SpyderD

I have a Holley 4777 model 650 DP with a large cam on my 350. Now I have read several times that Holleys have some screw to the rear and below the carb to open the secondaries a bit more at idle so I don't have so much of the transfer circuit open on the primaries. I've pocked around there plenty and no way can I find it.
My dilema is this - I would like to raise my idle speed a bit more from the 550-600 range. I have a large cam that seems happier at a higher idle speed )although it sounds downright mean at my current idle speed. I also have a slight hesitation (very slight) when I press the throttle about 2/3 of the way from idle or just off idle. I would like to turn up the secondaries a little bit then turn down the primary idle opening so the transfer circuit is not showing.
I get about 5 or 6" of vacuum off the ported source (about 12" from the manifold) at idle which is telling me to much of the transfer circuit is showing at idle (I think) from the idle speed screw. I have the idle mix screws turned out 4 turns for the best vacuum, when I turn it in clockwise all the way the car stalls, after about 4 turns out (to richen) it has no effect on vacuum. The darn thing wants to eat more gas at idle.
Now please note - maybe I am being a bit picky. The car idles fine altough loopy (expected in a large cam), never stalls, the plugs are a healthy off-white color, maybe a little bit too white, and the car can tear up the road when I step on it. I would always like improved part throttle response so that is why I want to fine tune the idle circuit. Also - I must have the only Holley double pumper in existence that runs LEAN (particularly at idle).
So - where is that mysterious screw? Maybe someone can provide a picture or diagram

comp

i believe you get to yours from the bottom of the baseplate

SpyderD

Do I have to take the carb off? I see screws to keep the secondary pump on, nothing else on the bottom, unless it is hidden.

clem zahrobsky

just bend the tab that makes contact with the stop screw on the secondary. holley sells a special part to change the idle speed on the secondary part# 26-137 :chevy


[Modified by clem zahrobsky, 5:55 PM 3/14/2004]

norvalwilhelm

On most holley carbs you take the carb off the intake, turn it upside down and there is a little screw, a set screw in the base plate that takes about a 1/8th wide slotted screw driver and you turn the screw in to increase the secondary opening.
The carb can not be adjusted without removing it.

Chuck Harmon

Hmmm, I just adjusted mine last night with the carb mounted. Not convenient to get to, but doable.

Chuck

SpyderD

Hmmm, still confused. Can someone offer me a "dummies guide to finding the secondary opening screw"? I've heard that you do not have to take the carb off.

Perfomer

I had this same problem, solved it by drilling one hole in each primary butterfly (1/64) this allowed more air at idle and let me back the idle screw off so that the idle circuit slots were not exposed. You might also put in a 4"hv power valve and to solve your off idle stumble go to a 30 squirter

clem zahrobsky

just bend the little metal tab on the linkage with a pair of needle nose pliers. with the engine off open the secondary and bend the tab that sets on the adjustment screw. if you remove the carb replace the screwdriver set screw with a allen set screw with the hex to the top so you can adjust it without removing the carb. like i posted above holley sell a new piece of linkage so you can adjust the secondary idle speed from the top. :chevy

Brettmc

Get this little jewel:

http://www.jegs.com/cgi-bin/ncommerc...81&prmenbr=361

It makes tuning you Holley a breeze. I got one a couple of months back...it's well worth it!!

Brett :thumbs:

The Dude

The screw head (well, it's not really a "head"--I should say the "slotted end") is located under the carb on the passenger side. Just look for the secondary shaft on the passenger side. There will be a 90-degree tab on the shaft there that looks like it's resting on the baseplate. Now open the secondaries. See the little "tit" sticking up? That's the bottom of the screw. There's no need on God's Green Earth to buy any kits or any additional parts to put the adjustment on top if you plan to make lots of adjustments to this for some reason. Just reverse the screw--take it out from the bottom and then screw it in from the top. I see this all the time and I do it myself on my 4150. Bang, boom there you go.

Now, having said that I have to ask this question. If, as you say, your carb is lean at idle, why in the world do you want to make it leaner by adjusting the secondaries to add more air to the mix? Usually this, among other things (like drilling the primaries, adjusting the secondaries and putting some wire into the idle feeds to restrict the amount of fuel at idle) is done when a car idles so rich that it makes your eyes burn no matter what you do.

Your hesitation off-idle is probably a momentary lean bog from your primary accelerator pump circuit that will just make the bog worse with the secondary adjustment you want to do. First try increasing the size of your primary shooter and then play with your primary pump cams. Each color has a different profile and two positions in which it can be mounted.


BTW, your idle circuit is just that--for idling. See, carbs have several circuits that come into play sequentially. BASICALLY, when you're idling, for example, you are on the idle circuit. When you hit the throttle, you get past the "transfer" and move onto the idle circuit and the primary circuit, which includes your accelarator pump, shooter and then the primary jettting, in that order. These all work together. Go deeper into the throttle and the idle and primary circuits are joined by the secondary circuit, which includes your other accelerator pump, the secondary shooter and then the secondary jetting, again in that order.

Tune circuit-by-circuit: Idle, Primary (shooters & cam), Secondary (shooter & cam) and then experiment with jetting, primary first and then secondary.

LOL. That was more than you wanted to know and it's just a very broad thumbnail. Have fun. :seeya


[Modified by The Dude, 11:35 PM 3/14/2004]

SpyderD

Dude - thank you for the information. I understand some of the basics of carb operation but I have yet to learn all the details. Yes it runs lean at idle but what I want to do is increase the idle RPM speed by opening the secondaries which, and maybe I am incorrect with my assumptions, will add BOTH more gas and more air to the mix to increase idle speed. The slight bog, and again this is my theory, is the result of the primary transfer circuit not bringing in the full amount of fuel and air is was designed to because it is already partially open at idle, bringing in both air AND fuel from the transfer circuits. Thus the transfer runs lean before the primary circuits have a chance to take over once I get off idle.

