So, I am a noob on this site but I have a Q-Jet question. I have a '72 Riviera w/ 455 and automatic. My Q-Jet is #7043244 LD & 1373 below. This looks to be a 1973 Q-Jet for a 350. I'm guessing this carb is limiting my HP. The Q-Jet decoder page says I need a #7042240 or #7042242 for a '72 455 auto. What are the differences between these two carbs? Also, what are the specs on the #7043244? Right now, it seems that my idle mixture is way rich. It will just about choke you if you let it sit and idle. I have tried to adjust the mixture screws with a vacuum gauge. That has been a while back and I don't remember how much vacuum it was pulling. The engine is stock. Any info would be great. Thanks, Dave
The 70 455 came with a 750 cfm qjet. Your 350 carb is a 750 also. If set up internally after a rebuild it would likely run fine. If you want the little extra that the correct 800 cfm qjet will bring they are plentiful. But get one rebuilt by one of the pros on this Board. It really makes a difference in how it will run and idle. These rebuilds involve more than the installation of an over the counter NAPA kit.
Other than the 50 CFM max airflow, The difference is in the "programming" or tuning of the carb. The ~240 carb would be set up a bit richer. Not just jets and rods, but the idle restrictions and such, internally. The ~244 carb would work fine on your car with the right rebuild. ANY Qjet you use will need to be tuned to your application to get best results. Get Cliff Ruggles' Book on rebuilding the Qjets. It is amazing the amount of power that can be drawn from the Qjets with the right mods. It is a good read.
When I first got my Skylark, it had a 350 Q-Jet on it and I could never get to idle correctly. After adding the correct 455 Q-Jet plus the RobbMc fuel pump, I can get scratch from the tires just by punching it if I'm going 50 MPH or less after adding the new fuel pump. When the 4bbl kicks in and the 200-r4 down shifts at the same time, hold on. Scare the crap out of me the first time I did it.:laugh:
I agree, the 350 carb will never be right on your 455 unless you drill out the idle air passages and re-jet it.... Lots of trial and error, better off finding a 455 carb.
I disagree. The 750 cmf carb is more than enough carb for a stock 455. The car will run better with a WELL tuned 750 Q-jet. This has been proven to my by a Stock Eliminator racer. Both on track and on Dyno.
Thanks for the input guys! Is one mixture screw for air and one for fuel? I read that 3 turns out is a good starting point. Also, I read that 2.5 air and 1.5 gas is good. I just don't know which is which. The cars runs alright and idles alright but the exhaust fumes make me dizzy. It has me considering putting some cat. converters on it. I was planning on adding some shorty headers from TA Performance and replacing all of the exhaust system. This being the case, should I seek out the 800 cfm carb or stay with the 750?
I agree.... But the CFM really isn't the issue. As mentioned above the air / fuel idle circuits for the 350 engine are not optimal for the 455. if you jet up (on the primaries) for the 455 without drilling out the air idle tubes, you will run into a over rich air / fuel mixture at idle. The secondaries can be another can of worms too. The 800 CFM can at times perform better at the gas pump (unless you're a lead foot), as the larger primaries delays the transition into the secondaries. Good for dedicated street cruisers; maybe not a really concern for street/strip. Tommy
Like many of us are saying it will never run right with that carb. You will need a 455 carb to get good idle and performance, either 750 or 800 cfm is fine. I prefer the 800 cfm units since they have larger primaries and have more performance without cracking the secondaries open. In a drag race the 800 cfm carb may have little or no advantage over the 750 cfm unit however on the street the difference is night and day! Better acceleration at part throttle and less need to punch the gas pedal. Now you asked about the idle mixture screws, well this is how I set them and it will make your car run better. However the reason it runs so rich is because the 455 demands more air than the 350 carb allows at idle... You will want to get a 455 carb. You can set the idle mixture screws to get highest vacuum readings (and smoothest idle) with the wheels blocked and e-brake on, vacuum advance plugged, at the rpm at the spec. for that year and motor. 1.disconnect and plug the vacuum line to the vac advance canister 2. block all 4 wheels 3. connect vacuum gauge 4. start car and put it in gear 5. adjust idle mixture screws to achieve max vacuum and smoothest idle (make sure they are equal turns out and start at 2 full 360 degree turns out) 6. re set idle speed (it should rise with the right idle mixture setting) 7. go back and check initial timing 8. As an extra step I then go back and fine tune the idle mix screws again but this time just adjusting a 1/8 of a turn to get it perfect. Some people like to set them 1/4 turn lean (in) once max vacuum is achieved.
