Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: Marx3 on April 07, 2010, 01:34:55 PM
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Hi, would this carb be a good object for a 468 pontiac (1973 455 ) ?
engine has 7M5 114 cc heads, rpm intake, and a 278/290 adv. dur. Crane cam, M21 and 3.55 gearing.
Upon disassembly i discover that this carb has the 3 brass tubes in the airhorn, plus that extra jet/rod in front of the powerpiston ( I thought I would never see this )
Now I have doubt as to how good this carb would be, even if it is an 800 cfm unit.
If it is good, I am thinking recipe 2, as of now ( 650 Holley ) it pulls 14-15 Hg at 800 rpm.
thanks, Jacob
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No one knows if this carb is worth rebuilding for bottom-end Pontiac power?
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Those carburetors are just as good as any other late model large cfm q-jet. They are more difficult to set-up, using the early styel APT system and auxiliary Power Piston. We block off the auxiliary PP and install larger jets and metering rods, the rest of the rebuild, recalibration, etc is the same as any other late model unit, and well described in our book.....Cliff
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Great, so it will get recipe 2.
It has:
jets: 60
rods: 35J
Hanger: N
Rods: DH
Which of these would you change to what, if all bleeds an channels goes according to recipe 2?
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btw, I have your book ( my Qjet-bible! ), and have already built 4 or 5 carbs to recipe 1 and 2, I just need guidelines for the jet/rod-size, and I will be quiet :-)
If I need jets/rods for this rebuild, I will order it from you.
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I'd install .073" main jets, and 44L metering rods. You can fine tune with the APT system if/as needed.
Block off the aux jet, and the vacuum to the aux power piston.
The rest of the build is the same as any other late model q-jet, except some 75 models still had the upper idle airbleeds in the airhorn instead of in the main casting....Cliff
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Okay, I will place an order for jets and rods!!
I thought upper idle airbleed was prefered in the airhorn?
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Doesn't really matter, but you MUST account for the difference in size, as the main body upper bleeds are "indirect".....Cliff
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Okay, got it!
So does the fact that it has primary POE affect the any sizes on the secondary side? and what should be done to the primary tubes?
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No, the primary POE does effect the primary side A/F when the throttle is depressed quickly, low vacuum situations, etc. The jetting I recomended is for the primary POE intact. If it were not being used, I'd use a 41 or 42 metering rod instead....Cliff
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Okay, I will block off the aux PP. and leave the POE intact.
You mentioned that this carb should have the APT located somewhere in the front area of the mainbody, but I really cant see anything that could be an APT screw etc.
Is it still worth the rebuild, without APT and will the jetting you recommended still apply?
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The APT system is the black thing with the single rod hanging out of it that goes into the single jet in the front left (as viewed from the front) corner of the main casting. It has a brass cup in the airhorn right over the adjustment screw.....Cliff
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this thing?
If yes, what do you normally do to gain acces to it from the outside? Or are you just supposed to take the airhorn off for adjustment?
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There is a brass cup plug in the airhorn that can be removed to access it....Cliff
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Are you supposed to leave the hole open, or put back the plug after adjustment?
I guess the tip in procedure is still the same.
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Oh, nevermind. I found it pops back in...
And still guess tip-in is standard procedure.
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Hi, got the carb installed and adjusted, car runs and pulls way better than with the newly rebuild Holley 750 that was on before,
but... the engine pulled 14-15 Hg with the holley, at about 700-800 rpm, and it pulled about the same with the Qjet, but the idle quallity was not that good, I kept climbing and falling again. Especially if you flipped the throttle, the idle would take a long time to "calm down" again.
The mixture screws did not have much effect, I could kill the engine by turning them all in, but I did not feel that I had great control...
With the engine idling at 800, if i closed the choke flap sligthly rpm would climb alot instantly, so I figured it was lean in idle, got the idle tube to .040 and the DCR to .058 as kind of in between recipe 2 and 3. It helped a bit, but no much.
Also I did not feel I had any control over the APT mixture when performing the tip in procedure...
What could be wrong?
( It is important to mention that the engine is freshly rebuild professionally, but we have strong doubt to the quality of the work as the engien has run troublesome ever since it was rebuild. )
So do we have a carb problem or an engine problem ?
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If closing the choke flap shows an rpm increase, and smoother idle, add some idle fuel, either larger idle tubes and/or a tad more idle downchannel.
The front located APT system adds fuel under the jets, directly to the main system. We try to set those carburetors up so that they are really close with the APT fully seated, then add in a tiny amount of fuel if/as needed via the APT.....Cliff
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Okay, great, I like it when my assumptions tunr out to be correct!
How big would you suggest for the tubes/dcr?
Funny regarding the APT... I found it to operate the best with the screw bottomed out..