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Old 02-28-2011, 02:39 AM
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Tuning a quadrajet

I have a stock q-jet on my 454 (79 C10 pickup). Its a q-jet from a 1978 HD fed emission 454 suburban. Its very simple. No EGR and all that extra stuff. The only vac stuff is for the trans, distributor (through a thermal delay valve on the t-stat housing) and a line to the air cleaner (which is plugged since I have an aftermarket air cleaner) and one to the choke pull-off.

Now for the question. Its a 454, I know its not going to get good mileage, and maybe its just the fact that I am used to the fuel gauges on a suburban (which have double the fuel capacity of the trucks, thus making the gauge move twice as fast on the truck since it has only half the fuel capacity) BUT it kinda smells like its burning rich and its using more gas that i expected (again it may not be doing as bad as it seems tho),

I would like to learn more about how to turn this carb. I have rebuilt it myself, and it runs fine so its not like I know absolutely nothing about carbs but I dont pretend to be an expert either. I do think it might be running rich. I have adjusted the idle mixture screws evenly to achieve the best idle quality. There is no evidence of flooding (like from the float being too high or something) and the choke is not hooked up.

There is no black smoke (that I can see anyways) but the exhaust just smells rich. Also there is no catalytic converter, so maybe thats just how non-catalytic converted exhaust smells? I dont know. I dont really think so, I think it smells rich.

I dont really know what to do about this. There aren't conventional jets in these things, just metering rods. (which if you are reading this you probably know that and know what they do and how they work etc) As far as I know the metering rods have never been changed for any that would make it run any richer.

Is there a way to lean it out a little without changing the metering rods? Id like to tune it for better mileage, power isnt a big concern. I mean I bought the 454 for the power, but maybe I could lean out the primaries and richen the secondaries and ultimately end up with the same air/fuel mixture at WOT, when the power is needed. I dont NEED the power "most of the time". And it stays in the primaries while driving around town.

I think this is ultimately why a q-jet has such small primaries and large secondaries.

Any thoughts, tips, tricks?

PS When I rebuilt the carb I noticed that someone had already been in there and epoxied the Welch plugs in the bottom of the float bowl. I still think they are leaking some because it takes a bit of cranking to get it to start (as if the float bowl was dry) after it sits a while, perhaps overnight or over the course of a couple days. Haven't really paid much attention to it tho, just enough to notice it.

Also since this engine is not original to this truck (it had a 250 I6 originally) it did not have a return line from the pump back to the tank but it did have a vent line from the charcoal canister. Because of this, the return line port on the fuel pump is plugged and the vent line from the tank is open to air.

I dont know if this causes any rich running or fuel mileage problems or not.

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Old 02-28-2011, 05:26 AM
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One thing you can do is remove the plug on the top plate(you have to remove top plate) just to the front of the choke housing. After doing this, you can adjust the primary metering rod depths. During idle, or any time other than WOT, the primary metering rods are pulled down into the main jets by vacuum to lean out the mixture, same principle as a power valve. Alot of people will say you need a tool to adjust it, but if you cant get one, i have took vacuum tubes out of a carb, and squeezed them with pliers to fit the stud where the primary piston bottoms out. Turning it down lets the needles fill more of the jets to lean it, while backing it out lets it run richer. Also you can drop down a size or two in the jets. KEEP TRACK OF TURNS. If it worstens, go back to where you were or the other direction. Hope this saves you a buck.
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Old 02-28-2011, 07:32 AM
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Let me tell you, 454+4:10 gears = no mileage. I think the best you can hope for is 10 mpg with the seconday's diconnected.
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Old 02-28-2011, 08:18 AM
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I'll get to the Qjet part in a minute, but I wanted to point something out...

Two things about plugging the extra line on the pump. 1) it WILL blow off and start spraying fuel, and 2) it is a required bypass. Without that return circuit, the carb will see full pump potential which can be excessive.

The correct solution is to get the correct pump. Go to the parts store and ask for a fuel pump for a 454 truck that isn't HD, or possibly a 454 from, let's say, a 74 Impala. When they ask, tell them no A/C, no 4x4, no whatever. That should get you a pump without that return circuit.
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Old 02-28-2011, 08:24 AM
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Quote:
Originally Posted by curtis73
I'll get to the Qjet part in a minute, but I wanted to point something out....
Where's the Quadrajet part?
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Old 02-28-2011, 08:35 AM
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Does the Qjet carb have a double capacity accelerator pump. After 170 temp it cuts the pump volumn in half. If the soleniod/sending unit is bad it could hurt . The choke setting can hurt, and the secondary plates stuck shut from a old bent shaft could in effect cause the same higher fuel consumption on a 454. Those trucks get poor mileage. My 85 305 C10 gets about 17-18 highway. That is with cruise control set steady speed at 60-65 any faster and the frontal wind resistance costs more fuel.
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Old 02-28-2011, 02:40 PM
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Quote:
Originally Posted by sqzbox
Let me tell you, 454+4:10 gears = no mileage. I think the best you can hope for is 10 mpg with the seconday's diconnected.
Yes I know it wont be great mileage. I dont know if i am making 10 even tho. No speedometer working yet ( trans is from a truck with 3.73 gears and 235 75 15 tires, this truck has 4.10s with 245 75 16 tires. Needs a different speedo gear which i DO have, but it also needs a different speedogear housing which i DONT have yet.

