The Stovebolt Site Forums The Shop Area Engine Shop 261 alternative carb...

Hoping to get some info on an alternative carb I could use on my 261 rebuild. I am fairly new to the game, I am having my 261 rebuilt at a local shop.(engine did not have a carb when I bought it) From what I know the 261 carb is unique with 1 3/4" throttle body vs 1 1/2"

http://devestechnet.com/home/thevenerable261

What are some alternatives I could use that would be as good or better the than original 261 carb?

Thanx in advance!

Take the time to read through this very thorough on-topic discussion started by Jon "carbking" on Selecting a Different Carburetor. Lots of great info on your question for the 261.

I ran a model "B' from a '67 292 for a couple years ran, changed main jet & got it to run pretty good. I have been running a Daytona UN2 the last 2 years. I like it, better response to light throttle opening.

Have you had to make any mods to the Daytona or did you just run it as is straight out of the box?

Has adjustable throttle lever, had to modify fuel & dist vacuum advance line, different air cleaner as it has the same air horn size as a pre 1960 carb. Adjust main jet as per supplied instructions.

I'd like to do a dyno run on that Daytona carb- - - -you're probably losing 20 HP or more on the top end.
Jerry

I 'm very happy with it, it pulls 6% grade better than any other set up I've run in this rig since I got it back in service in 1987.

Jerry, what would you be running today on a 261?

I have an 848 head that I hope to drop on the 261.

I'd modify an intake to accept a Rochester 2 barrel for a 265 or 283 Chevy V8, or a similar Stromberg for a mid-1960s MOPAR 270 V8. The stroker GMC 270 I'm building will use an intake for a 302 military engine that has the same footprint as one of those carbs. I'll probably use a Stromberg for a 318 MOPAR V8 on that one.
Jerry

And just a friendly note -- While certainly extensive, Deve's article (linked in the OP's first post) contains a few inaccuracies. For example, the first 261's, like mine, do *not* have the "Captain's Bars." Their absence does not mean that it isn't a 261. Casting numbers and date codes are the best, most accurate way to ID a 261. On the technical side, Jerry probably has a few errors he can spot.

For anyone reading this who hasn't yet sourced a 261 for their AD truck .... the early 261's (from the 1954 mold) were built for the late ('54 and '55 1st) Advance Design trucks. As such, they need no modifications (like the water pump relocation plate, etc) to fit into an AD truck.

Back to the OP's original question ... I am running a 261 with an 848 head. It did ... Okay ... with a Rochester B from a 235. But I have since upgraded to a Carter YF with the correct throat and jets for a 261 and it is doing great. A plain ole 235 Rochester B will get you by (I drove mine that way for about 4 or 5 years not knowing any better ... ) while you search up a proper carb.

As stated above, Carbking has excellent information on which carbs to look for.

Jerry's suggestions would, of course, be the platinum solution but not all of us have his machinist skills. He might consider elaborating on that just a bit ...

An engine is nothing but an air pump. Every two turns of the crankshaft, in theory, it flows the volume of air the pistons displace. In practice, that figure is impossible, due to various fudge factors such as turbulence, the shape and size of the intake and exhaust passages, AND- - - -the efficiency and size of the carburetor. To figure the absolute maximum "cubic feet per minute" (CFM) an engine can flow, multiply half the cubic inch displacement by the RPM you want to run, which gives you "cubic inches per minute". Divide that figure by 1728 (cubic inches in a cubic foot) and you get "theoretical CFM" For a more realistic figure, use 90% of that number as a general rule of thumb. For instance:

half of 261 cubic inches = 130.5 X 4000 RPM = 522,000 cubic inches per minute

Divide by 1728 = 302 cubic feet per minute 90% of 302 = 272 CFM

Ideally, a carburetor capable of 300 CFM would be all a 261 engine at 4K RPM would ever need. "Bigger" is definitely not "better" where carburetors are concerned!

Instead of building a carburetor with one big barrel, putting two smaller ones side by side will probably give better throttle response, and better fuel mixing, since the velocity through each venturi will be higher, and carburetors mix fuel and air by creating a vacuum in the venturi that pulls fuel through the jets and mixes it with the air stream. To fit a 2 barrel carb to a stovebolt single barrel intake requires some creative milling and welding, since an adapter from a 2 barrel carb to a 1 barrel manifold creates all sorts of problems.

