Seems like a simple question, but not finding a simple answer: What size of primary jets and secondary rods should I be running? The car is a '73 Riviera with a modified 455. The engine doesn't seem to have the power I'd like and will ping a bit a mid-throttle. I'm running the "good" 93 octane and the pinging is really affected by not getting the 93 or letting it get old. I'd like to richen it up. I've got a few Qjets in the garage but would like to use the stock carb. This one has the choke coil i the manifold. I can get the model numbers later...I should have those, I know. The engine was rebuilt last year and I've been working through issues The full description is here: GlenL Rebuild The basics are a 455 and... TA Perf TA_290-94H camshaft TA Perf TA_1607 pistons in 10:1 and 0.030 over I'm running a '74 HEI unit. Gotta get that set right, too. Any advice or pointers welcome
Set up the distributor first. Carb tuning is impossible without good spark and timing. * Install a Mr Gasket curve kit. Install the weights and the center peice, which will lock out some of the extra timing spread in the HEI. Try for 12 degrees idle without vacuum and a maximum of 32-34 peaking at about 3000 rpm. * Give the Quadrajet a good inspection. Jetting needs to be about .073 to .074 primary with .043-.044 rods. Secondary will be about right with DA or CY rods. There is much more to do, but a Cliff Ruggles book would be the best guide. I'd favor one jet size rich on a real heavy car with highway gears (.074 jet), and not exceed 34 Degrees mechanical timing ever (except vacuum advance). * Also keep the temp at 180 range. NO heavy throttle with temp above 190 degrees!
Thanks for the info, Gary. That's an actionable plan. A problem with the dist is that it's an early model and the advance kits don't fit it. An adjustable vacuum can might help on idle as the engine doesn't make enough vacuum to pull it fully open. I might just need to get a new dist and take it from there.
Good info given. I will add a little. If you have original quadrajet, it will be an 800 cfm carb and an excellent choice to build from. It would have 73 jets in it originally and. .044 primary rods with double taper. Probably had CT secondary rods which are .0774. Work on your timing as mentioned and then get the carb dialed in. The combo of jets and rods will work fine with a stock to mild engine, but the air restrictions may need opened up some with the cam and other mods you have. Does it idle ok now? You also want to check and verify that the choke is releasing all the way and not restricting the secondary bottom plates from opening. That is a severe performance reduction.
Thanks for the info. It idles OK, or almost OK. It'll need a bit of gas when cold and it idles well when warm. Stock torque convertor. It does fine when not in gear as I've got the throttle adjustment opened up a bit. Yeah... get the dist setup right and then re-adjust the idle. I'll get my carb numbers together...
The carb on the car is a 7043240 which is the correct carb for the car. In a box I've got a 29241 (aka 7029241) which is for a '69 Riv. Swap to this one? I wonder if earlier means dirtier and more power. Also have some Olds carbs for '75-76 455 and a 70422 1 so maybe I missed a number. It has the olds choke with crossover connections and I don't want to mess with that. Is there a good reference that really tells what these are? I've got a '73 Buick chassis manual around somewhere. Just haven't found it. Thanks.
The 7043240 is one of my favorite Quadrajets to build. It is 800 cfm, the largest quadrajet made for Buicks, and most any other engine. That is your best carb to build for your engine in my opinion no matter what others you may have. I have run these well over 500hp engines and building one now for a 640hp BBB. The 7029240 is 750 cfm and the 7043240 has some design changes by Rochester that was an improvement over the earlier years. When adding cam, compression and other things to your engine, you reduce the total vacuum and you need to modify the internal parts of the quadrajet for best idle. You can get them to idle by turning up the idle speed, but that will cause nozzle drip as it is pulling fuel from the mains when it shouldn't be. You would probably go to 74 primary jets to match the 44 primary rods, and maybe change the secondary rods if needed.
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So I've got the right carb to work with. I found my "73" chassis manual and it says "1971" on the front so no specs for this carb. From what people have said it should already have the right jets and rods. Or am I missing something there? How is it best to move ahead with this? I've rebuilt a Q-jet in the past so not worried about that. I've got the chassis manual and Roe book. I was thinking to swap parts at the time. Or is it better to have it rebuilt by a pro? Would they adjust it better and tune it outside of just putting in different parts? I'm open to suggestions on where to send it. Thanks!
Modifications are needed to your idle circuits so it will idle properly without nozzle drip from opening the primaries to far. Need to pull the air/idle rods and drill them, base plate and bypass air. Lots of drilling out by hand. Add the adjustable part throttle screw to help in that range, have to pull the original from the base plate.. You can go to a 75 jet and a AU rod or similar size for secondaries and that is a good combo for a healthy big block Buick. Get parts from cliffshighperformance.com. Don't use a parts store kit, they are junk. Need large needle/seat, and good accelerator pump. Change your power piston spring also so carb will work better with the lower vacuum. His kit will also have the plastic cam for secondary air valves that needs replaced. I always pull all the throttle blades and drill and install the bushings on the primary side also to prevent vacuum leaks. I hope this helps. If you are going to change up the components for the engine, the whole carb needs to be coordinated.
