Dual quads versus big hammer!!!...

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On a stock AVS or TQ, you can loosen the air door spring enough to where the doors just hang open.

Can you not do that with these carbs?
So the door is either hanging open, or just barely closed?
 
PRH said:
On a stock AVS or TQ, you can loosen the air door spring enough to where the doors just hang open.

Can you not do that with these carbs?
So the door is either hanging open, or just barely closed?
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I haven't got that far yet. Again still trying to clean up the primary circuit.... Only have my thoughts on the secondary circuit right now and again everything I'm thinking is probably the opposite of what I need to do LOL...
 
j par said:
So I'm seeing all this fuel flying and standing off. Especially when I jump on it at Cruise.. when I rev the car with the carburetors unhooked and do it with an individual carburetor the rear air door smack open. When I give the car full throttle at Cruise the doors just bobble and never smack open and I'm loosening the Springs to try and get them to pop open.. in that article where they're using two Street demons that seems to be the problem they were having as well. Again not only going leiner on the primaries I want to get those new doors hopefully tomorrow and start drilling them and seeing what happens when those doors actually do open when I slam the throttle???.. just out of curiosity I wondering if it's the opposite of what I'm thinking... it could quite possibly be the bottom opening up and letting the air and and the top staying closed and not letting air flow over the Ventura's and allowing more fuel to come in... Or maybe too much fuel is coming in and not enough air... Again they did mentioned in that article that they drill the holes because they couldn't get the doors to open no matter how light they made the springs.,
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Right. The air doors are acting like a choke. If you open the secondaries and the the air valves don't open, you effectively choke the carb.

That's what I said way back. The air valve has to open. It's not a vacuum secondary carb. I've seen BBC's on the dyno with the muched loved (which I hate) 3310 Holley never pull the secondaries open. Even with the weak spring. But it doesn't go dead rich because there isn't an air valve above the butterflies.
 
I don’t see any fundamental difference with this carb than an AVS, TQ, or Q-jet(there might be, but I don’t see it).
All of those have mechanical butterflies and spring loaded air doors.
I have always considered them to be “vacuum” secondary carbs, for lack of a better term.
It’s more like, secondary air flow “on demand”.
The air door opens only enough to satisfy the demand, to keep the velocity through the secondary barrels high.

I worked at a boat shop in the 80’s.
One of the models of small cruisers we sold came with a 305 with a q-jet.
These were pretty heavy boats that took a while to get up on plane.
At full song they were set up to turn about 4600rpm.
If you watched the secondary air valve in operation when going from low rpm to WOT, the valve would barely open at all(and it would slowly open up as the rpm went up).
If you pushed it open at that point, the boat would slow down, and never get up on plane.
If you loosened the spring tension on the door so it opened sooner, it took longer to get up on plane(too loose and it wouldn’t get up on plane at all).
At wot, at full speed(4600rpm) the air door was only open to about a 45* angle.
If you opened it up all the way, even with the motor already at full rpm...... the boat would slow down.
The factory setting of the door opening slowly, and not opening all the way at terminal rpm yielded the best performance in the boat (imagine that).

Even with the q-jet on my friends stocker...... we have the door set up with the spring tension pretty high.
 
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I think if you had Jpars car on a chassis dyno, and did some pulls starting at a pretty low rpm(2000-ish), the doors would hardly open at all, then as the rpm went up...... they’d open more and more(which is how it’s supposed to work).
The question would be.......how loose can the door spring be(how soon/far can the doors open) without negatively impacting the power.
There is a reason the doors are there.
 
PRH said:
I don’t see any fundamental difference with this carb than an AVS, TQ, or Q-jet(there might be, but I don’t see it).
All of those have mechanical butterflies and spring loaded air doors.
I have always considered then “vacuum” secondary carbs, for lack of a better term.
It’s more like, secondary air flow “on demand”.
The air door opens only enough to satisfy the demand, to keep the velocity through the secondary barrels high.

I worked at a boat shop in the 80’s.
One of the models of small cruisers we sold came with a 305 with a q-jet.
These were pretty heavy boats that took a while to get up on plane.
At full song they were set up to turn about 4600rpm.
If you watched the secondary air valve in operation when going from low rpm to WOT, the valve would barely open at all(and it would slowly open up as the rpm went up).
If you pushed it open at that point, the boat would slow down, and never get up on plane.
If you loosened the spring tension on the door so it opened sooner, it took longer to get up on plane(too loose and it wouldn’t get up on plane).
At wot, at full speed(4600rpm) the air door was only open to about a 45* angle.
If you opened it up all the way, even with the motor already at full rpm...... the boat would slow down.
The factory setting of the door opening slowly, and not opening all the way at terminal rpm yielded the best performance in the boat (imagine that).

