How To Quadrajet Carburetor Rebuild - Part 3 - Tuning

How To Quadrajet Carburetor Rebuild - Part 3 - Tuning

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a vacuum gauge and a straight slot screwdriver that's about all you need to tune to Rochester Quadrajet will dive into the details in just a moment I'm Tim Johnson and you're watching shop tool reviews so this is the third part of our three-part series so hopefully you've made it past one and two both cleaning and disassembling your Quadrajet as well as reassembling your Quadrajet so hopefully you're at this point now you're ready to tune and there's really not a lot to tuning a
00:32
Quadrajet a lot of times people they just kind of worry and they they just don't know what to do and I think it's a complicated process because it is kind of a complicated piece of equipment a carburetor does a lot of things there's a lot of passages as you know you've been inside this now but tuning it it's actually a pretty simple process and you only need a couple of tools and these aren't expensive tools a screwdriver a straight slot screwdriver you've probably got a bazillion of them you can use a carburetor tool they sell those flexible carburetor screwdrivers I've
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got one around here somewhere but always grab the straight slot because they're basically straight slot screws for the air fuel mixture for the idle passages basically and so you can actually get to them usually with a straight slot screwdriver no problem the second thing you need is a vacuum gauge don't cheap out on this I bought a Bosch vacuum gauge at the parts store for about 20 bucks comes with the hose comes with a couple adapters a tea adapter also a larger fitting that you can put on larger ports which is what we'll do so
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again you know a twenty dollar tool and something you've probably already got in your toolbox so buy a decent vacuum gauge by the way this will also test fuel pressure for mechanical fuel pressure not for knots for fuel injection motors or any high pressure fuel pumps but basically some of your standards you know non PSI on under 10 psi on under fuel pumps and you can test your fuel pressure as well with the same vacuum gauge so very handy tool to have and once you have it it's it's you'll find a lot of uses for it so tuning at
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Quadrajet really consists of three items number one is the choke number two are the air fuel mixture screws which are here in the front that connects to actually your primary ports here and primary throttle blade areas and number three is the actual idle screw for the actual throttle blades that actually set wherever the throttle blade position is so those are the only three things and number one is choke for a reason we're not going to forget about the choke because the choke does a lot more than just helping you to start your car and I'll show you that here in just a moment
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so here's your choke whether you've got an electronic choke or a hot air choke like I have here we're basically a tube comes up connects right here to this port and actually inside this thing is just a big spring and that spring operates this little lever in here that connects to this actual choke plate here so that basically closes off the air when the car is cold or the vehicle is cold and draws more fuel in so you got a higher fuel to air ratio and then once
03:10
that car warms up it starts opening up like this and then finally it stands straight up now basically adjusting that choke you're adjusting that spring I guess that little lever in there and you can see as I turn this it actually operates that plate there so we want to adjust that we've got just about just starting to open there on the spring and so that's good there and then you'll see you've got these little plates and little screws right here it's gonna put
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those in and by the way you don't need a lot of tension on these to actually hold that choke into place so basically you'll see right here on this on the plate of this choke on the face of this choke it says rich this way lean this way so clockwise is lean counter clockwise is rich and again I like to usually set it right as the choke the choke plate is about to close so there's a little gap sitting there and you can see here I've got a little white mark
04:11
here and you can see I'm right there almost in the dead middle of my choke adjustment and then I can just tighten these screws up and by the way you don't have to tighten these a lot they've got those little pressure tabs on it that's keeping keeping pressure on that little elastic dial but really it's not not a lots different whether you've got an electronic choke or an electric choke or whether you've got a hot air rise or choke like I have it's still operating with that little spring this isn't dialing in this this choke plate here now why do we want this to
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work number one is cold start so when the car is called you see like it is right here that this plate is closed and so it's gonna draw in more fuel than air and it's going to actually burn that and start warming up and as the car warms up that spring in there as metal gets hot it expands that spring loosens up it kind of loses some tension and as it lose and loses tension as that hot air starts coming to the carburetor coming in that choke area starts opening that
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plate up now here's the other key area what we want to make sure is when we say get one 150 160 degrees if you're running a 160 degree thermostat you especially want this to be the case so at a 160 degrees you want this straight up let me show you why let's say that we're mostly open but we're not and that's the most it ever gets let's say at operating temperature that's where the the choke plate sits looks like okay that's decent you know the primaries are still going to get plenty of air but let's say you're ready to make that pass you may be you're passing a
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tractor-trailer or you know what have you just wanted to stop on it open up those four barrels and let it eat well let's see what happens when you stomp that four-barrel the secondaries aren't opening because this little lever right here from the choke so once that choke opens all the way up now those pry those secondaries will come in and you'll get the full effect of that four-barrel especially on a Quadrajet you see how much bigger the secondaries are than the primary so you really feel that difference when you call in those
