Holley Idle mixture screws non responsive?

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Dart_440

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I scooped up a complete 383 which the owner knew nothing about other than it was rebuilt before he got it and ran great when pulled. I since have changed the intake to a performer RPM 383, with a new 750 vacuum sec, 4160 holley. While I had the intake off I checked out the cam and I believe it to be the mopar purple 484 shaft, I checked the timing chain and the cam was installed straight up (no advance), I calculated the compression a little over 9:1. The cylinders still had the cross hatching on them. So all in all the motor looked great.

I have it running.. but there are a few minor issues. One the idle mixture screws are non responsive. no matter where they are.. they don't really change the rpm or vacuum reading. If I turn them in they will kill the motor thats about it.

Also, the vacuum is about 11-12 psi in park, and about 7-8 in drive. Is this normal for this cam? I thought there would be more vacuum, so I have taken the intake off 3 times now and redone the gasket. The vacuum is always the same.

I changed the power valve to a 3.5 since I had one.. no change.

I have set my timing about 14 degrees, plus the mechanical and vacauum advance in the distributor.

So any ideas why the idle mixture screws don't do anything? And also does this vacuum reading sound about right for this combo??

Thanks Bryan
 
Could be because the primary throttle plates are open too far just to keep the engine running. You are actually idling on the transition circuit, not the idle circuit. The idle mixture screws only work on the idle circuit. You mentioned the carb being a Holley...there's a little screw under the secondary throttle shaft to open the secondaries slightly , and allowing the primaries to be closed down and allowing the idle circuit to function again.

Also, check your timing to ensure maximum initial timing.
 
What is the timing set at right now?
I would 1st
set the idle mix screws so they are 3/4 to 1 turn out and even on both sides, then set the initial timing to 18* and lower the idle blade setting down and see how it idles after that, but watch the total timing you don't want to have more than 38* [or around]
 
I am not 100% sure on the timing right now I have been messing with it so much I will need to go back and check it all out. But I know its at 14* plus mechanical and vacuum advance.

I am gonna lock it in at 18*, pull the carb and check the transition slot and adjust the secondaries if necessary.

Does the vacuum reading sound about right for this combo? I thought it was kinda low.
 
You really can't do anything until the timing is set correctly.

Sounds like a combination of the above. Your timing is out of whack so to compensate you have adjusted the idle screw, so the primary plates are open so far that you are beyond the idle circuit control.

Make sure you are setting the timing and carb with the vacuum advance unhooked and plugged.
 
And thats all I have read as well.. so I originally set my timing at 16 plus the vacuum and mech advance was getting me all in around 38. Still my mixture screws were useless so that is when I started messing with the timing to try to get some response from the screws. I will lock it in again tonight and see if I can check the transition slots.. maybe open the secondaries if need be.
 
Get the initial up to the 18-22 range and try again.

If the engine shuts down with the screws all the way in, they are working.

What is the idle drop from out to in gear? It may be bleeding mechanical advance in and when it drops into gear,that advance is gone, compounding your issue. Is the initial timing at idle the same in neutral and in gear?

If you can't get to the secondary adjuster, use a feeler gauge and open it about .004-.005, that should be about 1 turn of the adjuster if it's 24 pitch. Definitely check were the plates are in relation to the transfer slots.
 
There was about 300-400 drop from out to in gear. I can no longer check that because I have since pulled the motor and build and engine test stand to try to work some of the bugs out while I paint the engine compartment.

It seemed like a lot to drop?? Is that normal?
 
How far are your mixture screws adjusted right now? Re set them to 1 turn out from the seated position.

I may not be reading it clearly but I hope you are not saying that you set the initial timing at 16 with the advance unhooked then when you plug it in it jumps to 38?

Another thing to check…. Being that it is a new carb. Do you have all the unused ports plugged?

A 750 should just drop on that engine and run in my opinion. The problems are probably coming from other related issues.

In other words… I think you may be trying to adjust the carb to cover another issue.
 
yes 14* initial and then once the vacuum and mechanical advances kick in at about 2000 rpm I am around 38 or so.

I start the screws at 1 1/2 turns out... but from there they do nothing either way until almost completely closed then the motor will die.

I have all the unused ports plugged.

I agree I thought I should be able to drop it on and run it.. and its been anything but that.

I keep feeling like I have a vacuum leak but I have done the intake 4 times and always get the same vacuum reading.
 
Actually before I speak out of place.. I know my initial right now is about 14* but I need to verify the mechanical and vacuum, I am using a pertronix distributor and do not recall the advances.
 