No, they both are screws that meter the amount of Idle air-fuel mix that flows. The actual Idle Air/Fuel mix is set by the Idle tubes (fuel) and the Idle air bleeds (air) inside the carb.
If its that rich something isnt right with that carb, regardless of what kind of carb it is. A properly rebuilt 750 carb should run alright ,it won't be what should be on there but the physical running of the engine should be ok,in my opinion. Make sure your ignition is working properly,could have a weak spark or something of that nature also.
A 455 needs more air at idle than a 350. That is easy to understand. When you put a 350 carburetor on a 455, the throttle blades need to be farther open so the engine will have an acceptable idle. If the timing is relatively retarded, the throttle blades need to be open even more. The primary side of the Q-jet is very efficient with the triple venturi. It does not take much to initiate fuel flow from the mains. What happens is, instead of idling on the idle system, the engine idles on the mains. The mixture needles become unresponsive, and the mixture goes rich from the raw fuel being dripped from the mains. (nozzle drip) We have seen this many times before. Get a 455 carburetor.
Possibly, I would just find oone on e-bay or better yet here on the board.... Place a wanted add on here in the parts wanted section and then sell your 350 carb to someone who has a 350. Set the idle mixture screws like i posted above and then at least it will drive better till you get a 455 carb.
The idle mixture screws have no effect under the scenario I described. The only way to get that carburetor to run right on his engine would be to re-curve the distributor so that he could run a lot of initial timing. Then he MIGHT be able to close the throttle blades enough to run on the idle circuit of the carburetor. Maybe not. Best to get a 455 carburetor.
I agree, I just wanted to see him get the 350 carb run as good as it could until he makes the carb swap.
My 1979 350 Carb runs as good as any Holly or big Thermoquad I've ever had on my 455. Before I installed it, I did a second recipe as noted in Cliff ruggles book and already had my distributor at 20 derees max advance. With 14 degrees idle and 34 max it runs "like fuel injection". I had to run a 4 hole spacer instead of the open spacer because the carb had to be squeezed due to a freak air leak in the baseplate. I backed the idle screw completely off and set the idle. If I used the idle screw, it caused nozzle drizzle as too much air went down thru the front barrels. If I had not modified the carb, There's no way It would have run without the idle screw holding it open and it would have poured the gas through it. I also had to drill two .100 air bypass holes into the base plate to let air bypass the bores with the throttle closed tight and get the idle circut opened up. I have many details in this section from a year or two back if it still pulls up. I don't know how to install the blue letters on here to lead you to it. I also unwarped the carb first just to insure no funy side effects that warping causes such as running two air horn gaskets which lifts the rear rods up and affects the secondary metering. You should contact Ruggles to get tapered rods as the stock ones on your carb are STEPPED and will require bigger jets than he recommends and also realize his recipes may be for 800 carbs as far as primary jetting is concerned. He also makes tapered rods for the secondarys which are much better than the originals which make abrupt changes as they rise on secondary opening of the air valve. He sells a complete kit like no other, if you are really set to do this yourself. Years ago I had my Q-jet modified by two different experts and the idle was fair, but since I did Cliffs recipe, The difference is awsome, trust me. A modified Q-jet is all the same reguardless whether is was a 350 or 455 because you change EVERYTHING.
I also run a modified '79 350 carb on my 455. Most responsive carb I've ever had on it. Love the small air bleeds for idle circuit to main jet transition with lower vacuum of moderate cam. Opened up everything else. Most Qjets have idle air bypass passages, but the throttle body gasket blocks one or both. Also was super easy to change over to a factory looking electric choke from a Dualjet of the same era.
