How to 'tune' or rejet a quadrajet? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 05-12-2009, 05:27 PM
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How to 'tune' or rejet a quadrajet?

I have a quadrajet that I want to put on a 350 but I think it came from a 305. Do I need to 'rejet' it or anything?

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Old 05-12-2009, 06:10 PM
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Quote:
Originally Posted by maxpower_454
I have a quadrajet that I want to put on a 350 but I think it came from a 305. Do I need to 'rejet' it or anything?
First off just try it, the Q-Jet is pretty forgiving and if the 350 isn't a wild build it may have enough ability to cope. If it doesn't, Jet is the current purveyor of Q-Jet parts and you can get primary jets and metering rods from them as well a secondary rods. The secondary jets are fixed and shouldn't be messed with unless you're really accomplished at modifying Q-Jets, but in any case unless your changing to alcohol or some other exotic substance for fuel the adjustments needed should fall in the range of the metering rods. The other major adjustment is the spring under the metering piston, this can also be adjusted to dial in different metering rod settings for the manifold vacuum conditions, but unless you've changed cam timing from what the carb was originally set up for this shouldn't be an issue.

Bogie
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Old 05-12-2009, 06:54 PM
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Well I actually swapped it in and took it for a ride. It has great off idle and part throttle performance so I don't think i will mess with the primaries. I have this in a pretty much stock engine in a dirt track car that will see a lot of WOT. How will I know what secondary metering rod is in my qjet now? I'm thinking maybe I should just put the next larger one in to be safe. Putting in a bigger secondary metering rod will richen it up at WOT, correct?

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Old 05-12-2009, 07:14 PM
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i never change anything when swapping from one size to another all the q jets are pretty lean jetted anyways, especially the later years they are,
the secondary rods you can check without doing anything except rmeoving the small flat head blade screw that holds the secondary rod hanger on the top of the carb between the two secondary air valves cant miss it

jut pull the screw and lift the hanger up with the two rods, straight up

are two-letter codes on them like AA or BC etc.

are charts online etc to decode them, certain ones are better than others of course.,

i collect any and all q jet carbs i come across either to go through or for parts only. so i have amassed lots of trays of parts and lots of carbs over the years, and i only run q jets on my gm v-8 engines

there are billions of informational pages out there on the web for these carbs. lots of info, you cna find out anything about them.

good luck
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Old 05-12-2009, 07:43 PM
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Quote:
Originally Posted by maxpower_454
How will I know what jet is in my qjet now? I'm thinking maybe I should just put the next larger one in to be safe.

First start with a fresh set of spark plugs.

When you fire it up and take it out you're looking first for lean signs, these will be backfires thru the intake, pinging when you get on it, possible hard hesitation when you whack the throttle open. These are the first apparent signs the mix is lean, don't worry as long as you don't keep hammering the engine in this condition it'll be fine. If it doesn’t do this, drive it for a few days then pull the plugs. White with splotches on the insulator is telling you its way too dry and is detonating whether you hear it or not, slight discoloration of the insulator in brown, gray, with perhaps a bit of yellow to orange tells you the mixture is pretty good. Black says it's a little rich, caked black quite rich, and wet black way too rich. Rich isn't as immediately destructive as lean but it begins a washing of the upper cylinder lube from the cylinder walls which increases the wear rate between the walls and the rings so it's something you want to attend to when you see it. At the start up, rich will feel bubbly, like the engine is almost right but seems to be full of liquid, which it is.

I wouldn't go diving into jets and rods till I went thru this start up phase to see what it wants. Then it gets more subtle, a metering rod carb is more tunable than those with just straight jets, so you have to be careful not to get in too deep to fast, its really easy to get lost in Q-Jets and Carters. Remember jets are only one solution on a Q-Jet, equal to them is the metering rod and the piston control spring, this not only sets the effective jet size, but times those sizes on engine need by virtue of manifold vacuum. For a Holley there are main metering jets and the power valve/power jet combination. For the Q-Jet all that and more is built into the main jets, metering rods and the piston that moves the rods. The rods have 2 to 3 different sizes on each shaft each is positioned according to engine load. The jet is one size; the metering rod passes into the jet effectively reducing the size of the jet. High manifold vacuum pulls the control piston down against the spring putting the thickest diameter of the rod into the jet; this makes the mixture for low speeds at a nearly closed throttle. Then there is a slightly smaller step, as the manifold vacuum decreases the spring pushes the control piston up pulling the smaller diameter step into the jet, this provides the mixture for cruise as the throttle is getting more open. The next step is WOT, it is the smallest diameter, when the manifold vacuum falls toward zero, the spring pushes the control piston all the way up exposing the jet to the least restriction. This is high speed enrichment and corresponds to the more familiar Holley which would have main metering and the power jet both feeding the main well at this point.

On the secondary side a Holley works pretty much like the primary with events being controlled by a one size fits all jet, some models include a power valve and accelerating pump, but most don't. The Q-Jet remains more complicated here operating in a similar manner to the primaries with a large fixed jet and a metering rod. Only here the metering rod is driven by a flapper air valve hinged above the throttle plate. There are different cams available to adjust the open close profile of how the metering rods move in relation to the air valve. The air valve is controlled by a spring or counterweights depending on year and model of the carb. They also can be changed to tune the air valve. As the secondary throttle is opened which is mechanical, the air valve is subjected not to vacuum, but the engine's flow demand. So if you whack the throttle to WOT at idle, the air valve doesn’t move as there is no air demand for it. But do that at higher speeds and that valve will begin to open once the primary circuit can no longer keep up to demand. Then it will open proportionally to the need of the engine at any moment moving the secondary metering rods with it. These are usually tapered but some are stepped similar to the primary. The further the air valve opens the smaller the taper becomes on the metering rods and the more fuel is allowed to pass thru the jet.

That my friend is a Q-Jet without having to buy the book. However, I highly recommend this for bathroom reading http://www.amazon.com/Rebuild-Modify.../dp/1932494189 Mr. Ruggles knows his stuff and trust me the Qjet is a place where you want to know a lot before setting foot inside.

Bogie
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Old 05-14-2009, 10:23 AM
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secondary metering rods

To answer one of your questions, bigger (fatter) secondary rods will lean out the secondary side. However, changing the rod hanger can also effect mixture. As was mentioned before, Cliff Ruggles book really spells out how the QJet works and how to modify it.

Jack.
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