Best Way to set the idle mix on a 4160

[steve meltzer]

I've seen some people advise to adjust each mixture screw (from 1.5 turns out) to it's max vacuum. This makes sense to me, but on the Holley video (not Holly does Houston), they say to adjust to max vacuum, but that both sides should be equal. I find it hard to believe, that in the real world you could make both sides exactly equal, and that the max vacuum on each side should be your goal. thanx. Basic question, I know, but....thanx steve

[DanEC]

You can set the idle mixture with a vacuum gage or a tach. I've gotten to where I prefer a tach in recent years but I still use a vacuum gage sometimes. You should tune each side to achieve the highest vacuum or idle rpm separately - going back and forth a couple of times. On most motors the adjustments will end up pretty close to each other - but yeah, there may be a little difference in where the screws end up from side to side.

[steve meltzer]

thanx so much. that's what i suspected...max vac. s

[patrickt]

If you're between 3/4's of a turn out and 1-1/2 turn out the car should run decently. You can also try the old trick of adjusting the turns so that when you pull the rubber plug off the intake manifold port the RPM's stay the same. The most important part is not to have your idle turned up so far that you're too deep in to the transition slot. Or, you can do what I do to get it just perfect......... Make sure your primary butterflies are at the bottom of the transition slot, back your idle screws out about 3/4's of a turn from seated, then adjust the idle speed by cracking the secondaries with the hidden set screw. Then take your car out for a steady slow drive with the throttle lightly applied and steady and feel for your lean surge, which you should have. Back the screws out 1/8th of a turn and repeat until the surge is gone. The object is to have the leanest possible idle mixture setting that allows the car to run nicely.

[steve meltzer]

Patrick, i know we talked about replacing the factory slotted screw for the idle on the secondary. did you use a socket cap (with hex for an Allen key) or what? 1/4-20 or? thanx. steve

[patrickt]

Originally Posted by steve meltzer

Patrick, i know we talked about replacing the factory slotted screw for the idle on the secondary. did you use a socket cap (with hex for an Allen key) or what? 1/4-20 or? thanx. steve

10-32 cap screw. For use with an Allen wrench.

[steve meltzer]

thanx. s

[steve meltzer]

ooops! tryin' to have them on hand in advance...gotta length? now, i think, i'll stop pestering you. (temporarily!) s

[patrickt]

Alright, here are my tips for you, or anyone else that's reading this, to do this "fix" both with the carb on the engine, and with it off. Now remember, the goal here is to have your primary throttle blades set properly with regard to the transition slot, and your primary fuel/air mixture screws set properly so that your idle quality is nice at all under hood temperatures and so that your "light throttle" cruising has no lean surging. The idle RPM level will be set by the new secondary screw that you will be installing. The problem that you are trying to correct is that the density of the air that is going through the carb is WAY different when it is really, really hot as opposed to when in it nice and cool. Unfortunately, your carb only gets one single setting. Computer controlled fuel injection it ain't.

First, buy four 10-32 cap screws, a 3/4", a 1", a 1 1/4", and a 1 1/2". Then figure out which one of those four screws will be the most convenient to access with an Allen wrench once it is mounted to the same depth as the existing screw. Depending on your manifold and Turkey Pan combination, you might find that drilling an access hole in the Turkey Pan, that allows either a longer screw to poke through, or for the Allen wrench to come up through. Choose the screw that makes it easiest for you to get to, as you will be making slight idle adjustments once or twice a year, based mostly on the seasons. Inserting this screw can be done with the carb on the engine or off, but it is obviously easier to do with the carb off the engine. Insert your new screw with the same number of threads protruding up that the old screw had.

If you want to "get fancy," buy a 6 lb. spring, like you see in my pic above, and a washer for the screw. When you insert the new screw, just have the washer and spring "hanging loose" on it. Once the screw is adjusted properly, you can add that copper sleeve, that is just a piece of 3/8" copper tubing. It is cut to just the right length to apply good spring pressure. You get it around the cap screw by using your Dremel to cut a slot in the tubing so that it just barely slips on, and around the screw shaft, and then you use a pair of pliers to lightly squeeze the tubing tighter so it never falls off. That's how you get a perfect, custom fit. I also like to use a couple of drops of VC-3 "adjustable thread locker" on the threads. It doesn't harden, and allows you to make adjustments on the screw. https://www.vibra-tite.com/products/...-3-threadmate/

If the carb is off the car, flip it over and adjust the primary butterflies so that there is a "square" or less of the transition slot exposed. If the carb is on the engine, and you don't want to pull it off, then back the idle screw off on the driver's side until you see a "visible opening" between the screw and the idle lever. Then, place a pencil, eraser side down, in to the primary side of your carb so the eraser is sitting on the butterfly valve. Put a sticky tab on the point end of the pencil. Then slowly tighten up the idle screw until you see the pencil and sticky tab start to move (meaning you are now opening the primaries). Give it another 1/4 or half turn, but no more. Now lightly seat both idle mixture screws and back them out 3/4 of a turn.

