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Old 12-10-2011, 09:36 AM
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Q Jet mods for use on 355 hp ZZ4

Hi all ,

I thought this would be an easy one but so far, nothing has come up on searches .

I'm using the oem Q Jet from my 1969 Corvette on the swapped motor. Original carb has #67 main jets, 38B primary rods, AN secondary rods and an "H" hanger. This was a replacement carb put in around 1974 and from the spec charts I see they put the carb with the 38B's are for the manual transmission - I have the TH400. The 38B is richer so maybe for all these years the mistake was to my benefit? I know I need jet and rod changes. I would think there would be pretty much a standard answer for this combo like #73 jets and DA rods? Can anyone give me a little help on this? Thanks much, Fred

ps - project attached
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Old 12-10-2011, 10:28 AM
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.073 or .074 main jets with rods .026 tips and around .043 or .044. at the widest part of the taper. On secondary side H hanger is good, longer tip secondary metering rods with .052-.055 tips. You can turn them down in a drill press and a small file if you are good. Personally I prefer the newer castings (typically '75 or newer) that have the adjustable part throttle. I have never really seen any real advantage of using carb off a hp vehicle over a common pickup truck as a foundation for a well built carburetor. Good Luck. I've built literally hundreds of these carbs and found a lot of cool tricks that work for a lot of different applications. If you need a good resource the Cliff Ruggles book is a good place to start. The formula I gave you would be hard to improve upon.
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Old 12-10-2011, 07:16 PM
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This is the setup I am using on my 355 ZZ4 with vortec heads and 2004r trans in a 77 Corvette. The carb # is
17056230 76 Camaro 5.7 Mt.
With these spec. It has worked very well. You are better off with the 75 up castings as previously stated

Carb # 17056203 1976 Camaro 350 5.7

Idle tubes .035 #65

Idle restrictors .046 #56

Idle screw holes .085 #45

Upper air Bleeds .065 #52 changed .070

Lower air Bleeds .074 #49

Pump Nozzle .072 #50

Jet .077 #48

Primary rod .49K

Secondary Rod CH

Hanger G

Bypass Air

APT 3.81 changed 3/4 turn up
I measure this with a pair of calipers form the surface of the card to the flat of the apt screw
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Old 12-11-2011, 11:10 AM
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Thanks much both of you guys for your help . I'll let you know how I make out

Regards, Fred
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Old 12-11-2011, 02:10 PM
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If your carb has the huge upper main air bleeds (like .090 or bigger, I've seen a lot of early castings like this from the factory) Go ahead and use the larger .077 jets and .49 metering rods. If your upper and lower main air bleeds are around .070 (size them up to that if they are not) go with the smaller .073 or .074 jets, otherwise you run the risk of running too rich with a bad bog. Without APT you won't be able to tune it out.
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Old 12-12-2011, 08:53 AM
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I will check the size out - I have also just sent for Cliff's book .
Thanks, Fred
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Old 12-16-2011, 09:25 AM
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Looks like I have the small air bleeds - a 1/16" drill bit is too big for it to fit. Am now reading Cliff's book. I still can't get a positive ID on the carb. There is a 70# on the "top" of the carb that I didn't notice until I cleaned it up and put a magnifying glass on it. The other # is stamped on the side.
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Old 12-17-2011, 02:49 PM
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Thats an incomplete number for sure. In case you don;t know Cliff has his oun forum. You could ask over there.
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Old 12-28-2011, 08:52 AM
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I've decided to try #69 jets (from #67), keep the 38B primary rods, go to DA secondary rods, get a .120 needle/seat and go to the "smaller" float - and see how that works out
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Old 12-28-2011, 07:24 PM
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Quote:
Originally Posted by TWINRAY
Looks like I have the small air bleeds - a 1/16" drill bit is too big for it to fit. Am now reading Cliff's book. I still can't get a positive ID on the carb. There is a 70# on the "top" of the carb that I didn't notice until I cleaned it up and put a magnifying glass on it. The other # is stamped on the side.
You should see somewhere on it that it's made by Carter under license. Most will have the float vent w/the wire linkage attached to the accelerator pump, this is a relatively early Q-jet, pre-'70. The stock jetting should be close, jumping two sizes larger on the jets- unless the primary side was very lean- may be too much.
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Old 12-28-2011, 09:23 PM
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It's still a pretty conservative setup going up two jet sizes, with the 38B metering rods I think you'll be fine.
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Old 12-29-2011, 11:47 AM
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I have the small air bleeds. I think the jetting will be fine - the DA's may be a different story . I also went to a G hanger from my "H". Before I know anything, I have to start it . My automatic called for 42 primary's - when the shop put the "new" carb on (when changing from the Holly spread bore), they put the manual (more richer version) version on FWIW - whatever, it ran fine for the past 40 yrs .
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Old 12-29-2011, 12:03 PM
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Let us know how that works for you. I think that your performance engine could benefit from a little more fuel at part throttle, and you could have gone to .130 inlet valve, but every engine is different, so it might be just right.
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Old 12-29-2011, 12:18 PM
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thanks SB. I wanted to get the carb together and couldn't get bigger inlet's. I ordered .120's from Summitt and they are on BO. For the time being, I will put the standard ones in from the NAPA rebuild kit. If I have to take the carb apart again, it will be easy enough to do some more swaps .

Fred
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