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Old 04-29-2020, 07:06 PM   #1
47fordv8
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Default Primary carb- Front or back?

Since my last post, I found two problems. A sticky valve (MMO works!) and a bad condenser. Running better but still backfiring through exhaust a little. Advanced Dist. about as far as it will go, vacuum brake working fine, no leak.

My question is- on a regular dual intake, running two 94's w/ progressive linkage, front primary w/ 51 jets and a 3.5 power valve, rear is secondary without idle mix screws, .048 jets, no power valve (plugged). Which works or runs better, having the front carb as primary or the rear? Easier to get at idle screws if primary on rear, but any other differences? Thanks again!
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Old 04-29-2020, 07:10 PM   #2
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Default Re: Primary carb- Front or back?

I don't think it is ever going to run correctly with progressive linkage on a dual intake. Why does it have progressive linkage? I don't see any advantage and a lot of negatives.
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Old 04-29-2020, 09:21 PM   #3
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Default Re: Primary carb- Front or back?

"Regular" doesn't fully describer the manifold. Usually, it just means you can run a generator in the stock position. Are the carbs just moved closer together or are they biased towards the back, with the front carburetor in just about the same position of the carburetor on a a stock input? If it is the first type, a progressive setup will not work very well, if at all. If it is the second type. it is going to be a compromise however you run it.

On the second type of manifold, with progressive linkage, run the front carb as the primary. Hopefully, when you hit the throttle and the second carb opens, most of the flow from the front carb will be diverted to the front 4 intake ports, while the newly engaged rear carb will start feeding the back four ports. If you run a straight linkage with this kind of manifold, I think that you can see that there are problems with the fuel distribution given the location of the carburetors vis'-a-vis' the intake ports. Either way, it is not an ideal situation.

Since you spend most of the time driving on one carb around town, I'd try a progressive linkage first, as the primary carb is close to the stock location.
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Old 04-30-2020, 12:46 AM   #4
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Default Re: Primary carb- Front or back?

Normal practice with a dual intake manifold is to run both carbs in synch. About the only way to really run a progressive dual carb setup is to run one of those Y adapters into a stock intake. How else can one ever achieve a correct fuel mix balance?
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Old 04-30-2020, 09:47 AM   #5
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Default Re: Primary carb- Front or back?

To be clear, this is the type of dual manifold I am referring to that can be run with a progressive linkage; as a matter of fact, they may have been designed for it. Look how close the front carburetor is to the stock position, right in the middle. If you run the front as the primary and the carburetors are set up properly, this would probably run pretty well 90% of the time as fuel distribution would be close to that of a stock manifold. When the throttle is opened fully, the back carburetor would open, providing extra mixture. The distribution would not be ideal, but it might help a bit. The performance increase would not be great.

Being a fan of 4 BBLs and "Super" duals on flatheads I have never run one of these. I would be willing to bet that it would run better with progressive linkage than it would with straight linkage. At least with a progressive linkage, it would have decent fuel distribution when running on one carburetor, while it would be bad all of the time running running a straight linkage. These manifold were always meant for looks rather than performance, making the best of a bad situation.

All of that being said, any dual manifold with evenly spaced carbs requires straight linkage.

All dual carb manifolds are not created equal. What type do you have?
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Old 04-30-2020, 09:55 AM   #6
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Default Re: Primary carb- Front or back?

Aesthetics, that's what it's all about. How to put "DUAL" carbs on a stock engine so you can impress your fans. The best way is to place a block off plate under the rear carb, and don't run any gas to it. Engine will be happier.
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Old 04-30-2020, 09:58 AM   #7
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Default Re: Primary carb- Front or back?

Direct linkage is best for two carbs in my opinion. Idle off of both, and don't understand the logic of smaller main jets when blocking a power valve ? Should be the opposite.


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Old 04-30-2020, 10:12 AM   #8
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Default Re: Primary carb- Front or back?

I'm with Ron, I would block off the rear carb if you like the look. Just don't see any benefit to trying to run it.
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Old 04-30-2020, 10:45 AM   #9
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Default Re: Primary carb- Front or back?

So you guys just gloss over the kind of manifold he is running and make some knee-jerk recommendation. No disrespect, but how would blocking the back carb on a "Super" and running on only the front one work for ya'? We have to know what he's planning on running.

As to the one pictured running better with the back carb blocked off, yeah, it would probably run better that way, but what's the fun in that? If I had one of those, I'd at least run it both straight and progressive and get the real skinny on it. I just wouldn't waste any money on one.
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Old 04-30-2020, 01:23 PM   #10
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Default Re: Primary carb- Front or back?

"Look how close the front carburetor is to the stock position, right in the middle. If you run the front as the primary and the carburetors are set up properly, this would probably run pretty well 90% of the time as fuel distribution would be close to that of a stock manifold."

Don't think it is knee-jerk to make a sound suggestion to get it running correctly. The rear carb is not going to add anything, if it is a relative stock engine, expect headaches. If someone wants to mess with it, more power to them. But I think it is a solid suggestion to just block it off, enjoy the look and drive the car. I would be willing to bet it would perform better with it blocked, but just my opinion.
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Old 04-30-2020, 01:39 PM   #11
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Default Re: Primary carb- Front or back?

Yes, this is the intake I am using. Front carb is centered, rear is way back. Currently using front as primary w/ 51 jets and 3.5 PV. Engine is .030 over w/ stock ford crank w/ max-1 cam.

Seems to run decent except for idle and some backfiring through exhaust pipes. Difficult to get at idle screws w/ this set-up. Back carb butterflys seal tight in bores and have no idle screws, but still get air pulled in at idle w/ throttle closed.
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Old 04-30-2020, 01:48 PM   #12
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Default Re: Primary carb- Front or back?