Start the car and adjust the idle level by turning your newly installed secondary screw. Once you have a decent idle, take the car for a nice gentle drive where you see how she cruises at, say, 40 MPH, with your foot just ever so slightly giving gas. You should experience "surging" because of the lean mixture. Back each idle mixture screw out 1/8 of a turn and do it again until you have a nice clean, even engine performance with no surging when she is cruising with your foot just lightly on the throttle.

If your idle is too high, back your new secondary screw out a bit, if it's too low, screw it in a bit. You may also turn your regular idle screw in a bit. BUT NOT MUCH -- remember, we don't want to go in to the transition slot. Less is more in that regard. If you have "ham hands" you can visually see how much you're adjusting the new secondary screw by placing a clothes pin with a stick attached to it on the driver's side secondary throttle shaft. As you tighten the secondary screw in, while standing on the passenger side of the car, you will see the stick move clockwise. When you loosen it, it will move counterclockwise. It doesn't take much of a turn to that new screw to make significant changes to your idle. For a big FE, with a Holley 4160 on it, and an engine compartment that gets pretty darn hot, this is absolutely the best way to get your carb set perfectly.

[steve meltzer]

WOW! Patrick, thanx for the schooling. much appreicated. i'll have to take the car back off the car to get the second, errant, gasket in,so i'll R&R the screw then. s

[steve meltzer]

oops. i didn't see the picture you referenced in your long and detailed instructions. thanx. s

[patrickt]

I can not emphasize enough the role of the transition slot, and the positioning of the throttle blades along that slot, in getting your engine to run right. You have a big FE, a healthy cam that lowers your vacuum, along with hot under hood temps. If you have your throttle blade up deep in to the transition slot, then you're negating the effect of the idle mixture screw and actually defeating the whole purpose of the transition slot to begin with. The transition slot is really a beautiful little engineering design. When your throttle blade is down at or below the bottom of the slot then the slot is actually feeding fresh air in to the emulsified fuel. But as you open the throttle, and the blade moves up the slot, the portion of the slot that is below the blade grows larger and brings in more pure emulsified fuel instead of air -- it "transitions." It's a beautiful, simple design.

[steve meltzer]

Patrick, thanx again. let me digest this and get some wrench time in, hopefully, this weekend. my life and job are interfering with my fun. s

[patrickt]

Originally Posted by steve meltzer

Patrick, thanx again. let me digest this and get some wrench time in, hopefully, this weekend. my life and job are interfering with my fun. s

Alright, get cracking... and remember the Club Cobra Credo: "See One, Do One, Brag About One."

[steve meltzer]

Hmmm....vaguely like my residency training...see one, do one, teach one...
and our credo: we teach the unteachable, train the untrainable and accept the unacceptable.

s
(now sporting a tee-shirt with a frog that says "don't croak on my shift")

[steve meltzer]

OK, carb off, but I can't get the primary butterflies to close anywhere near the "perfect little square" that closes the transition slot the way it should be. Primary idle speed screw is all the way out. thanx. s

[steve meltzer]

Me bad and dumb...choke was in play! Now my problem is that the blades don't close evenly. It's slightly tighter on the left (driver's side). Can this be adjusted so that they close evenly? I would hope so. is it the "stake" screws that screw into butterfly itself, on the bottom of said butterfly? thanx, supernovice, steve

[patrickt]

OK, I think you are saying that your throttle plates are either not closing evenly, or they are out of whack somehow, or that they are not both even with their respective transition slots. If that is true, they don't really "adjust" all that much, but maybe you have some carbon build-up or something. Now I have heard of carbs that don't have the transfer slots even and that's because of a milling QC issue from Holley. What I would do is follow the "generic" throttle plate instructions that are here: http://www.jegs.com/InstallationInst...-26-93-100.pdf and just clean them up good, maybe do a little filing if they are rough on the edges, and then perform the "light bulb" test. Just get them as smooth as possible and, if the transition slots are not that even, then so be it. Just set the biggest one's opening to a square and leave the other one wherever it lands. Smoothness in the opening and closing is the key though.

EDIT -- Here's a nice thread, with lots of pics, dealing with the joy of throttle plate fixin' on our 4160 carbs: http://www.mastercraft.com/teamtalk/...ad.php?t=54417

[steve meltzer]

they're reasonably close, but i'll take a look and keep you posted. thanx again. s

[steve meltzer]

Just wanna confirm...this is a full manifold vacuum port. right? thanx. s