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Discussion Starter · #1 ·
Ok, this is the way I do it:

Turn one screw to lean out the mixture (clockwise) until engine rpm drops. Then back out the screw (counter-clockwise) to richen the mixture until the maximum idle speed is reached. Adjust for the highest rpm or vacuum then reset the idle speed. Repeat on the other screw, of course. Both screws should be adjusted nearly the same when you're done.

Am I missing something here? Do you have a better way to do it? I've got to think this has been covered before but I did a search and could not find specifics. If you have a link, let me see it please.

Mac
 

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I personally bounce back in forth between the two screws. I found that if you adjust one screw till you get the best idle then go to the other screw till it idles good that going back to the first screw and playing with it a half a turn either way will again make a difference for the better. So i say go from screw to screw keeping them within a half a turn from each other. I run my engine as slow as i can with out the vacuum guage fluctuating. If the engine speed increases with the adjustment of the mixture screws i then slow the idle back down. Make sure to have good plugs and ignition system or adjustsments will be harder to do.
 

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Discussion Starter · #5 ·
I think setting the idle mixture would be basically the same on all 4 barrel carbs, but I currently have an Edelbrock 1406 so if you know anything peculiar to that make and model that would be good to know.

Mac

Mudshark said:
What Carb???
 

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Discussion Starter · #6 ·
Yeah this is what I try to do also - but I have heard that some folks turn the screws in all the way then turn them back out 2 1/2 turns so that they start at the same place.

chrysler300le said:
I personally bounce back in forth between the two screws. I found that if you adjust one screw till you get the best idle then go to the other screw till it idles good that going back to the first screw and playing with it a half a turn either way will again make a difference for the better. So i say go from screw to screw keeping them within a half a turn from each other. I run my engine as slow as i can with out the vacuum guage fluctuating. If the engine speed increases with the adjustment of the mixture screws i then slow the idle back down. Make sure to have good plugs and ignition system or adjustsments will be harder to do.
 

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1) Is this carb on a pollution controlled vehicle?

Why do I ask, becuase according to my edelbrock big book, it won't pass emissions.

2) WHat I would do: I would start with a rich mixture and work my way down into the nice idle.
 

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Discussion Starter · #8 ·
Regarding emissions, that is by the book but it will pass emissions. I have a buddy with a Sun machine/computer that I have set up my carb to pass emissions - but this post is not really about emissions, it is about best performance. Now that it has passed emissions again for this year, I wanted to set the idle mixture the right way again.

Mac

Mudshark said:
1) Is this carb on a pollution controlled vehicle?

Why do I ask, becuase according to my edelbrock big book, it won't pass emissions.

2) WHat I would do: I would start with a rich mixture and work my way down into the nice idle.
 

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I tune with a vacuum gauge: Bottom out both idle mixture screws, and back them out 2-1/4 turns. Hook up vacuum gauge. Start engine, get fully warmed up. Drop curb idle as low as it will go and still run. Then I back off each idle mixture screw 1/4 turn at a time (alternating screws) until I read max vacuum on my gauge. Reset, idle speed, and I am done.

I usually take a look at my plugs after the first tank of gas after an adjustment. Vacuum gauge always gets me nice, light tan plugs.

-SM
 

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I use a vacuum gauge and set them for maximum vacuum, but just a little on the lean side of that. I mean, the guage reading is still at max, but I lean them both just a bit (till right before the vacuum drops).
 

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i reach over to my projection ECU and turn the knob that says "idle"
 

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Remember all absolutely all you are adjusting is mixture AT IDLE. This has very little to do with how the carb performs. 2 turn out from bottom is close enough to make the carb accelerate and run correctly. Using the vacuum and idle RPM method of zeroing it in works well, but it is preferance which is best.
 

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Discussion Starter · #13 ·
I know. Jets and metering rods and inner springs take care of a lot of the rest, however, if the idle mixture is not right it can have an effect on the overall performance of the carb.

Mac

ramchargertodd said:
Remember all absolutely all you are adjusting is mixture AT IDLE. This has very little to do with how the carb performs. 2 turn out from bottom is close enough to make the carb accelerate and run correctly. Using the vacuum and idle RPM method of zeroing it in works well, but it is preferance which is best.
 

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Well, I have always been told / taught by many more knowledgable than I to do it this way:

1) make sure both idle mixture screws are out roughly two turns & disconnect the vac advance

2) hook up tach and vacuum gage

3) start the engine and let it run till it is at normal temp. and check for fuel leaks and vac leaks.

4) check timing

5) set idle speed as desired

6) adjust mixture screws in equal amounts, about 1/2 turn or less at a time, until the highest rpm reading is obtained

7) repeat steps 5 & 6 to verify correct idle setting of mixture screws

8) with the correct timing and idle speed, now adjust for the highest vacuum reading.

9) Adjust both sides equally, and don't move them more than 1/4 of their total (ie: if they are out 2 turns, don't move them more than 1/2 turn either way)

10) repeat this until 1/4 of a turn makes a noticeable difference in either idle speed or vac reading difference.



don't forget to reconnect the vac advance when you are done.


 

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i usually use the lean drop method.

get engine warmed up, set idle to around 750, unhook vacuum advance and plug port. check your timing.

back out each screw until maximum rpm is reached, then turn each screw in until you notice a drop in rpm, then turn screw back out 1/2 turn. has always worked well for me

eric
 

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If you have acces to a gas anylizer set the mix for lowest HC`s,CO% and highest CO2 and leave it at that, thats what I do and i find it idles best/performes best that way (I have the same carb BTW).
 

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Discussion Starter · #17 ·
ChrisLib: I had to do what you suggested to get it to pass our emissions test here - had a friend with an analyzer work some magic on it, however, I noticed some intermittent bogging down when flooring it at around 55 mph. that was not there before the emissions adjustment. I wonder if the differences in our carbs not working the same when set up for lowest HC and CO is in the differences in our altitudes? I am at a hair over 5,000 ft above sea level.

TheRamChargerMan: I did it as you suggested and it is running fine. Your method makes sense to me and I think the little bog is gone but have to test it over the next week or so.

Eric: isn't 750 rpm a little low for you? It sure seems low to me on my rig. My RC seems to feel best at around 800-850 in warm weather - or did you mean 750 while adjusting the idle mixture only, then turning it back up later?


Thanks to all,
Mac
 

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Mine is running a bit rich, when its warm it chugs a little before it shuts off..you can also see it in the air cleaner. I'm not saure what all that means above, where are the adjustment screws located at?
 

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mrpeabdy1974 said:
Mine is running a bit rich, when its warm it chugs a little before it shuts off..you can also see it in the air cleaner. I'm not saure what all that means above, where are the adjustment screws located at?
It depends on what kind of carb you have. Either on the front of the carb at the base or on the sides of the carb.
 

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Mine idles at about 500-600, it works good.  Does it seem too low?
 
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