Dual carburetors on 250 I-6?

[JohnTaurus]

Engine is the 250/4.1L Inline 6 (non-integrated head) in a 1974 Custom 10. Keeping the 6 is important to me, just thought I'd mention that first.

I'm having issues again with the carburetor after having it rebuilt last year. I'm also aware that the intake manifold was cracked due to extreme heat created by defective emissions equipment in the exhaust manifold (which has been replaced). I had a knowledgeable guy do a patch job on it, and he advised to replace it as soon as possible.

So, aside from replacing the carburetor, I need to also address the intake manifold. Unfortunately, they're not nearly as easy to find as the exhaust manifold was.

What I have found is an Offenhouser dual carb intake manifold for the 230/250/292. It comes with the linkage and such (but not the carburetors themselves) for about $550. Its new, never installed.

Right now, the truck is my daily driver, and the current issue is bogging down when maintaining lower speeds (35-50) and bogging down when accelerating strongly. I have never been able to "floor it" with this pathetic 1bbl no-name POS carburetor, or it falls flat on its face. However, I usually was able to give it 3/4 pedal and it would accelerate acceptably, getting up to speed in a reasonable distance, and holding it there without issue. This is becoming more and more difficult for it lately. It still idles well, and the rest of the engine seems healthy.

So, am I right in assuming that the dual carburetors would give me better acceleration, a long with having new intake manifold and new carburetors ending my woes for now?


I can afford it, but I'm wanting some opinions on this upgrade, and advice in general on this situation.

(A V-8 swap is out of the question.)

 

[mcarlo86]

I think @DoubleDingo is our resident 6 cylinder expert. Pretty sure there are aftermarket manifolds for single carbs as well.

 

[DoubleDingo]

You'll still get the LS swap it, or big block it...lol... If you want to keep it stock, look at 12Bolt.com, he probably has one laying around. Even if his website doesn't show one, contact him. I have heard nothing good about the 1 barrels. There are other intake options out there. Aussie Speed, Clifford, and of course the Offenhauser you mentioned. Dual carbs will improve the engine performance a lot. Synchronizing the carbs is a different story. It can be done, though. Triple carbs is even better, as you are then giving each pair of cylinders the same amount of fuel/air, but then again you have the carb syncing. Aussie Speed you can get the intake and put a Holley right on it, but it would have to be a smaller Holley. Clifford has kits that come with Weber carburetors, and there are good reviews on those. If you have the money and want better performance, check out the Aussie or Clifford. No matter which way you go, you'll have to fabricate a throttle linkage setup.

 

[DoubleDingo]

I think @DoubleDingo is our resident 6 cylinder expert. Pretty sure there are aftermarket manifolds for single carbs as well.

Not an expert, just read a ton of stuff and bought accordingly...lol...

 

[JohnTaurus]

I think @DoubleDingo is our resident 6 cylinder expert. Pretty sure there are aftermarket manifolds for single carbs as well.

The ones I can find are for a 4bbl (which would be overkill IMO) or a triple carb setup (likewise). A single 2bbl carburetor intake would be ideal, but I do not believe it is available.

I'm open to suggestions, but I have scoured the internet to find solutions to this problem.

Elsewhere I have seen people put a 4-to-2bbl adaptors on the 4bbl intake, but I'm also having a hard time finding one of those adapters! And it looks as though it would be just as expensive to do that.

 

[DoubleDingo]

Clifford would be the easiest solution for a two barrel setup. https://cliffordperformance.net/store/ols/categories/chevy-250292

 

[DoubleDingo]

There are 4bbl to 2bbl adapters, just not for Rochester carbs. They are for Weber carbs typically.

 

[mcarlo86]

I agree with @DoubleDingo 's post about checking 12Bolt.com. Call him, I have heard really good things about that guy.

 

[Keith Seymore]

We had a 3x2 setup on our '69 GMC.

It was a straight 6 popped out to 305 cu in; dad added the extra pads to the intake manifold and used 3 Rochesters with a Pontiac progressive linkage. He fabbed up a dual exhaust as well.

Had a 3:11 low gear Saginaw with a floor mounted Hurst shifter.

K

 

[Paladin]

Well, "Big Block that Bitch" !!!

 

[DoubleDingo]

The ones I can find are for a 4bbl (which would be overkill IMO) or a triple carb setup (likewise). A single 2bbl carburetor intake would be ideal, but I do not believe it is available.

I'm open to suggestions, but I have scoured the internet to find solutions to this problem.

Elsewhere I have seen people put a 4-to-2bbl adaptors on the 4bbl intake, but I'm also having a hard time finding one of those adapters! And it looks as though it would be just as expensive to do that.

I haven't done any recent searching, except to provide you the link, on Clifford. I know they used to have provisions for heat under the carb. Not sure if the new ones do, or if there are heat options. You definitely want heat for better atomization. A holley 390 seems to be what the L6 crowd goes with when swapping in a clifford or offy. 12Bolt has a conversion kit that converts a 4bbl holley to a 3bbl. It block off one of the primaries, and then the secondaries provide the WOT. So many options.

 

[WamboJambo]

A holley 390 seems to be what the L6 crowd goes with when swapping in a clifford or offy. 12Bolt has a conversion kit that converts a 4bbl holley to a 3bbl. It block off one of the primaries, and then the secondaries provide the WOT. So many options.

Was about to say that as well, the 0-8007 Holley 390 cfm is often recommended to mate up to the quad barrel Offenhauser intake and is said to deliver good results

 

[JohnTaurus]

Clifford would be the easiest solution for a two barrel setup. https://cliffordperformance.net/store/ols/categories/chevy-250292

Thanks man, I'm checkin it out now. The website is cumbersome to use, your link took me to the homepage and I had to bounce around before I found the 250/292 product list.

I appreciate the advice, guys!

 

[WamboJambo]

I personally am running an Edelbrock 1403 (500 cfm). It's definitely too big and runs a little rich, and I've been stepping it down with a carb jetting/metering kit as well but there's only so far you can go with that. Definitely runs a lot better than it did with the old monojet and stock intake/exhaust manifolds though. I also imagine if you want to put a little money into it and get the lump port kit and a mild street cam installed, it could make better use of a bigger carburetor without as many negative effects.

 

[JohnTaurus]

I personally am running an Edelbrock 1403 (500 cfm). It's definitely too big and runs a little rich, and I've been stepping it down with a carb jetting/metering kit as well but there's only so far you can go with that. Definitely runs a lot better than it did with the old monojet and stock intake/exhaust manifolds though. I also imagine if you want to put a little money into it and get the lump port kit and a mild street cam installed, it could make better use of a bigger carburetor without as many negative effects.

Yeah, see that's what I'm afraid of. I don't know enough about what I'm doing here to find the right solution for my needs.

I don't have unlimited funds to build the engine up to match the carburetor. I could (and would love to) do the lump kit with a 2bbl single carburetor, but I think that's as far as I'm willing to go for now. And, I'd rather just take care of the intake/carb issue right now.

My ideal setup is EFI, but I am somewhat apprehensive about that as well.