Q-Jet rebuild for performance

[MaineShark]

There was some discussion a while back about how to rebuild a Q-Jet with performance in mind, and several good articles and links were posted. I can't find the thread, and my former computer died so I lost all the bookmarked links...

So let's start from scratch:

I want to rebuild a Q-Jet for performance. The first question is what core to use, if there is any difference. I have one that came out of an '84 pickup with a 350. On the plus side, it was completely free. On the possible negative side is that I don't know what sort of smog modifications may lurk within, and how well they can be circumvented. So does anyone know what cores are good to start from?

The second question is what modifications to make. The only one I remember off the top of my head was something about making sure that the throttle plates and the air door could all open fully, and I'm sure there are others.

The third question is where to start tuning from. What tools/supplies are needed to adjust the jets and such, and how do I go about getting the carb into proper tune?

Thanks for any help.

Joe

 

[72Mako]

Joe, sorry I don't have any links. However when I rebuilt the old Qjet on my vette and later on my Chev truck, I found a really good book to assist. It's called Rochester Carburetors. Author is Doug Roe and it's from HP Books. It cover stock rebuilds as well as tuning/modifying for performance. 176 pages. Saw you weren't getting ant replies, so hope this helps. Charles

 

[72Mako]

Joe, sorry I don't have any links. However when I rebuilt the old Qjet on my vette and later on my Chev truck, I found a really good book to assist. It's called Rochester Carburetors. Author is Doug Roe and it's from HP Books. It cover stock rebuilds as well as tuning/modifying for performance. 176 pages. Saw you weren't getting ant replies, so hope this helps. Charles

oops, thats any replies.:L

 

[Culprit]

The book 72Mako mentioned is a good book. I've read it, but haven't put it to use yet (need to get the brakes put back together first). From what I remember, most of it is geared towards first making sure that the carb is in good condition and working properly to begin with, and then chosing the right size metering rods and drilling a series of holes in them so that the secondaries kick in in more of a progressive manner rather than all or nothing. The Q-Jet design is so solid, that I don't remember any other major modifications that the book recommended.

Semper Fidelis,
Culprit

 

[silver 80]

joe
shoot me an e-mail I have some papers on this subject but can't seem to upload them here.

 

[bradfordsvettes]

a good rebuild on a q-jet works wonders the big block carbs flow more cfm there is info out there for metering rods and jets for the pimary and rods for the secondaries. the gm parts are getting harder to find i do not know if anyone makes the pieces in the after market. my personal opinion is a q-jet set up right beats a holley all to h*ll steve

 

[VRGSVET]

Go to www.corvettefaq.com and click on "Intake". There are several good articles by Lars there. He's the Q-jet guru. Edelbrock sells most of the parts you need to build a performance Q-jet. Check out www.edelbrock.com or you can get their parts from any of the big parts sellers like Jegs or Summit.

 

[BeaterShark]

I definately recommend the Doug Roe book also.

From what I remember, I think he states that all are just about as capable except the computer controled ones.

Another thing worth mentioning is that the carb you mention probably has "truck" primary rods. These have .036" tips instead of the .026" tips used originally in Corvettes. The .026" tip rods are real hard to find.

Edelbrock still sells selective parts (rods, hangers, jets, etc), but their primary rods are the "truck" rods.

 

[MaineShark]

Thanks, guys. I'll check into those sources.

Joe

 

[VRGSVET]

I definately recommend the Doug Roe book also.

From what I remember, I think he states that all are just about as capable except the computer controled ones.

Another thing worth mentioning is that the carb you mention probably has "truck" primary rods. These have .036" tips instead of the .026" tips used originally in Corvettes. The .026" tip rods are real hard to find.

Edelbrock still sells selective parts (rods, hangers, jets, etc), but their primary rods are the "truck" rods.

I think you got a little mixed up on that. Read the Q-Jet Tuning Paper by Lars Grimsrud found at Corvettefaq.com. He says ALL primary rods have a .026" tip. They have different tapers for 74 and earlier and 75 and later. Both types are available from Edelbrock. Now, the SECONDARY metering rods come in three different tapers: long, medium and short. Lars says the long ones are most commonly available aftermarket, but the short ones are the best for performance tuning. He says you can often find the short ones in old truck carbs. But, don't take my word, read the article. Lars knows his stuff.

