Hey all! Im rebuilding the Q-Jet from my -79 Chevy C20, it is the original carb for the truck, #17059508. Someone has been there before me obviously, since there was a blue cup in the accelerator pump and half the washers from the airhorn bolts were missing. But other than that it is really clean and its all there. Im using Cliff Ruggles book for reference and its great, the only thing that confuses me is the APT setting. When I took the carburetor apart, I calculated the turns from the bottom-5 and 3/4 turns. I know it is preset from the factory for the application, but from what I could find from the internet , it is usually 2-3 turns from the bottom. I do have the factory shop manual, but there is no mention about that- other than it is preset-DON`T touch it! My question is: what would be a good starting point for this or could it really be 5 3/4 turns? The engine is 350+TH350, 3.73 and probably around 4400-4600 lbs. Any input would be greatly appreciated:TU:
what rod and jet combination is in the carb? Normally I would like to have an initial setting of say 3.5 turns or so but if the las rebuilder had too large a set of rods put in then maybe the APT got run out further to make up for it. 5.75 turns seems excessive to me, but if it positions the rods well in the jets, then the # of turns dont matter so much. I would start at 3.5 and try it. if its too lean at cruise then turn it out more. Dont forget before reassembly to cut a screwdriver slot in the top of the screw.
Hey Ken, thanks for the reply. I cant remember the main jet size right now(74 maybe), I will check that and the metering rod size when I get back to garage. If everything checks Ok, Ill put the APT screw 3,5 turns from bottom. I wonder if there is a spec sheet somewhere for any given application, i.e. jet size, metering rod and that APT adjustment of course ?. I tried to study the "1979 Chevy Light Duty Truck" shop manual, but I couldnt find such information, maybe I should try little harder. For easier future APT adjustment, I will try to remove the aluminum plug in the airhorn, right on top of the APT screw, any experience with this? Im little worried about breaking the airhorn, seems to be pretty tight fit. This is my first Q-Jet ever, Im really excited tinkering with this, I really couldnt have designed a better carb myself:laugh:!!
with the air horn off, just use a punch from underneath to tap the aluminum plug out. If the air horn is still on and you need to remove the plug, drill a 3-32 hole into the aluminum plug and use a 1" long sheetmetal screw or drywall screw to thread into the hole, then use a pair of dikes or pliers to lever the plug out via the screw. I am sure theres some spec for the rods/jets in the book somewhere, but I doubt it will give anything for the APT setting. Thats more of a tune-it-on-the-go thing. If its set too lean you will run hot and surge or miss at highway cruise. If its too rich, gas mileage will suffer. I would tune it as lean as i could without symptoms of overleanness.
This is great info, thanks so much Ken! I did find the correct jet/main rod sizes for my carb from Capital Corvette Club pages, it is 73/51 , Ill check what I got when I get back to it. One last question, if you dont mind, what type of screw in plug would you recommend in place of the aluminum plug?
I would use a 1/8NPT socket head plug youll have to drill the hole open a little (11/32" iirc) and tap it 1/8NPT 'til the plug sits flush.
Ill do just that, nice to deal with such a knowledgeable person! Thank you again and all the best to you, Antti
Most likely that carb is the single main airbleed model and using 51M series primary metering rods with .073" main jets. Those metering rods will provide .010" worth of adjustment as the upper section tapers from .051" down to .041", then "steps" to .036" tips. With that jet/rod combination you have limited control of A/F from rich to lean anyhow, and its going to like a pretty high APT setting. Id start out at 3.5 turns up from seated, and nothing wrong with having to go up to 4.5 to 5 turns if the engine responds well to it. For the APT plug hole, a 3/8-16 set screw works well as a plug. No need to drill/tap for NPT threads, just use a tap and tap deep enough so the set screw tightens up flush with the casting surface. The APT screw can be slotted with a fine hacksaw blade, or you can make a tool to turn it by grinding a notch in an old screwdriver using an cutting wheel or really good file. Check the power piston hanger arms closely, 99 out of 100 of them are bent if someone got there before you did. They MUST be level and exactly even or the APT settings will NOT deliver the desired results. I also highly recommend upgraded the pump and seal, most of those blue seals are not all ethanol compatible, we have our own seal that will hold up in this new fuel, the over the counter stuff is JUNK and will swell up, stick in the pump bore and fail over timeCliff
Cliff, thank you for your input, plenty of good information. Ill try to do my best to make it right the first time, it was pretty clean to begin with, but I had it washed in ultrasonic cleaner just in case. And what a relief it was when I saw that the greenish plating was still intact after that! I did put epoxy on the bottom well plugs, replaced the idle tubes, new float, needle and seat, new blue cup, garter spring, duration spring in the accelerator pump. A new choke pull off and all new gaskets of course when I start putting it back together. I will check the power piston hanger like you mentioned. All the parts and the book came from Greg Gesslers new Quadrajet Parts.com, Im confident he sells only quality stuff based upon my earlier dealings with him, most of my Buicks 494 parts came from him and Rob Chilenski. Cliff, people like you, Ken and many more here make this board so special, I did ask the same APT guestion on a local board here and the general consensus was: "who the h.... wants to use a Quadrajet anyway?" , so here we go! I tried to hold myself back, but your username kinda makes me want ask: would you happen to be Mr Ruggles himself?:laugh:
Well, how about that!:grin: Thank you for creating that book, it is truly a good one. I really enjoy reading it and it makes the infamous Quadrajet a little less intimidating for a beginner like me. On page 12 I even found an explanation why the base plate, around the idle mixture screws, looks like it does! A real hack job, yet fully functional. Im really looking forward the outcome, I should be Ok with help from you and Ken. Thanks again !
My time is pretty limited but I try to help out on a few of boards when I can. I start at 5am every day, hit the ground running and work till well into the evening. We run a long backlog on carb work, so try to help out with parts and tech advice, etc......Cliff
