[345 desoto]

I've got 2 Carter Competition Series 500 CFM carbs (real ones) on my DeSoto 291 with a 6-71 blower...everything new/rebuilt. I spaced the carbs tight together on the blower so that I can use a one piece Edelbrock/KN air filter. The carbs are so close together (bases touching) that I can't get a tool between them to adjust the idle mix on the rear carb. The carb linkage is progressive, and the rear will be the primary. In order to fire/break-in the engine, I closed off the idle mix /idle speed screws on the rear (primary) carb and ran just the front...the engine doesn't know the difference. Got it running/idleing fine...3 full turns out on each idle mix screw. Now, my question is, do I set BOTH carbs to the same idle screw (mix) setting, shut down the front carb and set just the rear carb to 3 turns out, or set BOTH carbs to 1 1/2 turns (idle mix screws) out? I hope you followed this...I had a hell of a time keeping it straight while typing it...

 

[sqzbox]

Wow, awful close there! Here's a pic of a friend's Hemi. If you can enlarge it you can see the spacing on his using the same carb's, but I don't think he has the same filter. I wonder why if the rear is the primary carb, one would even need A/F ratio screw's on the secondary. I guess it's because they both run on the idle circuit at idle speed. If I catch him in his garage I can ask him though. He has about 3o yrs. experience building Hemi's.

 

[ericnova72]

Both carbs at 1-1/2 turns out, although you may find that once you spread the mixture out between the two carbs it may take less, 1 or 1-1/4 turns on each screw.

 

[68NovaSS]

Why are you running progressive linkage on a front and rear dual carb setup???

 

[sqzbox]

I got a question too. Why change the mechanic's to accommodate the filter?
Why not change the filter to accommodate the mechanic's? Oops, that's 2 questions, isn't it. :drunk:
68NOVA, perhaps he's thinking along the lines of a 3/2barrel, go figure?
It can't be a mileage thing, or he wouldn't have put a blower on it in the first place. Right?

 

[68NovaSS]

I know! But on a 3x2 the center carb is giving a somewhat equal feed to all cylinders, even with the blower mixing it up, running off a rear carb only doesn't sound good, JMO and probably only a minor leaning issue.

My 383 SBC runs great with 2 750 BC's, both boost referenced, and not progressive.

 

[345 desoto]

68 NOVA SS - Progressive linkage has worked fine on every engine I've ever had 2x4's on...including my 11 second 562 inch Street Pontiac and my current 345 inch DeSoto Hemi. I only asked the question, because a blower is involved in my current engine, and I thought there might be something that I should know.

SQZBOX - Since I asked the question, it would be natural to assume that I'm watching this Post for some answers. Maybe you should consider asking ME why, instead of addressing me in the third person...which is rude. I machined the carb mount specifically so that I could use an air filter that works, and looks halfway decent...rather that a crappy looking fake injector looking scoop that doesn't filter worth a damn...

 

[68NovaSS]

Well good for you and your "11 second 562 inch street Pontiac" 345desoto, I applaud your accomplishments, sorry it's in the 11's. I wasn't criticizing you, I was simply making an observation and voicing my thoughts, they might provide food for thought, calm down.

 

[68NovaSS]

BTW, beautiful car!

 

[sqzbox]

Hey, sorry if I offended you. It's like you got up and went to the bathroom.
I couldn't wait till you got back, because I had questions. I was using 3-2's as an example of progressive linkage, but don't understand it on 2-4's as the rear carb is centered over the rear 4 cylinders. Every blown 2-4bbl I've ever seen has run off the primary's of both carb's. Seems to me that forced induction would involve equal air/fuel ratio that the rear carb couldn't provide
alone causing a lean condition in the front 4 cylinders. I offered help from a friend that has the same set up with no progressive linkage and it runs very well! Crappy scoop or not. I have an idea though, once you get the A/F ratio set on the front carb, swap it with the rear one and set the A/F on it in the front position. Maybe then they'll both be right. I think a blower act's different than a naturally aspirated engine and forced induction would be more cylinder specific with mutiple carb's. I'm seeing holes in cyl. 1-3-2-4 pistons at this point with progressive rear/front carb's. If I've got it wrong, tell me,
don't :spank: me!

 

[345 desoto]

Thank you all for the replies. Adjusting both carbs, equally seems to be the consensus. I've always adjusted both equally, even on a progressive set up, but as I said, I thought there might be something else involved here since this is a blower motor. As for running progressive vs straight linkage, I'll try both and see if there is any noticeable difference in performance/operation. Any ideas why straight linkage should be preferred over progressive?...

 

[ericnova72]

. Any ideas why straight linkage should be preferred over progressive?...

You want to put an equal amount of mixture in acrossed the length of the blower, not be leaner(or richer) at one end or the other.

 

[sqzbox]

WoW! you mean there's a vacuum signal on a blown engine? I did not know that. How much of a reading is there? I'm curios. I thought it was all pressure.

 

[ericnova72]

WoW! you mean there's a vacuum signal on a blown engine? I did not know that. How much of a reading is there? I'm curios. I thought it was all pressure.

Even a blown motor will have vacuum below the blower at idle, the blower is turning too slow to make pressure enough to overcome it's own internal leakage because they don't have an air tight seal between the rotors or the case.

This is entirely dependent upon the individual blower used and its internal clearances, and the rpm turned(under or over driven).

 

[345 desoto]

You want to put an equal amount of mixture in acrossed the length of the blower, not be leaner(or richer) at one end or the other.

Now THAT makes sense...

 

[sqzbox]

ERICNOVA . . . Ok, Thanks for the info. I've got more questions that you probably don't have time for, so could you give me a link or something so I can read up on them?
I curious about any mod's that need to be made to the engine, if any, in order to run a blower. Is a 350ci a good candidate for one? I'v seen some front mount fan belt driven one's that fit under the hood from the factory and after market that looked pretty simple to add on with the air hose that ran from the supercharger to the carb. Thanks, SQZBOX

 

[sqzbox]

345 DESOTO, that's what I was trying so say in my post (#10) but couldn't get the word out right. Though I don't know much more that 10 % of how they work. I asked my friend Charlie who has a blown bored Hemi about your question and progressive linkage and that's what he said like ERICNOVA about the dual carb's. He said if there was just one carb, everything would be different. I said dam, 4 barrel's at once? then 8 at WOT, mileage is not an issue here, is it. He laughed at me and said, " if you want mileage, go buy a VW" Funny stuff there huh? He also told me how to set the A/F ratio on close mount's by swapping the front and rear after setting the front one. Hey, good luck and keep us updated on the puffer! SQZBOX :thumbup:

 

[ericnova72]

ERICNOVA . . . Ok, Thanks for the info. I've got more questions that you probably don't have time for, so could you give me a link or something so I can read up on them?
I curious about any mod's that need to be made to the engine, if any, in order to run a blower. Is a 350ci a good candidate for one? I'v seen some front mount fan belt driven one's that fit under the hood from the factory and after market that looked pretty simple to add on with the air hose that ran from the supercharger to the carb. Thanks, SQZBOX

Some of the best info available is at the BDS website(Blower Drive Service) for tech. www.blowerdriveservice.com