The Stovebolt Site Forums The Shop Area Engine Shop Dual carbs - which carbs do I need to use?

Fixing to start overhauling my 261, I'm planning to step it up a few notches. I want to run dual carbs on it but I don't have any idea what carbs I need to start looking for. Any help would be much appreciated.

Planning to raise the compression a bit (not too high - 9ish), change the cam and put dual exhaust on it.

2 stock Carter 1 bbl. originally used on the 216 is a safe choice.

you might consider upgrading the camshaft also.

my reference info for zenith carbs comes from the carburetor shop files


http://www.thecarburetorshop.com/TruckkitsG.htm#TkitGMC

and other files on his site. any model zenith that is specific to the 248 engine will work, obviously w/ some exceptions like the alcohol carb and a few others.

dual carbs?

You might not want to use two Zenith carbs from 248 engines. You might be better off trying two Zenith carbs off of 228 GMC engines.

Also, maybe someone will post info on an adjustable main jet for those carbs?

Yes, I stated that.

I run two Carter W-1's ( 1941 #483 ) on my 250 inch inline with no problems. It takes lots or trial and error to get duals set right. Plan on plenty of test drives and a lot of fine tuning. Once they are set, you are good to go just as you would be with any single carb.

Thanks for the input. I have lots and lots of experience with multiple carburetor setups so getting them sync'd should not be an issue for me. I just needed to know what to look for on this particular application. I haven't decided for sure just yet, but I may end up just making my own intake and running a single 2-barrel. We'll see.

I must have been daydreaming,sorry .LOL

I am now running a GMC 302's Holley AA-1 2-barrel with adjustable main jets on my 261. It is the best carburetor configuration that I have tried on it since I rebuilt this 261 engine in 1972.

What kind of manifold does the 2 barrel sit on?

If the carbs are properly selected and adjusted; and the intake is decent, then the duals will give a bit more performance; albeit at higher cost, and more time.

One needs to look at all aspects of the engine, especially target RPM when selecting the carbs.

A single two-barrel will be somewhat easier to select, certainly less expensive, than the duals.

Jon.

A homemade one.

Yeah, I have about 7 different Rochester 2-jets in my possession that I could choose from. I have always had good luck with those carburetors. Really the only reason I am even considering a dual setup is just for curb appeal whenever I open the hood for curious onlookers. lol The truck draws lots of attention wherever I drive it.

I'm not sure that a modified original manifold, or same with adapter using a 2 bbl. will have the same cylinder-to-cylinder mixture distribution as stock.
This is also true of the 2 X 1 manifolds: most including Edelbrock, Offenhauser etc. have the carbs centered in the inter-port distance, but others including Fenton have the position biased to the end cylinders. I suspect the latter is a better choice.

If choosing carbs or a manifold for 2 X 1, the size you want has the SAE #2 2-11/16" bolt pattern. The SAE #3 2-15/16" pattern is best for larger engines and higher RPM.

The configuration of the intake manifold is as important as the choice of carburetors; as the air/fuel mixture MUST pass through the intake to get to the cylinders.

No actual test data to back me up, but I would (opinion) certainly agree with Panic about the bias to the end cylinders being superior to bias to the center.

There is test data to support this on dual quads on Pontiac V-8's.

If doing a homemade intake, remember that the air/fuel mixture delights in traveling in a straight line and hates going around corners. Also that runners to each cylinder should be as close as possible to the same length.

As to a 2->1 adapter; the results of testing we have done (and we have done quite a bit on this subject) have ALWAYS been uniform.......regardless of what 2 barrel may be used, it NEVER worked as well as the single barrel it replaced. Think NASCAR restrictor plate.

Jon.