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Discussion Starter #1
I have a 78 chevrolet pickup with the factory 350 with the factory Quadrajet four barrel.I believe the carb has been rebuilt in past few years.The problem is that it seems to run too rich.I'm trying to figure out what screw is the Lean/rich ajustment and which direction it needs to be turned.PLus any other helpful tips with playing with a Quadrajet.

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1970 Chevelle SS396
1969 Camaro
 

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1. See if the choke is working properly. When the engine is warm your choke should be in the open position.
2. With engine shut off turn idle mixture screws all the way in and back them off 2.5 turns and then start engine. Then adjust them to you hear the best possible idle. While doing this I think it is beneficial to have a vaccuum guage while adjusting to get the highest vaccuum reading.
3.Maybe the float is not adjusted right or the needle and seat might have dirt or something lodged in it. If you never have messed around q-jets before you might want to let someone look at for you that is experienced. The first two things I think just about anyone can do, the latter might involve taking the top of the Carb loose.
The two screws in the front are the mixture screws to lean turn in and to richen turn out.
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Malibu 307 (aka 406 smblk)
69 SS 396(aka 454)
Wolf in sheeps skin


[This message has been edited by plain 69 (edited 12-13-99).]
 

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What makes you think it's running rich?

When do you think it's rich (idle, under partial throttle, under full throttle)?

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68 El Camino...Slow, Much Work Required
98 Z28...Fast, No Work Required
Goodfellow AFB, San Angelo, TX -- Where? That's what I said..
 

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Discussion Starter #5
The choke on the this Carb is controlled by the heat tube.It seems to run rich at idle and at full thottle.At warm up you can smell the raw fuel around the truck.
 

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After you go through the checks that plain69 aptly points out, remove the carb and look underneath. Towards the primary bores you'll see two knobs sticking out. They're machining plugs. Notorious for leaking. They're usually epoxied up on rebuilds and I've always done that. Check to see if they're wet. If so, you found the prob. If not, it's time to take it apart. Before you do you may want to read my QJet Tips#1 post in Tips and Fixes on some general disaasembly instructions. It's hard to find, I put it up back in the summer but worth it.
 
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The air bleed adjustment screws on a 78 model Qjet are not visable, may be sealed in the base plate. I have seen guys hack open the base plate to expose the adjustment screws.

The last factory fully adjustable Qjet was made in 73, before they made it difficult to get at the screws.

You mentioned it had been rebuilt several times, someone may have opened the access to the screws.

Rich at idle and rich at wot, two completly different feed circuits.

Needle and seat or maybe that stock hard foam float is junk. Hard to guess at this.

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Wally
Gold #67
67 Malibu "Small Block"
90 SS454
71 Malibu "Small Block"
93 torch red vette



[This message has been edited by Wally (edited 12-14-99).]
 

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If you can get to the two idle mix screws at the front of the carb in the baseplate, screw them all the way in while idling. if your carb is operating properly it should lean out and kill the motor. If it does, good news! just screw them all the way in (don't over-tighten) and back them out 2.5 turns each...Should be good to go.

If not, pull off the carb and check those epoxied passages as already posted here. If they're leaking, clean 'em and JB-Weld 'em.

Try it again. If it still runs with the idle screws in, carfully (wear a face shield if you have one) look into the primaries at idle with a flashlight...if you see gas seeping in anywhere on the top side, your float is sinking, out of adjustment, or you have some junk between you needle valve and seat. Bad news is, you have to pull the top of the carb off (not easy on a qjet) to fix this. Buy a carb rebuild kit and follow the instructions...replace the float too while you're at it.

Rich secondaries...You know this I assume because the engine bogs when the secondaries kick in??? If so, check the metering rods in the secondaries by taking off the screw in the center of 2 secondary plates...It's centered in a Y-shaped lever with the "Y" facing the front of the car (don't drop this screw!!!) pull this assembly up and check the rods...They should be straight and have a tapered end and also be equal length...Make sure the "Y" is sctraight and sits flat on the solid metal cam the screw goes into. These pieces control the amount of gas that comes in when your secondaries open...Finally check the vaccuum diaphram that sits on the passenger side of your carb...Have someone start your truck and whatch it...It should immediately pull the linkage going to your secondary valves toward the front of your truck...If it doesn't or doesn't hold the valves shut, check the vacuum line for leaks (should be about 1.5" long and hook to the front left side of carb about half way up from the bottom). If the line's okay, pop off the diaphram and replace it.

-good luck!

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68 El Camino...Slow, Much Work Required
98 Z28...Fast, No Work Required
Goodfellow AFB, San Angelo, TX -- Where? That's what I said..
 
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