I’m in the process of getting my 250 Chevy L6 motor ready to fire & I’ve been wanting to add a 2 barrel carburetor to it but I ain’t exactly got unlimited funds to buy a high dollar intake. i see they make an adapter to add a 2bbl carb to a single bbl intake but I’ve heard it’s ineffective much if at all because of the smaller bore on the factory intake. has anyone ever drilled the single bore on the intake slightly wider to allow the factory intake to perform better? ps looking at the adapter a little closer, it looks like it would need to be remed as well. pic of adapter
Doing something like this, you must be careful to not run into the exhaust heat chamber that surrounds the carburetor opening. Flip the manifold upside down to see what you're up against.
No, the intake opening is the restriction, adding bigger carb to a small hole won't help. I have a home made three carb intake using three Corvair single barrels you could modify for a two barrel, it bolts on to a stock exhaust manifold. Make you heck of a deal on it!
I did one for a flathead Dodge I'm building. Started with a piece of 1" aluminum. Drilled, ground and shaped the 2bbl side then bolted it too the manifold. Ground the taper, deep into the manifold, including breaking into the exhaust chamber. Not a problem as I won't be using exhaust heat, so I filled the chamber with epoxy. I'll use a split manifold with the center removed. Maybe add a plate to water heat the intake, maybe not.
The way carbs work is the venturi. The adapter would seem to create another venturi below the carb. It might provide a bit more flow, but not as much as a properly sized opening. Figure out how much CFM the engine pumps, and size the carb to match. You could do a larger single, a 2 bbl (current dream), more carbs on the log intake or individual throttles on each cylinder. All of it will be time and money. First, I'd do initial fire on the stock single. There is enough to worry about on a fresh startup. If you feel the need for more breathing after it's running and driving, keep an eye out for something like @Joe H is offering. The 250 is a good engine, but mostly suited for low RPM torque and limited top end. It can respond to mods for a higher RPM screamer, but would be more suited for drag or top speed runs in that tune, not street use. This page seems to have the basics covered. It might be AI slop, though. It does reference the HAMB as a good resource. Do a few searches here and see what others have done and their results with different intakes and carbs. https://www.junkyardmob.com/guides/chevrolet-250-inline-six-engine/
My Falcon 6 has had its integrated manifold modified to accept a Holley 2300 2-barrel pattern throttle body.
You could bolt the adapter up tight to the manifold and then grind them both at the same time, basically port-matching your intake to your adapter, which would give a smoother air transition from one to the other, but I doubt you'll be able to get a much bigger hole in there. Would help a little but, but probably not a ton. Was there no factory 2bbl intake for the 250/292 that you could buy? They put those engines in trucks well into the 80's I think. I can't believe they'd be crazy expensive, but I guess I've never looked for one.
Here's some modification info from back in 2013. https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=74173
The only 250s that came with a 2 bbl carburetor (Varijet) were the later ones that had a cylinder head with an integral intake manifold, like the Ford Falcon six.
The inside of the intake is totally destroyed from the cutting when the fellow added the two barrel in the video above, no way can that be good for air flow. The tri-power one is a much better option, but you need small carburetors. Three factory carbs will use a ton of fuel. I had three Corvair carbs, which would run, but not very well. They had no fuel enrichment circuits, so jet them lean for mileage, or rich for power, you couldn't have both at the same time. Dual Carter W-1's from 1948 Chevrolet 216 engines are just right, they have a great enrichment system and are sized just right for dual usage. The best thing for the inline six is spread the mixture out so all six cylinders get equal amounts of fuel/air. As designed with one carb, the center cylinders run rich, the outer 4 run leaner. It,s not so much the amount of fuel, but how it's distributed. One larger carb won't fix that.
Here is how I built my intake using hand tools, drill press and mig welder. The blue one is the triple set of Corvair cabs. It was a first attempt, smaller tube, square corners, no heat. The second one uses a little bigger tubing with rounder corners and a 1/2" deep heat chamber under the two carb openings.
For anyone considering using multiple Corvair carburetors, Rochester added a fuel enrichment system to them in 1965. The '65-'66 carbs are considered the best.
Joe - if you wanted to "play" more with your set-up, watch for three Carter type W-0 (zero, not ooh) carburetors that were used on the Jeep. The Jeep carbs would be tagged (if still present) 636s or 636sa. And as Carter stamped a code in the underside of the base, if the tag is missing, look for a stamped code of 505. DO NOT TRY TO USE THE CARTER W-0 AFTERMARKET CARBS FOR THE ENGLISH IMPORTS!!! While certainly superior to the original Solex carbs, they do not have the adjustability of the Jeep carbs. I posted here several years ago about using the Jeep carbs, but did not post how to identify them. One member found several of the W-0's designed for the English cars, bought them, and found they were a different animal. My apologies to that member. As I had only seen maybe a half dozen of the aftermarket versions in 50 plus years, I never dreamed someone would find three, the miracles of ebay. The Jeep carbs used to be quite expensive, but they have come way down in price over the last 15 or so years. Jon
I did it on a Chrysler 354 hemi 4bbl manifold to put a bogger carb on it. Used a carb adaptor plate to guide a hole saw using a drill press.
I used what I had, the Corvair carbs are 1961 versions and came out of the local high school shop cl*** when it was shut down. When I had them jetted rich, it ran pretty good, but at the drag strip it wasn't any quicker or faster then the stock set up with one Mono-jet carb. With out heat under the carbs, it wasn't the best for cool or cold drives.
I also cast a 180deg aluminum 4bbl intake for the TurboThrift series engines. I made it for a 230cuin engine I put in a '36 Pontiac.
The Jeep W-0 carburetors are basically a "little brother" to the W-1 carbs you are currently using, with the Carter metering rod technology. Jon
I would keep looking for a dual intake manifold, or make / modify one like others have done. Inline engines respond well to better fuel distribution, at least mine does.
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