A 300-400 drop in rpm is pretty big IMO. If you have a big cam and tight converter, it can cause issues. Check for that advance being pulled out in gear like I mentioned before. Have some one stand on the brakes and drop it in gear so you can see what it's doing with your timing light.

Check/adjust the float levels too. That's another first step. If they are too high, it may be dripping fuel into the engine.

Bump the timing up to 18-20 and see how it runs. Or, get it warm and keep advancing the timing until it kicks back on the starter, then dial it back a few degrees and see where you are at.

You really need to get the initial timing issues squared before worrying about carb idle adjustments.
 
If he has the primaries opened up that far, he may be getting some vacuum advance while idling too. When you put it in gear, the vacuum falls away and retards the timing even further, exaserbating the problem. Same goes for mech adv. if he has light springs installed.
 
ok so now I am really confused, I just went out and checked out my timing. I disconnected the vacuum advance and set it at 16*, I then moved up the rpms to about 2500 and the mechanical advance was all in and was a total of 45*. It is suppose to be a total of 24 advance plus my 16 would be 40* but I guess it's a little off. Anyway, I opened the secondaries a little so I could close the primaries some.. but never the less still no response from the mixture screws.

So what the heck do I do now?
 
Don't worry about total right now.

Get the initial idle timing set and the carb adjustments done. Leave the vacuum adv disconnected and plugged at the carb! Once you have that done, you can worry about restricting the mechanical so the total isn't out of sight.

Close the secondaries back down and start over. Adjust the floats, baseline the mixture screws and move the timing up another 4-8 degrees. Get it hot and see if it kicks back on the starter when cranking, if not, you're good.
 
Dart_440 said:
ok so now I am really confused, I just went out and checked out my timing. I disconnected the vacuum advance and set it at 16*, I then moved up the rpms to about 2500 and the mechanical advance was all in and was a total of 45*. It is suppose to be a total of 24 advance plus my 16 would be 40* but I guess it's a little off. Anyway, I opened the secondaries a little so I could close the primaries some.. but never the less still no response from the mixture screws.

So what the heck do I do now?
Click to expand...

What power valve is in it?
 
crackedback said:
Don't worry about total right now.

Get the initial idle timing set and the carb adjustments done. Leave the vacuum adv disconnected and plugged at the carb! Once you have that done, you can worry about restricting the mechanical so the total isn't out of sight.

Close the secondaries back down and start over. Adjust the floats, baseline the mixture screws and move the timing up another 4-8 degrees. Get it hot and see if it kicks back on the starter when cranking, if not, you're good.
Click to expand...

I will give it a go again this weekend after christmas, I am pretty sure I have plent of room to move up before the starter kicks.. I have toyed with it before. I will report back when I get some time to play.
 
As Crackedback said if the engine dies when you turn in the idle screws THEY ARE WORKING. You adjust them by turning them in until the engine slows down then back them out until it idles smooth but no more. He's also right about it being a big drop in rpm but as he said if the converter is stock that's the way they act with a fairly large cam like the 284/484. A buddy of mine has a Challenger with a 440 with the 284/484 cam in it and it only pulls about 8 inches of vacuum in gear and it wasn't that good until I modified the dist. weights so it could run more initial timing without going to much on the top end. He also has a stock converter and we have to idle it at 900 in neutral so it don't die when dropped in gear cause the stock converter is so tight it pulls down hard on the engine in drive. AT 900 in neutral it drops to about 650 in gear. He needs a looser converter. I had the same problem in my 360 at first and slapped in a 3k stall converter and now I set the idle at 775~800 in neutral and it only drops to 700-725 rpm when I drop it in gear.

You may have to modify the advance weights in the dist. so it doesn't have so much centrifugal timing and that'll allow you to run more initial. That's something you always do to a engine with a perf. cam.
 
A .484" has a pretty late intake closing point and with 9.0:1 CR it sounds marginal to me to get any decent driveability until you wind it up. If you can cram in as much timing as you want and it only improves, that may be a good indicator of low cylinder pressure, and in my opinion that's the beginning of your problem. Advancing the cam is a good way to get back lost cylinder pressure, increase manifold vacuum and also help ease your tuning efforts. 10-12 inches of mercury is pretty normal for big cams and having it drop in gear is also normal. The power valve you have seems about right.

If the timing is too retarded at idle it's possible the throttle blades are open too much to keep the engine running and that tends to pull more fuel out of the transfer slot, which bypasses the mixture screws. The proper position for the throttle blade in the bore is to have the bottom third or so of the transfer slot exposed to manifold vacuum and the ported vacuum port not exposed to manifold vacuum. Use that as a baseline to tune from. Also check the secondaries and make sure you are not idling excessively off those. Set the secondary stop screw just so the throttle blades don't stick in the bore.
 
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