Now changing the shooter and pump cam will increase the fuel in the primary (past idle/transfer) circuits, right? That is not where my problem is, I would say my jetting and pump shot are right on the spot, at least until I get my idle and transfer adjusted. I need to start on my idle first, then my transfer circuit, THEN the primary circuits.

Now saying all this, please correct me if my assumptions are incorrect. Also note that any changes I make at this point are fine tuning. I am very happy with the performance of my carb.



[Modified by SpyderD, 2:06 PM 3/15/2004]

Brettmc

If you have too much transfer slot exposed at idle, you will get bog or hesitation on off-idle tip in. I had that problem on my 850 DP. After I got the transfer slots exposure balanced between front and rear, it went away. Don't even think bout turning that little screw out and putting it in thru the top. That's still gonna be a pain to adjust. Get the secondary adjuster from Jegs. It's well worth the money spent in time and aggravation saved. It's really easy to install too.

Brett :thumbs:

The Dude

Only more air. It's like drilling the 1/16"-3/16" holes in the transfer slot of the primary butterflies. Now, if you shut the secondaries DOWN (i.e. close them a little more, if possible,) that may help your lean idle.

Right you are about the sequence. It's possible your bog is caused by having too much of the transfer slots exposed. But you say you're idling at 500-600 RPM. that sounds a little siow but if your engine's happy there, WTH. Some of the transfer slot should be exposed after you're done with your bench adjustments, anyhow--about .15" - .20". then, with the car running, adjust the idle screws back and forth for vest vacuum. Usually the engine will speed up after the first go-round. Then you adjust the primaries (closed) to slow it down and go back and do it again. Then adjust the primaries again. Keep going until ANY change to either of the idle mixture screws hurts vacuum.

Now saying all this, please correct me if my assumptions are incorrect. Also note that any changes I make at this point are fine tuning. I am very happy with the performance of my carb.

[Modified by SpyderD, 2:06 PM 3/15/2004][/QUOTE]

SpyderD

My car is OK at 550-600 RPM idle but I think it will be happier at higher RPM. My idle mix screws will LEAN out the mix but will not richen it up past 4 turns counterclockwise. So the secondary opening won't provide more fuel? Maybe I am approaching this wrong and I just need to go ahead and increase the idle speed even if it does expose the transfer slot (I just don't want it to start pulling open my vacuum advance wich is connected to ported vaccum)....also, I notice that increasing the initial timing helps but I have a fixed 20 degree mechancial advance so I am limited to about 16 initial or I start pinging at load.
The bog I can then adjust with accellerator pump as you stated.

The Dude

Cracking the secondary butterflies open with the adjustment will only let in more air. You usually do this after you've drilled holes in the primaries and/or restricted the idle feed passages in the metering block if you're idling really, really, eye-burning, smoke detector-activating rich. Doesn't sound like you want to be leaner at idle.

Get your timing locked down and try just setting the idle speed wherever you like it. Then do your mixture screws, go back and slow it down with the idle speed screw, etc. The .15" -20" of exposed transfer slot is like everything else--a guideline to make you remember that more closed is "better" than more open. Then drive it like that for a day or two and see what it tells you. It may want to diesel when you shut it off. If so, your idle may be too high. It may be fine. 'Sperimenting is all part of the fun.

As for reversing the secondary adjustment screw being a pain to adjust...well, you move the lever and turn the screw. It's can't get much easier than that, especially since youshouldn't have to be doing it that often. But then, it's not as cool as waiting for the UPS truck to drop off a package from Summit or Jeg's. Whatever works.


[Modified by The Dude, 4:04 PM 3/15/2004]

lostpatrolman

Im no carb expert, but i thought you should never have to pull your mixture screws out past 2 turns. If so you either need a rebuild or possibly a bigger carb. He is saying his are 4 turns out. Can someone enlighten me?

stickboy

If you haven'y done so yet, read this post from Lars:

http://forums.corvetteforum.com/zerothread?id=767841

This should help you out

SpyderD

Lostpatrolman - I used to have a 750DP that would never work right at any speed but for the last six months I have been running this new out of the box (actually, reconditioned) 650. I have a 355 cu engine.
I have no doubt I have the flow (large valve heads, headers, high compression, etc) to make use of the 750 but that would take even more tweaking to work at idle and off idle. The 650 works much better than the 750, at lest at "out of the box" settings.
After 4 turns out of the idle mix screws the vacuum and idle do not increase, which is telling me the idle mix is completely open. Like I said the darn thing wants more gas, my 750DP I used to have would be practically spitting out raw gas at idle and foul up plugs after an hour drive. This 650 is just the opposite.

Stickboy - ahh Lars the Carb God! I have practically memorized everthing he wrote. Good information, what that tells me is both secondary and primary need to be opened the same amount but I am wondering still if that will help me run richer at idle.

norvalwilhelm

Im no carb expert, but i thought you should never have to pull your mixture screws out past 2 turns. If so you either need a rebuild or possibly a bigger carb. He is saying his are 4 turns out. Can someone enlighten me?

Holley designed their carbs to run 1 1/2 turns out plus or minus very little. Most holleys run rich at idle and need more air added, or at least close down the transfer slots.

A carb takes only a few minutes to remove so if you have any work to do on it why not just take it off?? The more parts you remove the more confidence you build, the more experience you gain.
Opening the rear throttle plates more will lean out the idle mixture. Closing down the air bleeds with richen it up as will reaming out the idle circuit.
Like I said most holleys are too rich at idle.