Q-Jet Jetting

[hondarider188]

ok now i now that tuning in a Q-jet is not anything new but i havnt really been able to find the answers iv'e been looking for on here or anywhere else online. so no the specs on the truck, 1983 k10 5.7L engine with edelbrock sp2-p intake and a new(remanufactured) Q-jet and all along with headers as for the rest of the drive train th350 trans and 3.73 gears. so now what i want to do is get my carb tuned to the truck the best i can but i have no idea where to start on the jets or the rods or whats even in the carb now. I know that for almost all carbs and makes they have a reference chart as to the correct jets and rods for your elevation and setup but i cant find anything like that online. The carb was rebuilt by a company called vapex out of california and i really like the work they did to it and it works great but i would like to get the best mpg and performance that i can out of it.

[rich weyand]

Qjets need to be tuned to the specific setup; there's lots of air bleeds and such in there that have to be drilled to size for the specific engine, induction system, etc.  The factory set them up once for each model, and then copied that on the line.

The guy who does this better than anybody is Sean Murphy.  I think he has basically tuned a Qjet to every possible setup over the years and kept all his notes.

http://www.smicarburetor.com/products/sfID1/85/sfID2/86/sfID3/9/productID/428

[hondarider188]

I do know that the carb i bought was supposed to be for my truck. In the ad is advertised as 81'-85' chevy trucks( i beleive that was the right years or at least met the range for my truck) and was for 305 and 350 SBC with electric choke setup. and ill try and go out a little later tonight and get the  model numbers off the carb.

[rich weyand]

But you changed the induction system, with a non-stock intake manifold and headers.

[hondarider188]

Your right about the intake and headers but why can't I jet the carb according to what my setup is and achieve better performance and mpg?

[hondarider188]

ok i tried to get the numbers off of the carb but it appears they have been partially worn off or maybe removed but it also appears to have two sets of numbers and a few of them were there but it wasnt complete so here is how they appear exactly on the carb   1_ _ _ _ _20
                                                                                                                                                                                    2_ _ 8_ _ _p
on the top row it actually makes sense from what numbers are there.  and what would be the fourth letter into the top row their appeared to be a 5 but i wasnt sure but that would make it a 1705 model year(s) carb it is was for sure a 5. and the 20 would make sense as a chevy branch of general motors and the 0 as an automatic transmission but as for the second row i have no idea on it

[rich weyand]

Your right about the intake and headers but why can't I jet the carb according to what my setup is and achieve better performance and mpg?

You CAN modify the carb to get better performance and mpg, but there is a lot more than the jetting, as I said before.  There are air bleeds that are sized by inserting orifices and then drilling them out to proper size for the application, for instance.

[hondarider188]

Ok I see what you mean so if I can't identify the carb further and determine the actual application it was designed for I should look at getting a different carb. I hate to get another one though when I have a good carb already. So can't I just tune the carb in better for now and see how the performance is and wheather or not I need to get one with a better application for my truck.

[rich weyand]

You can run it and see what you get.  And you don't need a new carb: you can have that one rebuilt to the application.  They'll touch up a bunch of other problems that old Qjets get while they're at it.  That SMI page I linked above is for rebuilding and retuning your carb.

[hondarider188]

ok thanks rich i might give them a call tomorrow and see what they charge for it. If it is a ridiculous amount then i will probably try it myself and ill be back to get more info on your recommendations for a base setting.

[rich weyand]

Well, I am not the guy to ask for recommendations on Qjet settings for specific applications.  I just know a bit about the tuning procedure.

You should acquire and read the books by Ruggles and Roe about Qjets:
http://www.amazon.com/Rebuild-Modify-Rochester-Quadrajet-Carburetors/dp/1932494189/ref=sr_1_1?s=books&ie=UTF8&qid=1374813940&sr=1-1&keywords=ruggles
http://www.amazon.com/Rochester-Carburetors-Doug-Roe/dp/0895863014/ref=sr_1_1?s=books&ie=UTF8&qid=1374813967&sr=1-1&keywords=roe+quadrajet

[hondarider188]

Ok say i have been searching the web some more and if i remember correctly this is the carb that i had boughthttp://www.ebay.com/itm/ROCHESTER-QUADRAJET-CARBURETOR-fits-81-88-CHEVY-GMC-OLDS-305-350c-i-180-6892-/390621539140. but i could have swore that the one i had bought said truck not olds. and according to the add their was only one left when i purchased it so i imagine that i got the last one they had. but this was the exact same add except for the truck version. and i think i will go ahead and order the doug rowe book and see how it does.