Now- - - - -how about using two small carburetors on a custom manifold? That opens up a Pandora's box of unintended consequences. An inline six needs either one carburetor- - - -or three- - - -one for each pair of cylinders- - - -particularly if it only has three intake ports like a stovebolt. The biggest mistake most engine builders make is "too much camshaft" and/or "too much carburetor". Doing race engine stuff on a street engine is beyond stupid!
Jerry

Jerry mentioned CFM; if one is not familiar with "CFM" this link to my website may help:

CFM

And one more, although specific for 4-barrels, more information about CFM and Jerry's comment about over carburetion:

More on CFM

And this is at the end of the second link, for those of you who do not like links:

"Finally, there is an old story here in the Missouri Ozarks which, unfortunately, is ignored by many enthusiasts when choosing the correct carburetor size. Seems like a young hillbilly reached his 16th birthday having never owned a real pair of shoes. His uncle told him to go into town, and come up with a new pair of shoes, and the uncle would buy. So, much later in the day, the youngster comes walking back from town, with the shine of new boots visible through the dust of the dirt road. His uncle asked what size he bought, and the youngster replied "wal, the salesman thot I oughter wear a 9, but 10's felt so good, I got 11's!".

If acquiring a carburetor for a street engine, give the above some thought!"

Generally, on a street engine, over-carburetion is worse, sometimes MUCH worse than under-carburetion.

And Jerry, I must disagree on the 90 percent efficiency figure; most street engines won't make 90 percent.

Jon

A good choice would be a Roch. Monojet carb for a 292 engine. The best one is for a early 80's with electric choke heat. The earlier ones with manual choke would be good also. Monojets had some weird choke setups through the years. They have to be matched up with the manifolds.

George

Jerry, in your formula to calculate the maximum cfm, what figure do you use for the rpm? Is that the red line rpm? For example the 216 ID plate says it puts out 85 horsepower at 3300 rpm. Would 3300 RPM be the figure used in your formula? And I'm asking this because I'm working with a friend who has a Chevy 350 in a 72 blazer. He has a 600 CFM Edelbrock on it. Model 1406

Pick whatever RPM you intend to run as the redline. On the round track cars I figured CFM at 7K-7500. On a stovebolt, most owners get a case of the vapors if the engine gets up to 3K, which is at least 1500 below the point it would start to damage itself. They're too timid to ever approach anything resembling a real redline. 600 CFM is more than enough for any 350 in street trim. The theoretical CFM for a 350 at 7K is 709 CFM. A street engine will seldom, if ever see that speed without floating the valves.
Jerry

When you start talking about V-8's and 4-barrrel carbs, you should consider whether the carburetor is a square-bore or spread-bore.

The factory spread-bores on the 350 were 750 CFM, of which only 150 was on the primary side.

With a 4-barrel, CFM should be discussed on both the primary and secondary side, rather than total.

Jon

Many years ago there were log manifold kits for various V8 engines that you could weld together and mount some number of Stromberg 97 two-barrels on, usually six. Nile, a friend of mine had a 53 Chev coup powered by a 270 GMC, on which were mounted 5 Stromberg 97s on such a manifold. At the time I had a 53 Chev coup with a 303 Olds V8 mounting only 3 Stromberg 97s. We raced down the straightaway near the high school. Nile beat me. Hands down. Which goes to show you that 5 Stromberg 97 are better than three. Right?

Back in the day there were bone yards full of old Fords each with a Stromberg 97 that could be had for a couple dollars each. You could change the jets from the outside. So, many of these carbs on log manifolds ended up on hot rods built on the cheap and using trusty 97s. No way could we afford to do it like they did in Hot Rod Magazine with tuned stacks and Hilborne injectors.

Help to educate me here. Seriously, it seems to me that a sufficiently large plenum feeding 3 intake ports doesn't care how many carburetors are feeding the plenum as long as they flow enough air and mix the fuel and air to match the engine speed and load. Be it one, three or five. I'm running two Zeniths on an Offy manifold which seems to be a fairly large plenum with stubs that go beyond the end ports. Runs good. Goes up hill. Gets 10 MPG with a 3700 lb camper at 62MPH. Wouldn't beat Nile's Jimmy though.