My suggestion is to keep at it. I'd concentrate on how well the back carb seals at idle. You might try temporarily blocking off the rear carb to determine where the problem is. As to blocking it off permanently, I just don't swing that way. I also don't put faux Oldsmobile valve covers on an SBC.

One thing to keep in mind is "the majority of carburetor problems are ignition related and vice-versa".
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Old 04-30-2020, 02:36 PM   #13
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Default Re: Primary carb- Front or back?

Does it pop back through the exhaust when decelerating, or accelerating? If when accelerating, it is probably an ignition issue. If when decelerating, probably carb, like idle jets over rich or power valve leaking. Or you still have a little valve sticking.

When warmed up and at idle, put your hand on the rear carb to shut off all air. I you get a significant rpm change, you need to work on the throttle butterflies to be sure they are closing completely, and that both are perfectly centered in their bores. You might need a stiffer or extra spring to ensure they close completely.
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Old 04-30-2020, 02:46 PM   #14
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Default Re: Primary carb- Front or back?

Quote:
Originally Posted by tubman View Post
So you guys just gloss over the kind of manifold he is running and make some knee-jerk recommendation. No disrespect, but how would blocking the back carb on a "Super" and running on only the front one work for ya'? We have to know what he's planning on running.

As to the one pictured running better with the back carb blocked off, yeah, it would probably run better that way, but what's the fun in that? If I had one of those, I'd at least run it both straight and progressive and get the real skinny on it. I just wouldn't waste any money on one.

Well, I'll tell ya. Plenty of disrespect was taken. By me anyway. Next time we'll just send you PM's to resolve multiple carb issues.

Sal
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Old 04-30-2020, 02:46 PM   #15
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Default Re: Primary carb- Front or back?

Your never going to get the car to run well with the progressive set up it two deuce intakes were never designed for that. The two carbs running together when adjusted correctly will work fine. To get both carbs working as one will take time adjusting both so each carb is pulling the same amount of air at idel speed. To do this is simple by using a unisyn and vacuum gauge. Any idea one carb could be closed off so its not pulling any air at idel with a two carb progressive set up will never work especially with the 94 type carb just the air leaking past the throttle shafts on the carb you think can be shut off will cause problems. If its the looks your after do as suggested above put a block off plate under one carb. If your looking for the added performance you will gain than run both carbs together all the time the way the intake was designed to be used. When adjusted correctly fuel economy can be achieved as a results. I have tuned and continue to tune many stock and modified flatheads with dual carb set ups so what i have written is actual hands on experience many hours of gas smelling cologne in other words.
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Old 04-30-2020, 03:42 PM   #16
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Thumbs up Re: Primary carb- Front or back?

Quote:
Originally Posted by tubman View Post
So you guys just gloss over the kind of manifold he is running and make some knee-jerk recommendation. No disrespect, but how would blocking the back carb on a "Super" and running on only the front one work for ya'? We have to know what he's planning on running.

As to the one pictured running better with the back carb blocked off, yeah, it would probably run better that way, but what's the fun in that? If I had one of those, I'd at least run it both straight and progressive and get the real skinny on it. I just wouldn't waste any money on one.

Quote:
Originally Posted by scicala View Post
Well, I'll tell ya. Plenty of disrespect was taken. By me anyway. Next time we'll just send you PM's to resolve multiple carb issues.

Sal

Good one, Sal. Good for you....
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Last edited by petehoovie; 04-30-2020 at 06:48 PM.
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Old 04-30-2020, 04:47 PM   #17
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Default Re: Primary carb- Front or back?

I struggled for too long with similar afterfire thru exhaust. I have twin 94s, synced linkage, blocked power valves.....started with 50 jets, went to 52s...slight improvement but very white spark plug porcelain. Finally got it running well with 54 jets and backed off the timing to 1 increment advanced. Good luck
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Old 04-30-2020, 05:55 PM   #18
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Default Re: Primary carb- Front or back?

"Disrespect"? Just because I think you should know the type on manifold the guy is using before making recommendations about what the guy is asking? Give me a break.

You guys take the cake. Not a one of you has come out and said, "Yeah, I tried that and just couldn't get it to work, no matter what I tried". Sure, it's not an ideal setup, but I bet the original manufacturers made them so "weekend warriors" had a way to run dual carbs without all of the folderrol with generator brackets and everything else that entails.

Tell you what. Somebody send me one (for an 8BA) and when (or if) I ever get back to Minnesota, I'll give this a real test.
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Old 04-30-2020, 06:02 PM   #19
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Default Re: Primary carb- Front or back?

Quote:
Originally Posted by tubman View Post
"Disrespect"? Just because I think you should know the type on manifold the guy is using before making recommendations about what the guy is asking? Give me a break.

You guys take the cake. Not a one of you has come out and said, "Yeah, I tried that and just couldn't get it to work, no matter what I tried". Sure, it's not an ideal setup, but I bet the original manufacturers made them so "weekend warriors" had a way to run dual carbs without all of the folderrol with generator brackets and everything else that entails.

Tell you what. Somebody send me one (for an 8BA) and when (or if) I ever get back to Minnesota, I'll give this a real test.
Get off your high horse and read post 15 save yourself the trouble to try and prove me wrong!!
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Old 04-30-2020, 06:42 PM   #20
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Default Re: Primary carb- Front or back?

I don't see anything in your post that would cause me to change my mind. Let me ask you : Have you ever spent any amount of time with these "rear-biased" manifolds?

Look at the way the manifold is made and THINK. That front carb is in the stock location for a reason.
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