turns out from lightly seated. Adjust the mixture screw adjustments at that point, then setting the RPM where it belongs, then recheck the mixture screws. Make sure the accelerator pump linkage has zero slack at idle and some available additional travel when at wide open throttle to prevent damage. Done.
If you are still out of adjustment, the carb can come off to verify the transition slots are not over or under exposed. You can play with the secondary butterfly stop screw a little if you need more or less primary airflow to get the primary transition slot just right.
Then leave it alone.
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From: Charlotte NC
Just to add my experience to the mix - when all is said and done I like to turn my mixture screws in JUST A TAD for a slightly lean idle. To me it helps make the motor more responsive to cracking the pedal, keeps the plugs cleaner AND it is a little less rich out the back. We are used to the clean running exhaust of our DD so our "fun cars" usually bring tears from the fumes.
And there is no "normal" vacuum reading, depends on your cam. BUT my 70 stock LT1 350 pulls 22 in at 750 rpm. If you have a hot cam you may only get 14 in.
And there is no "normal" vacuum reading, depends on your cam. BUT my 70 stock LT1 350 pulls 22 in at 750 rpm. If you have a hot cam you may only get 14 in.
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Just to add my experience to the mix - when all is said and done I like to turn my mixture screws in JUST A TAD for a slightly lean idle. To me it helps make the motor more responsive to cracking the pedal, keeps the plugs cleaner AND it is a little less rich out the back. We are used to the clean running exhaust of our DD so our "fun cars" usually bring tears from the fumes.
And there is no "normal" vacuum reading, depends on your cam. BUT my 70 stock LT1 350 pulls 22 in at 750 rpm. If you have a hot cam you may only get 14 in.
And there is no "normal" vacuum reading, depends on your cam. BUT my 70 stock LT1 350 pulls 22 in at 750 rpm. If you have a hot cam you may only get 14 in.