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secondaries and open up the four-barrel if you will so adjusting that choke is really easy once the car warms up and gets up to temperature just make sure that that choke blade is all the way up if it's not again you can loosen these screws and just turn it one way tack so you'll see it finish opening up or the other way where it's closing so once it's up to operating temperature you want that dialed in to make sure that that choke blade is all the way up and you'll see this little lever here where it clears that secondary secondary blade there so that those will actually
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open and those are pulled open by vacuum where the secondary throttle blades are actually controlled by the by the throttle or by the throttle cable you can see the secondaries open even though they're opening here they're not opening here and the vacuum can't pull these open but again once that choke is out of the way now those secondaries can open so we've set up the choke we've got that dialed in now let's look at the air fuel mixture screws now you can start with setting these kind of all the way in then gently tighten those all the way
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down where they bottomed out you don't want to tighten those hard because you'll actually distort or deform the actual tip so close those all the way in so clockwise all the way in till they seat and then back about about three full turns three and a half full turns is a good place to start then you want to make sure that you've got enough throttle blade position with your idle screw here to where it's actually going to run so crank up the car and make sure you got the idle set that where it's running pretty good to where you know it's not stumbling on itself and not
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able to run so what you're going to want to do is to find manifold vacuum we don't want metered vacuum we don't want these small vacuum ports that are maybe hooked to like distributors and things like that that are actually metered by the carburetor we don't want that we want two manifold vacuum which is a constant vacuum regardless of the position of the throttle blades on the carburetor or where the carburetor position is so one of those things is where the PCV valve hooks into on the Quadrajet and that's right here brake
08:20
booster is another one however on the Oldsmobile that we're on brake booster actually hooks directly into the manifold itself now I also like to pull my manifold vacuum off of the carburetor not off the manifold itself so again pulling from that large PCV area is a great place to pull that from because you're getting that constant manifold vacuum now I'm using this little adapter here this got the big end on it so I can actually just put it here on the PCV port and then I can hook my vacuum gauge
08:49
to that and now basically we're just going to run the car and as we run the car what we're gonna do is adjust these air fuel mixture screws one at a time and we're gonna just to vacuum so basically closing these things in is going to shut the fuel off opening up is going to give me more fuel so let's start by closing them off say a quarter turn at a time if the vacuum goes up keep closing down so close another quarter turn and each time let let the engine kind of catch up a
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lot of times it will change but make sure it kind of settles down and and kind of levels out if you will so if you keep getting a raise in the vacuum as you change that idle the idle air mixture screw they continue to move in that direction because basically we're just wanting to tune to maximum vacuum so as that vacuum increases continue to move that way now once it reaches its peak and you turn it again and it moves the other way back it back to that prior
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position and that's where you want to leave it leave it right there so now we we should be somewhere around 17 18 you know 15 to 20 pounds of vacuum is where we should it be so then we want to move to the other idle mixture screw or the idle air fuel screw which is over here on the other primary blade or other primary port so it's the same thing here start with closing that down and if the the vacuum continues to increase continue to move that quarter turn at a
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time if you're having if it's if it's actually getting less then you want to back it back out and actually open those a quarter turn at a time so it's that easy really you're just wanting to make those turns until you've maximized that vacuum once you got both those dialed in where you've got the maximum amount of vacuum out of it then you want to move to your actual idle screw and set it to the RPM that you want it to idle at and that's really going to depend on your car you have an automatic transmission you don't want it
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to stumble when you put it in the drive do you have air conditioning that you're going to turn on an idle again so probably somewhere between 750 and 900 rpm is probably the sweet spot on a decent idle where it's not too low and not - hi one of the things that may cause you some issues is if you have a big cam with a lot of overlap which really and truly the car we're tuning on actually got a pretty big cam and it's got some decent overlap on it so you can see the kind of erratic idle and erratic vacuum there so it's hard to get a stable
11:23
vacuum and the bigger cam you have the more overlap you had the worse you're going to deal with that and in extreme cases you may get into drilling throttle blades and things like that so you get bigger air passages that actually come through they're not going to cover that here but I'm just telling you if that's where you're at if you've got that big of a strip cam or that big of a street strip cam or maybe you're running this thing's bracket racing on the weekends things like that or what have you then you may need to go to some extreme
11:55
circumstances outside of just setting the vacuum with the air fuel mixture screws and setting the throttle plate throttle blade position so that's really it with tuning the carburetor get that vacuum maximized with the air fuel mixture screws then set your idle on the actual throttle blades here and then you should be good to go and make sure your choke is working both at startup as well as when it gets up to operating temperature that it's opening up all the way and allowing those secondaries and open that's it a simple box back engage
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and a straight slot screwdrivers all we needed for this please keep track of us on Instagram Facebook and Twitter hit that like and subscribe button if you don't mind and hit the bell notification if you want to be notified of future videos and have a great day you

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