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peggyfran1
User
Feb 28, 2010, 8:14 AM
Post #1 of 5
(1443 views)
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i have a1985 gmc pickup, just put a new timing chain on. also a rebuilt quadrojet 4 barrel. im trying to adjust the carb, got it running fair but dont want to idle when i put it in gear. any one know how to adjust the carb screws would be helpful.
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 28, 2010, 8:51 AM
Post #2 of 5
(1437 views)
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Re: carb adjustment
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Much of the last Quadrajets blocked adjustments but you can adjust idle speed if that's all that's wrong.
Base idle which is commonly set to a "stall" or near stall to prevent run on is here.........
There are a zillion different adjustments going on with late ones and probably uses a "dashpot" for running idle speed. Could be electrical or mechanical with vacuum - can't know from here. If original the idle mixture screws would be blocked off with hard metal caps which could be removed with carb off, they can be chiseled out at the risk of wrecking it too - nice. They shouldn't need messing with but could need cleaning out which you really can't with them in place.
Each carb will have a # punched into it with specs unique to it. If you can find that # you can find adjustment specs of what is adjustable. Look at pic below for that # which may start with or be like 17051234 (just an example) and you may need to wire brush or light sandpaper area to read it. See location below.....
Do keep count/track of anything you mess with such that you can put it back to exactly where it was! That if some adjustment makes things worse!
There are very likely three idle speed adjustments, one for cold on passenger's side on a step cam, one set to a stall usually as discussed, and if the third would be as discussed while running.
There are high chances this isn't the original carb to the truck as the late carbs were such a pest some folks changed them for older styles, still involved but more ability to adjust. Carb must have both pull-offs wide open and be warmed up for final operating idle adjustment or it will end up wrong which is why you should keep track of any changes you make,
T
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peggyfran1
User
Feb 28, 2010, 9:12 AM
Post #3 of 5
(1433 views)
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Re: carb adjustment
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thanks it does have the two screws in the front. i got the carb from mountain man in ark. i have tried to adjust the two screws seems to help the more i turn them out. but still wants to die when put in gear. could be something else going on. it was popping through the tailpipe before i put the timing chain on.
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chickenhouse
Enthusiast
Feb 28, 2010, 10:14 AM
Post #4 of 5
(1430 views)
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Re: carb adjustment
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"i got the carb from mountain man in ark" I haven't sold any Q jets lately!
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Feb 28, 2010, 2:31 PM
Post #5 of 5
(1425 views)
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Re: carb adjustment
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Peggyfran: Again, keep track of adjustments by how many turns or whatever so you can at least go back to the start point. The timing chain just put valve timing back on target - a great thing for older even real chain engines.
The infamous Quadrajets were the staple of GM for ions. They had so many finicky details many new out of the gate weren't right. You need to have a basic idea that it isn't or is the issue. All tune-up items must be right, any vacuum issues solved and basic engine in good health or you could make adjustment that may cover up a flaw elsewhere. These carbs are mostly excellent but are like sewing machine repair to get right. I used to re-do them blindfolded back when more were out there. Not easy and intimidating to totally dismantle, clean, adjust off vehicle (so called rebuilding is mostly clean, adjust and new gaskets) and those kits are not expensive and some come with very nice details for the exact carb you have.
Whole new ones are still big bucks and not always the solution. Make sure vacuum lines, diaphragms are working and not leaking. Rear (secondary) pull-off usually had a metered leak - this one may not. Choke must heat up via electric heater on later ones and would feel warm/hot on the black case that's riveted in. You actually need to drill those out as they don't want you to mess with things on late ones.
If old the problem areas are diaphragm leaks, base gasket can leak, choke not heating (would run rich), float level off because of heavy- gas soaked floats s these should be a composite not brass like much older ones.
This and any carb is chiefly a bottom, middle and top. The bottom has direct manifold vacuum and is responsible for idle and first part of part throttle then middle (called intermediate) takes most of the share of distributing fuel, and finally WOT (wide open throttle) where the chief performance is the other two getting out of the way! Just FYI - it's a two barrel carb until you really get on it. Like carbs are, adjustments can and will effect the next setting so they need be done in order. Problems elsewhere just confuse tweaking adjustments.
Try to rule out other issues as possible and then move on to the carb,
T
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