holley 750dp question

[69chargerR/T]

I just installed holley 750dp carb on my 440,# 4779-9 with 4 corner idle screws,when I turn the idle screws all the way in,the right front and left rear screws all most kill the engine, it stumbles alot, but the left front and right rear slows the idle down a little bit and engine stumbles very little any one have any ideas what may cause this I was thinking maybe primary throttle open to far and opening to much of transfer slots ? If so should I open secondary plates a little so I can close primary plates I know I have to pull carb to check, with idle speed set how much of the transfer slot should be showing Also when I adjust idle screws for best vacuum and idle the screws are only 1/2 to 3/4 turns out,on my 770 avenger I took off the idle screws were all most two turns out, is the idle metering that much different between the two Carb has stock jets 71s front 80s rear, took car for ride car does run good. Took car to my shop put in new plugs drove car home about 12 miles, drove it about 35 to 45 miles per hour a couple times 50, pulled a couple plugs on each side the porcelain on the plugs is white and the outer ring is a little black. Should I drive the car longer maybe on the highway at a steady 55 mph to get a better reading, because  the porcelain on the plug didn't change color at all Any one have any ideas on this, the idle screws bother me because two screws make a big difference and two do not. Car has mp cam I think its the 4120235 .484 lift witch I will be changing but not right away, can this cam be causing my tuning problems also has eddy rpm intake stock 906 heads 1 7/8 comp header 3in flowmaster exhaust trw flat top pistons and 3:91 gear.   



                                                                  Thanks, Alan                                                                   

[jg68]

 What you need to do first is, check the primary idle speed screw to see how far it is open from fully closed, then even up both F&R throttle blades, there is a screw on the back pass side under the base for the sec. throttle adj., its most likely closed & this is causing the mix screws to be off, because its getting most of its air from the primary side, i would take the carb back off & open up both F&R about 3/4 to 1 full turn, then start at 1.1/4 turns out on each mix screw, put the carb back on & fire it up, if your idle is too high, just lower them evenly, sometimes its hard getting to that back screw unless you have a tall intake with a spacer, either way, just try & keep all adjustments even.

Whats happening is, you are drawing fuel from all 4 corners with this carb vs. a carb with 2 corner idleing, but this is a good thing, it gives you a crisper throttle responce with more aggressive camshafts

[firefighter3931]

What you need to do first is, check the primary idle speed screw to see how far it is open from fully closed, then even up both F&R throttle blades, there is a screw on the back pass side under the base for the sec. throttle adj., its most likely closed & this is causing the mix screws to be off, because its getting most of its air from the primary side, i would take the carb back off & open up both F&R about 3/4 to 1 full turn, then start at 1.1/4 turns out on each mix screw, put the carb back on & fire it up, if your idle is too high, just lower them evenly, sometimes its hard getting to that back screw unless you have a tall intake with a spacer, either way, just try & keep all adjustments even.

Whats happening is, you are drawing fuel from all 4 corners with this carb vs. a carb with 2 corner idleing, but this is a good thing, it gives you a crisper throttle responce with more aggressive camshafts

Good advice 

Like Joe said the secondary idle circuit needs some tweaking. I like to use a vacuum guage when adjusting 4 corner idle setups. 

We just installed a 750dp on a 408 E-headed stroker this w/e and it runs and idles like a champ. No bog or hesitation whatsoever with full throttle from a dead stop. Box stock jetting too.  The engine is crisper at idle, part throttle is much stronger and the rich idle condition is gone.
     

The Edelbrock is on the shelf.....probably headed for e-bay along with the Demon. 



Ron

[69chargerR/T]

What you need to do first is, check the primary idle speed screw to see how far it is open from fully closed, then even up both F&R throttle blades, there is a screw on the back pass side under the base for the sec. throttle adj., its most likely closed & this is causing the mix screws to be off, because its getting most of its air from the primary side, i would take the carb back off & open up both F&R about 3/4 to 1 full turn, then start at 1.1/4 turns out on each mix screw, put the carb back on & fire it up, if your idle is too high, just lower them evenly, sometimes its hard getting to that back screw unless you have a tall intake with a spacer, either way, just try & keep all adjustments even.

Whats happening is, you are drawing fuel from all 4 corners with this carb vs. a carb with 2 corner idleing, but this is a good thing, it gives you a crisper throttle responce with more aggressive camshafts

     ig68,thanks for the info,thats what I think that the secondary throttle plates are not open enough, and the primary throttle plates  are open too much. I'm going to pull the carb and check, should I close both throttle plates then turn the idle stop screws till they touch the throttle arm and then turn the screw another 3/4 to 1 turn is that what you mean. Or should I open both throttle plates evenly till the transfer slots are just showing ? Do you know how much of the transfer slots should be showing ?


