Ok, so I went out today and got a vacumn gauge to try to adjust my carb a little better, I still feel its not running right. Well after a while of messing around with the carb I end up with a extra sluggish barely idling engine. I just dont really know much about carbs and it seems like I just made matters worse. So I thought I would run here and maybe can find some help. Say you are completley retuning a carb, where should I start. Set the Idle screw then? And also does all the mixture screws going around the carb need to be set at the same rotations? I have a double pumper 750, and I thought I knew what I was doing, but obviously I guess I dont. Guys help would be greatly appreciated, I just want the beast to stay running.

http://holley.com/index.asp

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Did you get the carb used? If so I'd reccomend returning everything to factory specifications for a baseline. They (double pumpers) usually work pretty good out of the box. What is the List # on the front of the carb?

Hey guys, the carb is a 4150 series double pumper, it has 4 idle mixture screws going around and a screw on the throttle. It was running decent on the car until i screwed with it. Just I hear 2.5 turns on the screws is a good base to start with, is this correct? I checked out the holley site but even the instructions didnt really explain to me the proper was to adjust the mixture screws going around it

This mourning before I left for work I closed all the idle mixture screws on the blocks completely in, then I backed it open 1.5 turns out, and the thing sounded horrible, spitting and shakinglike it had a huge solid cam in it or something. I went all the way up to 4 turns going around it and I didnt notice improvement, just seemd to get worse. I manage to rev drive it back over herer to my place for it to sit now. Im kicking myself now for not writing down what the original setting was at whenever it was running half way decent. I dont have a tach so I guess i will buy one today and see if I cant get it any better. We didnt do much adjustments at first to the carb at all when we first put it on, anyone know how the idle mixture screws are set straight from the factory? my fuel pressure is also at 7 lbs so that should be fine. I went threw a few holley tuning guides on the internet but still havnt gotten any good results. I know this carb can run ok, because it was fine a few days ago before I ever touched it.

oh and sorry heres the part number 0-4779S

turn the screws in to about 3/4 turn and start from there. My 750 VS will not run at 1 1/2 turns out...it is just too rich. Make very small adjustments, about 1/8 turn and read the guage.

[This message has been edited by Greg Mc (edited September 02, 2005).]

on my old holley 750 i was running the idle mixture screws were only 1 turn out.

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1972 chevy camaro ss
425HP 350sbc old school build-up

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if it's to loud your to old

tried to adjust it again today with no luck. Is it possible for the floats to get out of wack from me just messing with the idle mixture screws?

<font face="Arial,Verdana" size="2">Originally posted by 5LiterEater:
tried to adjust it again today with no luck. Is it possible for the floats to get out of wack from me just messing with the idle mixture screws?</font>

nope !! http://www.nastyz28.com/ubb/smile.gif

Ok Thanks, One less thing to worry about. I have been adjusting all 4 points the exact same going around the carb but wouldnt the front 2 idle screws have more effect on the idle then the back 2? I am determined to get this straight this weeked, lol. My poor neighbors can just deal with a little tuning. Thanks again for all the replies guys, this is a learning experience for me.

If your timing is too low and the throttle blades are open to far to keep it running you wont get anywhere with the Idle mixture screws.

When its running like crap can you look down in the carb , do you notice any fuel dripping from any of the boosters? If so its probrably trash in the needle and seat on that float or a float to high, or fuel pressure.

5litereater,, look, here's the thing to keep in mind. ESPECIALLY with a Holley with the 4corner idle circut. Things 'should be' somewhat 'symetrical' but the Holley 4bbl is like having two 2bbls mounted on the intake. They are married only by the main body. And even tho they may not share the same meetering block and jetting, they should be very similar at idle. I mean if the right front primary idle air screw is opened 1 turn, the left front should be very nearly in the same place. It's not uncommon for the rear idle screws to be a little different than the front (not much tho), but they should be the same side to side. If your carb won't adjust and the motor run smooth that way,, something is amiss.

Get back to basics. remove the fuel level site plugs and you should have fuel at the bottom of the hole. If you shake the car, the fuel should just slosh out. Get the floats adjusted first! Then move on to the idle air.

But before you do anything, double check that you don't have any vacuum leaks. Make sure every fitting on the carb has a hose attached, or a plugging cap over it. You will never get idle air adjustments to make sense if you have a big gapping hole sucking raw air somewhere.

