Fuel and Air Screws - how to find the proper settings?

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j5ball

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The last thing I need to do with my engine is put on my elec distributor and ignition coil. I will have that done today. Once I get the timing set I will need to reset my fuel and air screws. Right now I have them set per the book - once the screws were set in I back out 4 turns (did this after I rebuilt my carb.) But how do I know these are right . .is this by ear? is there a tool? is there something I can look at? is this a feel thing? Thanks . . . JIMB
 
I read somewhere they should be about 3 turns out. Also I think your supposed to turn them in one at a time until you hear a change (like it starts to sound rough), then back it out slightly till it sounds better.
 
You need either a tachometer that reads down to 25rpm increments or a vacuum gauge. You didn't mention what carb so I'll assume either a Quad or Edelbrock. Note the rpm or vacuum reading and turn one of the screws in a 1/4 turn at a time till you notice a drop in rpm or vacuum reading then back that screw out 1/4 turn. Do the same with the other screw. If you notice your RPM climbing too high turn it back down and continue with mixture screws. If an automatic place multiple wheel chocks on tires and do this in gear with window open so you reach ignition key.
 
Slight hesitation of carb&engine out of idle state

I now have Hooker headers and Flowmaster exhaust installed. 1968 Pontiac Lemans 350 2bbl

Not sure the above statement has anything to do with my next question but wanted to put it on the table.

With my car running and idling at about 1200 RPM I can push the pedal or move the carb cable by hand . . there is a hesitation in the motor when I pull the carb cable. The engine catches up and is fine after that . . . but the slight hesitation is irritating me . . . and it only happens after an idle. If the car is already revved up and I pull the cable for more RPM the engine reacts nice. Thoughts?? Thanks . . . JIMB
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Why is your car iding at 1200 RPM, way too high? Assuming you have an automatic did you adjust the carburetor idle and mixture screws with the transmission in Drive and the wheels chocked for an ingear idle of about 700. THis will place the throttle blades in the proper position relative to the transfer slots in the carburetor. If you still have the problem after adjusting the carburetor, with the engine offf, look down the carburetor throat and see if you get a shot of gas when you move the throttle by hand. If you do not than the accelerator pump in the carburetor is shot. Inaddition make sure no active vacuum lines are disconnected and that the carburetor base bolts or nuts arent loose creating a vacuum leak.
 
I am turning the screws and can get the engine to idle at 780ish rpm in park. The manual says the cars should be reading 500rpm in park and 700rpm in drive with brake on. I cannot get the car in park to hold an idle under 780ish rpm. I looked at the timing and I think i am at 10 degrees TDC. The manual says I should be at 9 degrees TDC. Will the 1 degree make a difference in getting the car to idle at the right RPMs?


Suggestions for how to handle the screws at this point. Here is what I have been doing. Turn one screw 1/4 turn IN (to right) and wait for the engine to idle rough. If the engine does not idle rough I turn the other screw 1/4 turn IN. I continue this until I get a rough idle from one of the screws Once I get a rough idle I turn that specific screw 1/4 back out (left) then go to the other screw and try to turn IN 1/4 screw. Is this the right process ?
 
Not sure where you are reading about adjusting idle in park, it is done in drive or reverse, there has to be a load on the engine. In drive or reverse you need about 700rpm. I use a similar method to you but look at a tachometer set at a fine scale so that I can see a 25 rpm drop. If you see a 25 rpm drop turn the screw back out the 1/4 turn. Same as you first one screw and then the other. Once you get more adventurous, you should recurve your distrbutor. Mr. Gasket makes a kit with springs, weights and a stop bushing. This kit will enable you to advance the initial timing more and have the total timing come in sooner. It usually provides an obvious increase in performance.
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Like Melsg5 said, you should have the car in drive with parking break set and wheels chocked. You should also have your vacuum advance hose removed and the port plugged. Maybe go back to your timing now. With these conditions I have my initial timing at 14 deg. btdc. I know the manual calls for it to be lower, but mine idles much better where it is. Once you have your timing dialed in, then you can work on your carb. You should be using the set screw on the driver's side of the carb that is up against the throttle linkage. Turn that to get you down to like 600 rpm or as close to it. Now you can turn adjust the 2 screws at the front of the carb. On my car (also a 2 bbl) I have my points dwell set at 30 deg, 14 deg initial timing, idle in gear is 650 rpm, set screws about 2 3/4 turns out. Hope this helps. BTW check for vacuum leaks. That will prevent you from tuning your carb right.
 
Is there an idle speed screw on the linkage. It sounds like it needs to be set down. the mixture screws do not really control the idle speed
 
I cannot get my car to hold an idle in gear. Recap: 1968 Lemans, 350, Rochester 2BBL (rebuilt). I start with the fuel and air screws all the way in (turned in clockwise all the way.) When I back out (turn counter clockwise) one screw I also turn the other screw the same amount (so when one screw is back out say 2 1/2 turns the other screw is also out 2 1/2.) Backing out the screws, keeping an idle (in park) then putting the car in drive. Here are the combinations I have tried and results.