 

[BeaterShark]

I'm going off the top of my head here, but I'm pretty sure Lars is who told me about the .036" tip primary rods. I think they were stamped with a "D", where the .026" rods are "B" rods. I guess I should have stated that I don't believe all trucks Q-Jets had "truck" rods and I have no idea when/what they were originally installed on.

I know I measured at least one of my 75+ Edelbrock primary rod tips and it came up .036". These are marked with a "M", as I beleive all 75+ Edel primary rods are.

 

[VRGSVET]

Ok, take a look at page 151 in the book, Rochester Carburetors, by Doug Roe. It says, "All rods have a 0.026-in.-diameter tip at their smallest point, except rod 50D, which has a 0.036-in. tip." At the bottom it says, rod 50D is a "special rod used in GMC Motorhome - has 0.036-in. power tip." Like I said, I'm not an expert, but I know what I read. To me that says there is only one size primary rod out of dozens that had the .036" tip, and it was only used for GMC motorhomes. If you know something different I apologize, but I just don't want erroneous information posted out there. No offense intended.

Paul

 

[BeaterShark]

The Doug Roe book was published in 1981 (at least mine is). This carb we are talking about is a 1984 model. Also, in 1981, Edelbrock did not make Q-jets.

I still stand by the following sizes for the primary rod tip sizes based on the letter stamped:
B: .026"
D: .036"
M: .036"

I also *think* that the 75+ primary rods are no longer available from GM and that all 75+ Edelbrock primary rods are stamped with a "M".

I have also heard the slang "truck" used to describe the rods with .036" tips.

I probably did give mis-information when I said that all Edelbrock primary rods have .036" tips and I corrected my self on that issue by specifying the 75+ rods (stamped with a "M") have .036" tips.


I'll end my post with the same disclaimer that is in the Doug Roe book:
Notice: The information contained in my posts in this thread is true and complete to the best of my knowledge. All recommendations on parts and procedures are made without any guarantees on the part of me. I disclaim all liability incurred in connection with the use of this information.

 

[Evolution1980]

...I want to rebuild a Q-Jet for performance. The first question is what core to use, if there is any difference. I have one that came out of an '84 pickup with a 350.... Joe

Joe, I don't know a whole hell of a lot here, but since no one else mentioned it... You are driving a '79 right?
Didn't GM start making the carb with computer controls in '81 with the Crossfire injection? I believe by '84 that same/similiar carb had made it's way into most of the GM lines. I would think you want a Q-Jet that was pre-computer controlled, so you'll want to find one from 1980 or prior.

 

[MaineShark]

I haven't noticed any electrical connectors on it, so maybe the trucks didn't get the electronic controls? Or maybe it's just not the original carb...

In any case, once I have a chance to clean it off (it's pretty grungy), I'll check for casting numbers and figure out what it is, exactly.

Joe

 

[VRGSVET]

The Doug Roe book was published in 1981 (at least mine is). This carb we are talking about is a 1984 model. Also, in 1981, Edelbrock did not make Q-jets.

I still stand by the following sizes for the primary rod tip sizes based on the letter stamped:
B: .026"
D: .036"
M: .036"

I also *think* that the 75+ primary rods are no longer available from GM and that all 75+ Edelbrock primary rods are stamped with a "M".

I have also heard the slang "truck" used to describe the rods with .036" tips.

I probably did give mis-information when I said that all Edelbrock primary rods have .036" tips and I corrected my self on that issue by specifying the 75+ rods (stamped with a "M") have .036" tips.

You're right on both counts, now that I understand what you're saying. However, my book was copyrighted in 1981, but is a revision and includes information for carbs up to 1986. That doesn't mean it's correct, though. Also, you are right about Edelbrock rods, I checked. The 75+ rods do have a 0.036" diameter tip. So, now I'm confused. Were the GM rods in 75+ vehicles .026 or .036? Because, like I said, both Lars' paper and the Doug Roe book say "all" primary rods have a .026" tip and both sources covered carbs at least up to 1981, if not later.

 

[Stingray6974]

Computer carbs started in 1980 on some Oldsmobiles and California 305 Corvettes. All Gm cars in 1981 had them. 1981 was the only year the 350 Corvette used them. Pickups did not use the computer carb but in 1985 and 1986 used an internal solenoid to reduce accelerator pump shot after the engine was warmed up.

Mike