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Old 09-15-2003, 10:49 PM
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Q-Jet Metering Rod Idea

Ok I'm prolly gonna get a million posts tellin me how stupid I am for even thinking this up and I may be way off in the way my mind is thinking.... But anyways here it goes....

From what i understand there are 2 metering rods.. 1 for part throttle. and the second for WOT for use with the secondaries.. Well if this proves right.. Which it probably doesn't and this whole post is for nothing. But anyways back to the subject... If this is true. Then would it be possible to put one size of metering rod on the part throttle side to make it run a tad leaner and get better gas mileage. Then on the other side have a slightly richer rod for when the secondaries open?

I mainly thaught of this because I've been experimenting with 2 different sets of metering rods in my 305... One set gives me excelent gas mileage.. But not soo good power when the secondaries are open.. Where the other set gives me more ponies but less gas mileage... Was hoping to maybe have gas mileage to and from places.. Then when I open it up have the power there with a richer metering rod..

Also the side that sets lower is the Primary (part throttle) rod right? And the one slightly higher is for the secondaries?

Good idea? Bad Idea? Or just an idea i gotta experiment with on my own?

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Last edited by Silver_Camaro; 09-15-2003 at 11:20 PM.
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Old 09-15-2003, 11:57 PM
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Well, I think I understand your question.
The Q-jet has 4 metering rods. Two are inside the carb under the top lid, they are for the primarys, the front 2, small butterflys. They adjust up and down by engine vacuum to richen the mix during acceleration, and then lower down some during cruise.

The other 2 are for the rear 2 big secondary butterflys. They also move up in the jet when the secondarys are opened.

Both the front and rear rods are replacable. The front rods fit into replacable jets, the rears go into a fixed hole.


You can change the front ones to smaller diameter rods ( more jet area, richer mixture) or bigger rods, ( smaller jet area, leaner mixture.) You can also change the jets too.

You can also change the rear rods for a richer or leaner "wide open throttle" position.

Mark
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Old 09-16-2003, 07:47 AM
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I actually asked this same question about a year ago to one of the guys that I race with, and just as Jmark said, it can be done. The only problem is, that when you disrupt the balance between the primaries and secondaries, it's near impossible to get a smooth transition into your secondary circuit. There are 21 different hangers which basically control how the secondary metering rods sit, and then there are nearly 50 rods that control the rate of fuel itself. That equals up to over 1000 combinations, not to mention that you'll have to tune your air valves to open correctly in response to the huge amount of fuel. You'd be better off, in my opinion, to find a mid point as far as rods and hangers. If you have time, find a B, K or L, and a V hangar to give you the extremes and a midpoint, and then do the same with the rods you have. After tuning on my firebird, I was able to get about 15 mpg, and still turn out 400+ horsepower. Good luck and let us know when you do get everything tuned in.
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Old 09-16-2003, 09:10 AM
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I also "tinkered" with the q-jet secondaries for a racing application. I started by experimenting with rods only, and it seemed to change things more drastically than I wanted. Thought I was smart, so I changed jets then, trying to figure out the perfect combo. As adryan16 said, there are a LOT of combinations of rods/jets. Combine that with the need to gently lift the lid from a q-jet at the track, between laps, and I was getting a bit frustrated. I went back to the baseline combination and adjusted the rods and jets one change at a time as a set. That seemed to get closer to the results that I was after in the RPM range that I was looking for. Somewhere I found a chart that mated rods and jets for several applications, as well as some data on making changes. I'll dig around to see if I can find it. My advice is to change small, check the results, write it down, and change some more. Good luck. It just takes time, money and a good set of eyes and fingers to mess with q-jets!
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Old 09-16-2003, 01:53 PM
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You can adjust the primary side using PTA screw, a small amount changes the jet area a LOT so go easy. That will give you the good gas mileage. When picking a set be concerned with the tip diameter thats what will keep it from pinging at light throttle.

On the secondary side just find a set stamped DA theyll work good.
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Old 09-16-2003, 01:55 PM
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Just from changing the secondary metering rods I had a drastic change in milage and power... Milage went down and power went up... But what if i were to have a leaner rod on the side that sets lower.. And a richer rod on the side that sets higher? Is it possible to do this? And would I hurt anything if I tried it?
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Old 09-16-2003, 02:48 PM
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Most Q-jets are on the lean side to begin with, I don't think you'll be able to lean out the primary's any more? Your feeding all 8 cylinders with the frt barrels until you stand on it. It's kinda like 2 seperate carbs. can't steal from one side and give it to the other!
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