Thread: dual quads on a SBC...which ones to use with the least trouble?

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i dont need a gob of horses or the biggest carbs....i want a dependable street engine with 2 carbs.. using a low profile Edelbrock manifold on a 70 LT1 Corvette 350 with a mild cam profile for street use. I have heard the old standby Q-jet carb was hard to beat, but I am totally in the dark on this one...PLEASE HELP ME>>>somebody! Many thanks!

Never saw anyone run dual Q-jets, the twin edelbrock 500s on the bucket have'nt been touched since I bolted em on and set em almost 7,000 miles ago, still starting/running great

Originally posted by viking
Never saw anyone run dual Q-jets, the twin edelbrock 500s on the bucket have'nt been touched since I bolted em on and set em almost 7,000 miles ago, still starting/running great

i was going to say the same thing viking, never seen 2 Q-JETS on a car. i think its because if you needed to fine tune them its hard to find parts for them. if you have to, you can fine tune those EDELBROCKS all you wont to.

Think there might be a reason GM did'nt put dual Q-jets on lets say a vette , ya know they tried it.

Oh don't get me wrong, I'm not saying it CAN'T be done, only that there are better set ups out there


And that intake you posted looks like a C-26, I'm going to replace my tunnel ram with one and see what happens to my low end torque

Originally posted by Streets
Y don'tcha try Twin Predators IF ya want "That lQQk" and performance too.....

like this.

Originally posted by DennyW
I use different sizes to get what I need on low end, and a 30 mph punch. This is similer to what I use a lot.

denny, your suppose to bolt that to a intake, not the board.

Originally posted by DennyW
Well, that thing was ready to run, hahaha. had to hold it down, hahaha.

you D-man

Well, guys, sounds like you all have been down different forks of the same road...sounds like the 500 edelbrock will be my choice, due to size and adjustability. thanks for the history and the lessons and the advise....

Well the thing about carbs is...theres LOTS of info in the archives about them here. And I could go on for a while about the basic two styles out there, but its easy to find. A quick lesson though, a Holley has power valves which base the big fuel flow on well...flow and need and throttle position, and also the butterfly and idle circuit and well...so on...well actually, explaining the whole ideas could take a while, the Edelbrocks dont use this system, rather a metering rod system, some say is easier to tune and less finicky. They will work great for tuning and driveability, I like the Demon series too, coming out as a great all around carb too.

Hehe, there are actually several fuel delivery methods but for your purpose I think you will be fine with the average carburetor. I will indeed help and explain as much as I can recall, however it is quite late and I will try to do that sometime tomorrow for you. What you might want to do in the meantime is, if you can, check your idle vacuum because this will determine some things about your carb choice, especially a Holley. Also, do you have the manifold already? Because despite there being plates to adapt between the two possibly, there's square bore and spread bore style carbs as well. I am assuming you have a square bore being that you already have a Holley.

OK here it goes, the basics of both styles of carbs are quite similar up to a point, however when it comes to power enrichment that is where the big differences begin. SO, to show the similarities of most carbs I will start by describing the circuits that carburetors have.

The first is the idle circuit which supplies fuel when the throttle plates are only slightly open. The idle circuit, to a point, can be adjusted quite easily with the idle screws. The fuel comes out of a hole that is just below the throttle plates in the baseplate which is why you would not likely see fuel coming from the main jets under idle conditions. However, the idle circuit is controlled by three main things, idle feed restrictions, air bleeds and mixture screws. If you have a carb that is perhaps not suited to the task at hand, just bumping up the idle speed or opening the screws may not be enough because that could bring the next circuit into play.

