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Old 05-12-2009, 07:43 PM
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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 Mr. Ruggles knows his stuff and trust me the Qjet is a place where you want to know a lot before setting foot inside.

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