It might be making 10mpg IDK, but the exhaust smells rich to me still which is probably costing me some mileage.

With the cost of gas going up and up and up like it is I need all the mileage I can get.

I nearly soiled my pants when I saw 3.60 for diesel here, which is what my other truck uses.
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Old 02-28-2011, 02:43 PM
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Quote:
Originally Posted by spinn
Does the Qjet carb have a double capacity accelerator pump. After 170 temp it cuts the pump volumn in half. If the soleniod/sending unit is bad it could hurt . The choke setting can hurt, and the secondary plates stuck shut from a old bent shaft could in effect cause the same higher fuel consumption on a 454. Those trucks get poor mileage. My 85 305 C10 gets about 17-18 highway. That is with cruise control set steady speed at 60-65 any faster and the frontal wind resistance costs more fuel.
I think only the electronic controlled qjets have that double cap accellerator pump, This one is too old. No choke installed. The pull off is there but the flapper and rod are gone.

Not sure what solenoid or sending unit you are talking about - i think that might be an electronic qjet thing. Secondaries are opening and closing just fine as well.
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Old 02-28-2011, 03:56 PM
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Quote:
Originally Posted by Torque454
I nearly soiled my pants when I saw 3.60 for diesel here, which is what my other truck uses.
Yeah it's crazy! If Wall Street hear's someone fart, they'll raise the price of natural gas too. Between the middle east unrest and the Fed's buying up bad debt de-valuating the dollar at a time we are trying to rebound from a recession, the price of petrolium was a gimmie to go up.
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Old 02-28-2011, 06:06 PM
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Those old qjets only have a few adjustments.You can downsize the primary jet a little for mid-range,for secondaries you can the spring tension on secondary flaps it takes small allen wrench and a pocket screwdriver.Also the rear metering rods come in different sizes.
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Old 02-28-2011, 06:39 PM
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Quote:
Originally Posted by Torque454
I have a stock q-jet on my 454 (79 C10 pickup). Its a q-jet from a 1978 HD fed emission 454 suburban. Its very simple. No EGR and all that extra stuff. The only vac stuff is for the trans, distributor (through a thermal delay valve on the t-stat housing) and a line to the air cleaner (which is plugged since I have an aftermarket air cleaner) and one to the choke pull-off.

Now for the question. Its a 454, I know its not going to get good mileage, and maybe its just the fact that I am used to the fuel gauges on a suburban (which have double the fuel capacity of the trucks, thus making the gauge move twice as fast on the truck since it has only half the fuel capacity) BUT it kinda smells like its burning rich and its using more gas that i expected (again it may not be doing as bad as it seems tho),

I would like to learn more about how to turn this carb. I have rebuilt it myself, and it runs fine so its not like I know absolutely nothing about carbs but I dont pretend to be an expert either. I do think it might be running rich. I have adjusted the idle mixture screws evenly to achieve the best idle quality. There is no evidence of flooding (like from the float being too high or something) and the choke is not hooked up.

There is no black smoke (that I can see anyways) but the exhaust just smells rich. Also there is no catalytic converter, so maybe thats just how non-catalytic converted exhaust smells? I dont know. I dont really think so, I think it smells rich.

I dont really know what to do about this. There aren't conventional jets in these things, just metering rods. (which if you are reading this you probably know that and know what they do and how they work etc) As far as I know the metering rods have never been changed for any that would make it run any richer.

Is there a way to lean it out a little without changing the metering rods? Id like to tune it for better mileage, power isnt a big concern. I mean I bought the 454 for the power, but maybe I could lean out the primaries and richen the secondaries and ultimately end up with the same air/fuel mixture at WOT, when the power is needed. I dont NEED the power "most of the time". And it stays in the primaries while driving around town.

I think this is ultimately why a q-jet has such small primaries and large secondaries.

Any thoughts, tips, tricks?

PS When I rebuilt the carb I noticed that someone had already been in there and epoxied the Welch plugs in the bottom of the float bowl. I still think they are leaking some because it takes a bit of cranking to get it to start (as if the float bowl was dry) after it sits a while, perhaps overnight or over the course of a couple days. Haven't really paid much attention to it tho, just enough to notice it.

