Quadrajet Guru's Float Question
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Quadrajet Guru's Float Question
In 1968/early 1969 Rochester moved the fulcrom forward (towards the front of the carb) and used a smaller float which increased float bowl volume.
My question is ....did the Qj's that were built before this modification.. which were used on some 68/69 GM cars (including the Corvette 327/427) perform as well as the newer revised "forward fulcrom" setup?
Were the 68/69 engines equipped with the older style setup starving for fuel ect: ?
Thanks
My question is ....did the Qj's that were built before this modification.. which were used on some 68/69 GM cars (including the Corvette 327/427) perform as well as the newer revised "forward fulcrom" setup?
Were the 68/69 engines equipped with the older style setup starving for fuel ect: ?
Thanks
Last edited by Bob Onit; Aug 7, 2006 at 08:37 PM.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Originally Posted by Bob Onit
In 1968/early 1969 Rochester moved the fulcrom forward (towards the front of the carb) and used a smaller float which increased float bowl volume.
My question is ....did the Qj's that were built before this modification.. which were used on some 68/69 GM cars (including the Corvette 327/427) perform as well as the newer revised "set back fulcrom" setup?
Were the 68/69 engines equipped with the older style setup starving for fuel ect: ?
Thanks
My question is ....did the Qj's that were built before this modification.. which were used on some 68/69 GM cars (including the Corvette 327/427) perform as well as the newer revised "set back fulcrom" setup?
Were the 68/69 engines equipped with the older style setup starving for fuel ect: ?
Thanks
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
I didn't know about the repositioning of the lever,..news to me. But yes early Q-Jets used the brass float,..later floats were made from cellumar (sp?) material and is a bit larger. I don't see why either would provide a performance advantage,..really not the job of the float.
If you're running some horsepower and you're concerned about emptying the Q-Jet's fuel bowl, two things:
1) Get a larger needle & seat assy. AC Delco's part number 30-142 has a .135" opening and is what I use most the time. Stock is .125" on most Q-Jets. If you're still suffering from starvation (doubtful), purchase Edelbrock's part # 1980 which has a .149" opening,..this is used primarily for racing and can flood a street carb.
2) Purchase a low pressure, high volume fuel pump. Carter has some outstanding mechanical fuel pumps that come in at 6 psi and 100 GPH for about $65. Don't run more than 6 PSI,..Q-Jets don't like it.
Good luck!
If you're running some horsepower and you're concerned about emptying the Q-Jet's fuel bowl, two things:
1) Get a larger needle & seat assy. AC Delco's part number 30-142 has a .135" opening and is what I use most the time. Stock is .125" on most Q-Jets. If you're still suffering from starvation (doubtful), purchase Edelbrock's part # 1980 which has a .149" opening,..this is used primarily for racing and can flood a street carb.
2) Purchase a low pressure, high volume fuel pump. Carter has some outstanding mechanical fuel pumps that come in at 6 psi and 100 GPH for about $65. Don't run more than 6 PSI,..Q-Jets don't like it.
Good luck!
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Thanks Dark Blue
I am trying to figure out if I can run a 1968 QJ on my 70' El Camino SS
Im not sure if the float bowl capacity is going to be enough for the 454
Any suggestions... anybody?
I am trying to figure out if I can run a 1968 QJ on my 70' El Camino SS
Im not sure if the float bowl capacity is going to be enough for the 454
Any suggestions... anybody?
Last edited by Bob Onit; Aug 8, 2006 at 07:36 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,382
Likes: 6,402
From: At my Bar drinking and wrenching in Lafayette Colorado
First, the early Q-Jets did not use brass floats. All Q-Jets, including the ones built in 67 and 68, used NitriFill floats. Brass floats are aftermarket replacements installed by people who don't know correct Q-Jet setup.
I've run a lot of the early carbs with the first-design float, and I've never noticed any difference in performance capability throughout the rpmk range, even on the big engines. I don't like the early carbs simply because the float pin seat/fulcrum is small and very fragile, and the bowl tends to break out in this area. ut if you have a good noe, I would not hesitate to use it on a 70 BB application. Set float level to .375" for good street performance. If you drag race the car, you can bump it up to .25".
I've run a lot of the early carbs with the first-design float, and I've never noticed any difference in performance capability throughout the rpmk range, even on the big engines. I don't like the early carbs simply because the float pin seat/fulcrum is small and very fragile, and the bowl tends to break out in this area. ut if you have a good noe, I would not hesitate to use it on a 70 BB application. Set float level to .375" for good street performance. If you drag race the car, you can bump it up to .25".
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Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
if you are using the replacement rochester float gm part # 7033752 / 7046300 (most times the float has a #1073 on it) the float is set at 1/16" HIGHER than the original spec with the original float # 7034454 as per a rochester service bulletin that included with every replacement float in years past. this is because the float is larger than the original float that had a problem where it became fuel logged. the average spec is 3/16" to 1/4 max. with the GM replacement float. a 1969 or later carb that came with float # 7037316 now uses a 7037705 float and is set 1/16 lower than the rochester spec since the replacement float has higher density.
the early q-jets also used a smaller needle and seat that may not flow enough fuel for a performance engine so i would use a 0.130 needle and seat.