2 1/4 turns out - just dies.
2 1/2 turns out - 950 park idle - dies when moved into drive
2 3/4 turns out - 950 park idle - dies when moved into drive
3 turns out - 930 park idle - dies when moved into drive
3 turns out - 1170 park idle - holds for about 5 secs in drive then dies
3 1/4 turns out - 940 park idle - dies when moved into drive
3 1/2 turns out - 950 park idle - dies when moved into drive
3 3/4 turns out - 950 park idle - dies when moved into drive
4 turns out - 970 park idle - dies when moved into drive
4 1/4 turns out - 960 park idle - dies when moved into drive
4 1/2 turns out - 960 park idle - dies when moved into drive

From the assembly manual I see the Idle park RPM is suppose to be 800 RPM and the Drive RPM is suppose to be 650. But I now have 3 Inch hooker headers on my car and HEI ignition so I am not sure how much I should follow the 800 & 650 set up. I cannot get my car to hold and idle at 800 RPM. The RPM fluctuates by about 40 all the time - I have a digital timing gun.

Any help would be appreciated. Frustrated.

Thanks . . . JIMB
 
Maybe you should check for vacuum leaks at the carb and all your vacuum connections. Also make sure your float is set right. If it hasn't had one, your carb may need a rebuild. Can't tune it right with any of these issues. My 2 bbl had a couple of vacuum leaks, but it still idled as low as 600 rpm. Something isn't right somewhere.
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I just rebuilt the carb to specs. So I am going to assume I did it right for now. :) How do I check for vacuum leaks at the carb and for issues with my vacuum connections? While i was doing all this testing I had the vacuum line that goes to the distributor disconnected and closed off.
 
You are supposed adjust idle with the vacuum connected, only the timing is done with the vacuum to the distributor disconnected. It does sound like you have a vacuum leak, you did use a new gasket when installing the carburetor and it was the correct one? Take a can of carburetor cleaner and with the engine running in park spray the base of the carburetor to see if you get any changes in idle speed indicating a leak.
Once you have this issue resolved, start with the 2 front mixture screws turned counterclockwise about 3 turns. Set your idle speed with the screw on the side of the carburetor to 650 in gear. Now turn one of the two screws in the front clockwise about 1/4 turn. If there is an increase in speed lower it back down with the side screw to 650. Then go to the next front screw and turn it in 1/4 turn again regulate the side screw to maintain the 650. Continue going from side to side untill you notice a drop in engine speed and then turn the screw back out the 1/4 turn do the same with the screw on the other side.

You did adjust the point dwell to 30 degrees and then set the timing with the vacuum disconnected?
 
OK. Hooked back up the vacuum to the distributor. I did use a brand new gasket (came in the kit) for the carb to the intake manifold. I put in a HEI distributor so I do not have points and dwell anymore. I did TIME the car with the vacuum off. The book says 9 degrees TDC and I have mine at 10 degrees TDC. Will 1 degree make that much of a difference with the issues I am having. Off to test the vacuum leaks at the bottom of the carb. BRB.
 
Everything hooked back up. Carb juice sprayed at the bottom of the carb and no change in idle so I am fairly sure there are no leaks at the base of the carb. The car holds a real smooth idle in park between 800-825 RPMs (but it does fluctuate.) The car sounds right at about the 800-825 RPM mark. But when I put the car in drive it shuts down on me. I have the screws set at 3 turns out counter clockwise. I really can't manage the screws if I cannot get the car to idle in Drive or Reverse. Do I just crank up the idle till I can get it to hold an idle in Drive then manage the screws? AAAAAAAh:confused:
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If you rebuilt the carb and all the setting are right then I don't know. Maybe check the ends of the throttle shaft for vacuum leaks. And see if you can move the throttle shaft from side to side. Mine had a lot of play in it. It needed a throttle shaft repair kit. How about the fuel lines and hoses? Could there be a crack somewhere? Is your HEI new or used? Maybe its a spark issue. Sorry I can't be more helpful. Just throwing out ideas.
 
I appreciate all the thought Icefan. I will check the throttle shaft. I have a new fuel pump, new lines, new plugs, and new HEI. When the card is idling at 815 RPM in park and I push the throttle the engines revs up and down like I would expect . . no hesitation up or down in RPMs . .the car does not sound like its over fueled or under fueled. Its just when I move into gear and put a load on the car . . the engine shuts down.
 
Turn the mixture screws further out and set the idle speed in park to say 1200 then put it in gear and adjust the idle speed back down to 650 in gear. Then start turning the mixture screws back in 1/4 turn at a time. Are you sure you have no vacuum lines disconnected going to the interior of the car or to any engine components, no unpluged ports on the carburetor, a vacuum line going to transmission? The choke is fully open? Intake manifold bolts are tight?
 
Update. I turned the screws out 4 1/2 turns, I have the RPMs in Park idle about 1000 . . . sounds ok but you can hear its running high. It does hold an idle when put in drive/reverse - RPMs go down to about 800 RPMs. If I turn in the fuel and air screws (even a qtr turn) the engine shuts down. If I try to turn the idle screw out the engine shuts down. My numbers seem to be running about 200 RPMs to high. I have no idea what to do. I ran the car around the streets for 45 minutes and it runs great. Great pickup . . nice peel outs . . quick to 80 MPHs. I am running through some gas. Car kinda scares me:D
 
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