Also since this engine is not original to this truck (it had a 250 I6 originally) it did not have a return line from the pump back to the tank but it did have a vent line from the charcoal canister. Because of this, the return line port on the fuel pump is plugged and the vent line from the tank is open to air.

I dont know if this causes any rich running or fuel mileage problems or not.
CLIFF RUGGLES seems to really know Q-JETS.Has a good book about them.Rebuilds them for a living. Google his name.Some good posts here as well.LARRY
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Old 02-28-2011, 06:52 PM
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The 2 stage accelerator pumps were used about '85-'86 IIRC. They didn't have CCC control, although the electrical connector looks like one for the MC solenoid.

There WERE jets and rods on the primary side, the secondary side only has rods that can be replaced- the "jets" are fixed in place, non replaceable for all intents and purposes. This isn't a problem for tuning- there are 93 different secondary rods along w/20 hangars.

A leaky well can hurt economy like a beach! If you have a leaker, fix it.

If you have a too-short accelerator pump and the pump arm is adjusted for the biggest shot, this can hurt economy badly. Since there were at least 6 different pumps, this happens a lot more than many realize. The pump arm can be bent downward to take up some of the pump travel, that- along w/changing to the lower shot- will help matters.

The power piston spring acts as the power valve of a Holley. If the spring is causing enrichment at too high of a vacuum reading, this can really hurt economy.

Be sure the PCV valve and plumbing is working- w/o it the carb will be rich.
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Old 02-28-2011, 06:54 PM
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Quote:
Originally Posted by sqzbox
Yeah it's crazy! If Wall Street hear's someone fart, they'll raise the price of natural gas too. Between the middle east unrest and the Fed's buying up bad debt de-valuating the dollar at a time we are trying to rebound from a recession, the price of petrolium was a gimmie to go up.
Yeah. And the oil companies have some blame too. Last time gas went way up the oil companies reported record profits which means it wasnt just the cost of oil that went up, it was greed. So we got hosed two times - once for the cost of oil going up and once for their desire for more profit. As if they need it. How much money can they spend. At some point you have more money than you can spend. Perhaps they should give it to the government so maybe our debt to other countries wont keep gong up so much.
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Old 02-28-2011, 06:57 PM
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Quote:
Originally Posted by mcjones
Those old qjets only have a few adjustments.You can downsize the primary jet a little for mid-range,for secondaries you can the spring tension on secondary flaps it takes small allen wrench and a pocket screwdriver.Also the rear metering rods come in different sizes.

I'm not real worried about the secondaries. In fact I could probably richen them up a bit for some more power. I just want to lean out the primaries a little to lessen the rich mixture smell and increase mileage, even if it is just a little. It all adds up. Higher tire pressure, leaner fuel mixture, advance the timing a few degrees, etc.

When I need the power, I can get into the secondaries and let the **** hit the fan.
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Old 02-28-2011, 07:06 PM
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Quote:
Originally Posted by cobalt327
The 2 stage accelerator pumps were used about '85-'86 IIRC. They didn't have CCC control, although the electrical connector looks like one for the MC solenoid.

There WERE jets and rods on the primary side, the secondary side only has rods that can be replaced- the "jets" are fixed in place, non replaceable for all intents and purposes. This isn't a problem for tuning- there are 93 different secondary rods along w/20 hangars.

A leaky well can hurt economy like a beach! If you have a leaker, fix it.

If you have a too-short accelerator pump and the pump arm is adjusted for the biggest shot, this can hurt economy badly. Since there were at least 6 different pumps, this happens a lot more than many realize. The pump arm can be bent downward to take up some of the pump travel, that- along w/changing to the lower shot- will help matters.

The power piston spring acts as the power valve of a Holley. If the spring is causing enrichment at too high of a vacuum reading, this can really hurt economy.

Be sure the PCV valve and plumbing is working- w/o it the carb will be rich.
Well this is a 78 carb so I knew it didnt have the two stage pump, or any electrical connectors. I didnt know that there were any non CCCs that had the 2 stage pump tho.

I dont think I need to get into changing rods or anything based on my reading. As long as the secondary rods are correct and WOT operation is fine. Its the primary side I would like to lean out. I could possibly install a lighter spring under the power piston so that when i do let the **** hit the fan, it might make up for leaning out the primaries with the APT screw, but only when I have my foot in it.

I do need to check on that leaky well situation. I took note this morning when I jumped in the truck and noticed it took several seconds of cranking before It even thought about starting. Part of that could be the no-choke deal, but I kinda think the well plugs are leaking some.

I have no idea if it has the correct accel pump in it or if its in the right hole. I really think that part is ok but who knows. How do I check to make sure its got the right pump and the right pivot setting?


PCV is connected and working.
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