Assuming all that checks out. The general idea is to have the idle air adjustment screws ran in (clockwise) nearly as much as possible, and have the motor idle SMOOTH. When you adjust the idle settings you are going to have to adjust the idle speed (throttle blade opening). When you change the idle speed, you change the idle mixture setting so you have to adjust BOTH somewhat in harmony.

Get the car started and set the idle speed to something reasonable for your package. You didn't say anything about what you have there so lets assume it's a mild street application with an automatic,, adjust the idle to around 700-750rpm. Now the very FIRST thing you have to keep in mind is you can't wrench down on the idle adjustment screws. That needle will distort in the seat if you get heavy handed. When you run the screws IN to a SOFT bottomed out position, do it LIGHTLY. Start with the primaries out 1/2 turn, one full turn, whatever seems to allow the car to idle at all. Turn one screw in till the motor stumbles slightly and rpm's start to drop, then back it out (typically about 1/4 turn) to best idle. Now move to the other primary adjustment screw and do the same thing. You should be very nearly identical on both sides of the carb. If the pasengers side is out 1/2 turn,,, the drivers side should be 'close' to the same. Some older worn carbs will defy this and be sucking raw air in through a worn throttle shaft bushing so don't get crazy if they are not 'exacty' the same.

Time to READJUST THE IDLE SPEED!!! Get it back to 750rpm. Move to the secondaries and do the same thing,,, run the idle adjustment IN till the motor stumbles, and back it out 'JUST' enough for a smooth idle. Don't be too surprised if the secondaries idle smoother with the adjustment screwed in or out slightly in more than the primaries. It should be close' to the same as the primaries, but nothing says it has to be exactly the same. Remember the Holley 4bbl. TWO 2bbls on the intake.

Now re-adjust idle speed, re-adjust the primary idle settings. THATS YOUR BASELINE!!!
Now you are ready to hook up your vacuum guage and trim the idle adjustment to best vacuum. You should be within about a 1/4 turn or less from you baseline. If not, you have a leak or restriction somewhere in the carb.

Like Wilson has said. If you got the carb used, go to Holleys site and get jetting back to factory calibration. You should reduce jets one jet number for every 2000 feet above sea level. That's a STARTING POINT. BTW, are you sure that is a 4779-(S???) and not a number???? like "6"?) Make sure your initial ignition timing is correct for your package, and make SURE the vacuum advance hose is connected to the distributor (if so equipped). You can't fine tune the carb until the ignition is correct.

That should get you going unless you have other problems. Don't be afraid to stop by somewhere/someone familiar with tunning Holleys and have them check you work.

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Usually on a Holley, 1/2 to 1 turn max for best idle. Sounds like you have a blockage.

Hey Marv, thanks for the lengthy repley. I know it will be very useful today and hopefully I can get this knocked out. as for the carb number, that is what was listed on the holley site for it. Heres the direct link http://holley.com/products.asp?product=0-4779S I appreciate the help, everyone on here is always great at helping me solve problems

OK heres the update guys. I spent a couple of hours this mourning trying to get this thing straight. And I end up with a carb that runs ok until you wop on it, then some hard spitting backfires out the carb. I checked the carb floats, and Everthing seemed good, I shook the car with the plugs out and a light bit of fuel ran down the side of the carb. So then I set the timing to 10 degrees off the balancer. Seemed like the best steady Idle was around 1.25 turns out for the mixture screws. I used the vacumn gauge to check it afterwards and I was pulling around 10 out from the vacum, seems a bit low right? But turning the screws any more either way caused really ruff ideling. I checked all the vacumn lines all over, and everything is in place and no cracks on the lines. i set the idle throttle to around 1000 in park, and it drops to around 800 in drive. Fuel Pressure is at a steady 7psi. The idle sounds nice, then when you start to accelerate it begins to get throaty, and after a hard punch then it pops out. The carb is pretty much new with around 400 miles on it at most. I just put in a brand new set of plugs last weekend and all was burned equally. I know a few people mentioned an acel pump issue possibly or blockage? I would hate to go take my car somewhere to be tuned especially after me and my dad built the whole car originally off the frame, but its sorta looking that way at the moment and since Im out of state now, I cant get much assistance from him. If anyone has some more ideas this weekend let me know please. Thanks again for the intensive help

Ok, I figure I will attempt to pull apart the carb to have a look at it and clean it out good, Anybody have any tips for me on cleaning out a carb?

Dont worry about the 10 deg initial set the total timing at about 36 with the vac. advance disconnected and make sure there is no slack at all in the accelerator pump levers, maybe a little preload just make sure its not in a bind at WOT and that should do it unless the power valve is opening way too late.