                                                                                                                       Thanks, Alan

[jg68]

What you need to do first is, check the primary idle speed screw to see how far it is open from fully closed, then even up both F&R throttle blades, there is a screw on the back pass side under the base for the sec. throttle adj., its most likely closed & this is causing the mix screws to be off, because its getting most of its air from the primary side, i would take the carb back off & open up both F&R about 3/4 to 1 full turn, then start at 1.1/4 turns out on each mix screw, put the carb back on & fire it up, if your idle is too high, just lower them evenly, sometimes its hard getting to that back screw unless you have a tall intake with a spacer, either way, just try & keep all adjustments even.

Whats happening is, you are drawing fuel from all 4 corners with this carb vs. a carb with 2 corner idleing, but this is a good thing, it gives you a crisper throttle responce with more aggressive camshafts

     ig68,thanks for the info,thats what I think that the secondary throttle plates are not open enough, and the primary throttle plates  are open too much. I'm going to pull the carb and check, should I close both throttle plates then turn the idle stop screws till they touch the throttle arm and then turn the screw another 3/4 to 1 turn is that what you mean. Or should I open both throttle plates evenly till the transfer slots are just showing ? Do you know how much of the transfer slots should be showing ?


                                                                                                                       Thanks, Alan

Allen, about an EVEN square is what you like to start off with on the transfer slots, but you want to make sure both F&R are even on turns, so just start in the front & start closeing the throttle untill its fully seated, you'll notice the throttle bracket stop moving, i useally work the screw back & forth a few times to find out exactly were it stops, the back will be trickier, because all it has is alittle (flat head) screw under the base by the sec. arm (where the cam is installed), do the same thing with the rear, i would open them both up 1 full turn, now if its idleing too high or too low, of coarse you'll have to re-ajust, just do it in 1/4 turn incraments, now holley sells a nice sec. throttle (idle) adjuster for 32.95 part #26-137, this will make life much easier

What i can't understand is why holley is so slow to offer these parts on there 4 corner idle carbs , the o'le mighty dollar i'm sure

[69chargerR/T]

What you need to do first is, check the primary idle speed screw to see how far it is open from fully closed, then even up both F&R throttle blades, there is a screw on the back pass side under the base for the sec. throttle adj., its most likely closed & this is causing the mix screws to be off, because its getting most of its air from the primary side, i would take the carb back off & open up both F&R about 3/4 to 1 full turn, then start at 1.1/4 turns out on each mix screw, put the carb back on & fire it up, if your idle is too high, just lower them evenly, sometimes its hard getting to that back screw unless you have a tall intake with a spacer, either way, just try & keep all adjustments even.

Whats happening is, you are drawing fuel from all 4 corners with this carb vs. a carb with 2 corner idleing, but this is a good thing, it gives you a crisper throttle responce with more aggressive camshafts

     ig68,thanks for the info,thats what I think that the secondary throttle plates are not open enough, and the primary throttle plates  are open too much. I'm going to pull the carb and check, should I close both throttle plates then turn the idle stop screws till they touch the throttle arm and then turn the screw another 3/4 to 1 turn is that what you mean. Or should I open both throttle plates evenly till the transfer slots are just showing ? Do you know how much of the transfer slots should be showing ?


                                                                                                                       Thanks, Alan

Allen, about an EVEN square is what you like to start off with on the transfer slots, but you want to make sure both F&R are even on turns, so just start in the front & start closeing the throttle untill its fully seated, you'll notice the throttle bracket stop moving, i useally work the screw back & forth a few times to find out exactly were it stops, the back will be trickier, because all it has is alittle (flat head) screw under the base by the sec. arm (where the cam is installed), do the same thing with the rear, i would open them both up 1 full turn, now if its idleing too high or too low, of coarse you'll have to re-ajust, just do it in 1/4 turn incraments, now holley sells a nice sec. throttle (idle) adjuster for 32.95 part #26-137, this will make life much easier

What i can't understand is why holley is so slow to offer these parts on there 4 corner idle carbs , the o'le mighty dollar i'm sure

ig68,thanks again, thats what I thought you meant, I will pull carb tomorrow make adjustment I want car to idle at like 850 or so, so when in gear it will idle about 700 to 750 rpm. After I adjust throttle plates if my idle rpm is close to what I want, say with in 50 to 100 rpm can I just turn the primary idle screw to get the rpm to what I want ? It seems it would be a pain to adjust both idle screws to get the idle exactly too the rpm that I want

                                                         Thanks, Alan       
   

[jg68]

Yeah Allen it is, so if your close, then just work with the primary throttle

[69chargerR/T]

Yeah Allen it is, so if your close, then just work with the primary throttle

  jg68, thanks for your help  I checked out that secondary idle adjuster, looks like it makes the job a lot easier Got one coming from summit !!


                                   Thanks alot, Alan