Bob, also i think i noted in past posts that the pull clip for the needle goes thru the holes in the float arm, it does not as per GM bulletin #206 72-t-2.
henry @ oles carb
the early q-jets also used a smaller needle and seat that may not flow enough fuel for a performance engine so i would use a 0.130 needle and seat.
Bob, also i think i noted in past posts that the pull clip for the needle goes thru the holes in the float arm, it does not as per GM bulletin #206 72-t-2.
henry @ oles carb
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by olescarb
the original float that had a problem where it became fuel logged.
the early q-jets also used a smaller needle and seat that may not flow enough fuel for a performance engine so i would use a 0.130 needle and seat.
Bob, also i think i noted in past posts that the pull clip for the needle goes thru the holes in the float arm, it does not as per GM bulletin #206 72-t-2.
henry @ oles carb
the early q-jets also used a smaller needle and seat that may not flow enough fuel for a performance engine so i would use a 0.130 needle and seat.
Bob, also i think i noted in past posts that the pull clip for the needle goes thru the holes in the float arm, it does not as per GM bulletin #206 72-t-2.
henry @ oles carb
I tested it in a small cup of fuel overnight with it weighted down and it doesnt leak.
They floats are old but never used, the numbers on the boxes are no longer readable and since the float is installed and the carb is re-assembled I would have to pull the air horn ect: to tell you what float I used.
As far as the needle and seat goes. again Im not sure what size it is but it has no windows in the seat and from what I have read, they will fill the bowl quicker than the seats with the windows (the slits on either side of the seat)
Yes I hung the needle in the center of the float rather than using one of the holes on top.
I guess all thats left is to mount it and try it.
The El Camino is at my Mothers house so I wont be able to get to it until later tonight or tomorrow eve.
I used 78/51 for the mains and CJ sec rods (thats all I had available)
Im thinking it might be too lean on the primary but well find out!
Thanks
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
a 7028212 would have had a 71 jet with 46 rods from the factory that is where i would start, do you have a LM-1 (wide-band o2 ) yet? almost every fuel system guru
has that or a exhaust gas analyzer, if not you are not tuning you are just guessing
float level @ 3/16 if it has the replacement float 7033752/7046301
henry @ oles carb
has that or a exhaust gas analyzer, if not you are not tuning you are just guessing float level @ 3/16 if it has the replacement float 7033752/7046301
henry @ oles carb
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by olescarb
a 7028212 would have had a 71 jet with 46 rods from the factory that is where i would start, do you have a LM-1 (wide-band o2 ) yet? almost every fuel system guru has that or a exhaust gas analyzer, if not you are not tuning you are just guessing
float level @ 3/16 if it has the replacement float 7033752/7046301
henry @ oles carb
has that or a exhaust gas analyzer, if not you are not tuning you are just guessing float level @ 3/16 if it has the replacement float 7033752/7046301
henry @ oles carb
unbelieveably hot
I think it had 78/39 mains
I have no idea what it was used on
I set it up as close as I could for a 70 Big Block according to Lars paper
"7040201 CHEV 70 CHVL, VETTE BB 4-SPD 78/48"
I diddnt know I was supposed to set it up to
"7028212 -Chev 68 327/350 AT 71/46" specs
I thought it was best to set it up by the application, not the year of the carb as my El Camino is a 454.
Ill have to see if I have the jets and rods you suggested..I probably have a set
I do not have an O2 sensor as as far as I can tell it will take a lifetime to figure these things out so Guru is something Ill never be as far as Carbs are concerned
Learn something every day
Thanks
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
a 68 or 69 has a totaly different main well (main jet) air bleed diameter than later carbs so the jet package it quite different, if i were you i would consider getting a big block core to start with and then fine tune from there
henry @ oles carb
henry @ oles carb
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Thanks Henry
I really dont need another carb for the car but seeing as I have quite a few QJ's and many parts I thought I would try and build a spare for it.
It also helps me learn about these carbs
Thanks again
I really dont need another carb for the car but seeing as I have quite a few QJ's and many parts I thought I would try and build a spare for it.
It also helps me learn about these carbs
Thanks again
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Brass float verification...from Doug Roe's book "Rochester" p.7, "Floats on early carbs (Q-jets) are made from thin brass stampings soldered into an airtight assembly. These can develop leaks, fill with gasoline, and sink...Nowadays most floats are made of closed cellular material (sorry, I stated 'cellumar') that doesn't leak but sometimes slowly absorb gasoline and lose their buoyancy".
Doug Roe was the Chief Performance Engineer at Chevy back in the day.
Doug Roe was the Chief Performance Engineer at Chevy back in the day.
Last edited by 73, Dark Blue 454; Aug 9, 2006 at 01:12 AM.
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Hey...... thats the same page where I read that the non windowed seats move more fuel than the windowed seats.
What a coincidence ... I couldnt remember where I reaad about the seats..
What a coincidence ... I couldnt remember where I reaad about the seats..