The transition circuit supplies fuel before the main metering system comes into play but after the idle circuit thus the name. The problem found by just adjusting the throttle plates position aka idle speed screw is that it can bring this transition circuit into play before it is required and richen the mixture which might tempt you to turn the idle screws in, which would lean it out for the idle circuit but then seem a bit lean or doggy when you tip in because the transition circuit is already supplying its fuel. Mind you it would be a slight feel under tip in because the main metering system may pick up the slack...but its better to have a well tuned carb than just have something that sorta works ok. Anyhow this circuit also works with feed restrictions that are built into the carb. BOTH the idle and transition circuits work off of vacuum, when there is vacuum there is flow, when you punch it these circuits do not flow fuel.

The main metering system is the meat of the carb, where the majority of the fuel will come from under most conditions. The venturi, the jets and the air bleeds are the main compenents. On a Edelbrock or Rochester or a Carter for example there are metering rods which richen the mixture under load, because they are based on vacuum, the less amount of vacuum you have the more fuel that will flow. I will get into this more thoroughly shortly.

There are also accelerator pumps which supply fuel under pressure to compensate for big changes in airflow (namely less airflow) but when the engine is under load. They are controlled by check valves that dont allow fuel under conditions where it is not necessary, but when required allow it under pressure.

Now heres the main difference between the two basic types, and the reason behind some peoples' reasons for not using or tuning certain types of carbs. The power system or enrichment system on a Holley is controlled by a power valve which supplies extra fuel to richen up the main metering system. This valve opens up under a set vacuum amount. SO, if you punch it and the valve is set to open at say 8 inches of vacuum then thats when it will supply a huge shot of fuel. BUT lets say you have a huge cam or two carbs then maybe your vacuum at idle isnt very much and under normal load the power valves are working, or you get LOTS of enrichment under some normal load because you have two carbs with two power valves that are set at a fairly high vacuum amount. The power valves are also metered by channel restrictions and can be changed. So it is possible to choose a different power valve, one that opens at a higher manifold vacuum and ones that open at less and so on. The ratings are usually stamped on the valve as well. A stamping of 3.5 would mean 3.5 inches of mercury manifold vacuum and so on.
ON other carbs such as I noted before the enrichment is done with metering rods, basically a tapered rod that is held down with high vacuum and pushed up by a spring. With less vacuum the spring overcomes the weight of the rod and allows more fuel to pass becaue the rod is tapered. Different springs and different rods can be used and this is often toted as much easier to tune because well..it is. Power valves can blow out and supply a big shot of fuel all at once, while metering rods do not blow out, are easier to change and allow a gradual increase in enrichment. If you are new to carbs and want to tune them it would be easier to use an Edelbrock or other non power valve carb for this reason alone. Things like jets and idle circuits etc are changed in much the same way.

The main circuit does the majority of the fuel delivery and thus the commonplace task of changing jets to get more power, tune it up etc. Working with the idle circuit and accelerator pump circuit, transfer circuit usually require more skilled carb tuners to fix because drilling, slotting etc is required to tune those out and for the most part are not required today on the street because for one, they may not change much in a street/strip situation and also the amount and quality of aftermarket carburetors does not really justify taking the time to do these things.

Now that you know some basics (I hope I got most of it right..someone out there want to second this info? I know I missed lots of parts, floats and setting the levels, and how-tos on adjustments, needles etc) you can begin looking at a carb you think would work best. For your situation I would recommend something like an Edelbrock just because it is easier to tune for you and many models are offered and new parts are available thus making it easier to set up for a dual carb system. I'm not sure how much you know but you will have to choose between some things such as a mechanical secondary or a vacuum secondary. I dont know or remember what your intentions were for the car except that you want some power, to keep it under the hood and look cool so likely not the strip. My recommendation is for vacuum secondaries and maybe electric choke, simplifying the installation some since mechanical chokes might be harder to adapt in that situation. And base the carb flow (cfm) not necessarily on your engine size but its air flow needs. This does take into account its power lvl some but more so your main limiting factor is the rpm and displacement. I cant really think of anything...I'm sure I forgot lots but I dont think I can put it all down or claim to know everything in one place, nor can I remember anything at this point so maybe I'll check back later, good luck, I hope I represented good info and most of all have fun.