Trying to set my new 750 DP

[71chmark]

After awhile (5 miles) my car started stalling at stop signs.  The Holley directions said the fuel should just dribble out the sight hole and that's what it did.  First I tried the idle mixture adjustments.  The directions said to adjust to 1 1/2 turns open and adjust for best vacuum from there.  I don't have a vacuum gage so I thought I would just see what happens.  They were originally set at 1 turn on the front and 1/2 turn on the back.  That didn't help and then it would make the car jerk like crazy in fourth gear.  I checked the plugs and they were fouled really bad.  By brother in law told me the fuel shouldn't come out the sight hole so I adjusted the float to just below coming out the sight hole.  That fixed the idle problem (finding the two plug wires I hadn't hooked up right after cleaning the plugs probably helped too).  It still jerked alot in fourth gear so I set the idle mixture screws back to where the were originally and it runs a lot better though it still jerks very slightly in fourth gear. I'm not sure where to go from here.  I've never messed with a carb before so I'm in virgin territory.  I also can't tell you how many times I shifted out of fourth looking for fifth even though I only have 3.23 gears.  I'm going to get some new pictures and post them on the restoration page later.  Did anyone make it through this novella?

[422STROKER]

I would pick up a vacuum guage, it's about 10 dollars at your nearest Vatozone or similar parts store. 

Tom

I thought the 2 idle screws were a pain on my Vac sec carb, you got all 4 to do.

[MEK-Dangerfield]

Idle screws should only be dealt with when the engine is plenty warm. They would cause your stalling at a red light if they were too lean. Up until now, I have just listened to how the engine was idleing, as I tried to turn each inward. Turning inward will make the mixture leaner, so if you go too far, the idle will drop fast.  I have to think you aren't far from reaching your goal. The jerking you speak of, and the black plugs, tells me your jets are too big in you primaries.


  Mike

[nivvy]

Idle speed screw is different from idle mixture screws... your 4 idle mixture screws should ALL be the same amount out...... (Square)

You should not see any fuel dribbling down into the carb...


From a carb tech site:

If the mixture screw is more than 2 1/2 turns out turns then the Idle jet is too lean (too Small). When the mixture screw is less than 11/2 then the Idle jet (IFR) is too rich (too large). These assumptions are based on the fact that the Idle speed screw setting is not opened more than 11/2 turns. If the Idle speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. This is what causes the visible rich condition, and confirms the need to increase the jet size.

[matt63]

I'm certainly not an expert (and still struggling with my 750 tuning) but it is my understanding that the idle mixture screws don't have anything to do with how the car runs at part or full throttle.  I have found that float level makes a big difference on plug fouling.  I wonder if a low float level could cause stalling as you come to a stop as well as part throttle jerkiness?  I have also read that the idle screws should be turned in until the engine rpm drops then turned back out 1/2 turn.  This seems to get things close too. 

[Chryco Psycho]

I need to write a book on carb tuning there is so much to it
 What Stroker is saying is essentially true , you may need to tune the idle circuit , especially with big cams & low manifold vacuum but 90% of all carbs do not have an adjustment to do this , the new Holley HP , Proform & AED carbs have removable bleeds NOT jets to adjust this , what it does is control how much fuel is pulled in with vacuum by creating an air leak in the idle circuit .For example on the same engine making say 16" of vacuum if you use a smaller bleed the vacuum has more effect on the amout of fuel pulled in & will run rich , conversly if you use a larger bleed the vacuum has less effect & runs leaner , now assuming the carb was tuned rigth from the builder for 16" of vacuum & you engine is only making 6" of vacuum [in gear] the engine will idle badly requiring more RPM to keep it running But if you install a smaller bleed the lower vacuum will pull more fuel & compensate . This is easy to check by simply putting your fingers over the bleeds for a couple of seconds blocking the bleed  & see if the car runs better or worse . If your carb doesn`t have removable bleeds my trick is to insert a small diameter piece of wire into the bleed partially restricting it , this will often make an engine idle Far better immediatly . Idle mixture does effect part throttle as well as the circuit doesn`t quit working in part or full throttle but the effect is minimal , often when I am breaking in engines at 2000 rpm I will adjust the mixture in or out to see if the engine smooths out , , if you have to turn the screws in the jetting is too rich so down jet 1-2 sizes , or the opposite if turning the screws out help step the jets up 1-2 sizes . BTW Matt you are correct turn the idle mixture screws in evenly until the engine changes & drops RPM then turn it back 1/4 turn or so to smooth the engine back out