Even with the q-jet on my friends stocker...... we have the door set up with the spring tension pretty high.
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If you put two of any of these carbs on a TR you don't have the air flow to open the air valves.

I understand what you are saying, but they can't be classified as vacuum secondary carbs.

If under certain loads and engine RPM, the air flow isn't enough to pull the air valves open, but they butterflies are open. You now effectively have a choke.

We are assuming a lean bog, and I'm assuming a rich choke condition.

I said that many posts ago, when Jpar posted the results of the other dual quad application.

Running two fours doesn't play by the same rules as single 4 stuff.
 
I understand what you are saying, but they can't be classified as vacuum secondary carbs.
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Huh.

You’d think a carb company would know how to classify their own carbs......

59F3EBAA-EA9B-4C90-A4B5-EB9C0FA2790B.png
 
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PRH said:
I don’t see any fundamental difference with this carb than an AVS, TQ, or Q-jet(there might be, but I don’t see it).
All of those have mechanical butterflies and spring loaded air doors.
I have always considered them to be “vacuum” secondary carbs, for lack of a better term.
It’s more like, secondary air flow “on demand”.
The air door opens only enough to satisfy the demand, to keep the velocity through the secondary barrels high.

I worked at a boat shop in the 80’s.
One of the models of small cruisers we sold came with a 305 with a q-jet.
These were pretty heavy boats that took a while to get up on plane.
At full song they were set up to turn about 4600rpm.
If you watched the secondary air valve in operation when going from low rpm to WOT, the valve would barely open at all(and it would slowly open up as the rpm went up).
If you pushed it open at that point, the boat would slow down, and never get up on plane.
If you loosened the spring tension on the door so it opened sooner, it took longer to get up on plane(too loose and it wouldn’t get up on plane at all).
At wot, at full speed(4600rpm) the air door was only open to about a 45* angle.
If you opened it up all the way, even with the motor already at full rpm...... the boat would slow down.
The factory setting of the door opening slowly, and not opening all the way at terminal rpm yielded the best performance in the boat (imagine that).

Even with the q-jet on my friends stocker...... we have the door set up with the spring tension pretty high.
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I think the big difference in this analogy is this isn't a boat! LOL
The cars a tad bit snappier and the RPM's don't take long to move upwards...
 
PRH said:
Huh.

You’d think a carb company would know how to classify their own carbs......

View attachment 1715485385
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Marketing. It's an air valve secondary. It has mechanical secondary throttle blades.

They can call it a pink elephant. Doesn't make it one.

The entire aftermarket calls them vacuum secondary. Doesn't make it correct. You can not mechanically open the secondaries and call it a vacuum secondary.

Again, marketing. What companies like these do best.
 
PRH said:
Got it....... only you know what it really is.
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No, most of us do. It is what it has been for decades. Again...marketing.

Doesn't change the fact that the issue isn't opening the air valves too soon.

We will k IQ more tomorrow unless it rains on Jpar. They are predicting snow here Thursday night and on through the weekend.
 
I see..... like where they point out:

Air Valve - These secondaries have an air valve, similar to a choke blade, above the barrels. As carburetor vacuum increases, it opens the air valve.
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Like as I mentioned previously...... air flow on demand.

The root cause of the doors not opening at wot and low rpms is very simple.
The motor doesn’t need 1250cfm of carb capacity at 2000rpm.
The two primary sections are apparently enough to keep up with the air flow demand of that motor during those conditions.

I’m not saying modifying something to trick the doors into opening sooner won’t help the situation...... you’re just covering up a shortfall of that carb design for that particular application.
 
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PRH said:
I see..... like where they point out:



Like as I mentioned previously...... air flow on demand.
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Jesus...did you read it ALL? There are THREE types. They lay them out.

My point is it's not just me that knows the difference. If you don't know it or won't admit to it that isn't my problem.

They operate on two entirely different systems. One system opens the throttle blades and the other opens an air valve. One has mechanical secondary throttle blades, the other has vacuum operated throttle blades.

There is a HUGE difference.

Edit: forgot to mention the comparison to a choke plate, which is what I suspect is happening. Can't have that with a vacuum secondary carb.
 
they work like a vacuum secondary . If the secondary air valve opens to quick it bogs if to slow its lazy. the nice thing about the primary rods is you can change the jet size without changing the actual jets just by metering rod size.
if the secondary air flap opens too quick it bogs because there isnt enough fuel to cover up the hole created by the throttle plate opening because it doesnt have a secondary acc pump.How many holleys or avs or afb have you had to change a large amount of jet if there wasnt something wrong with the motor?Point is the calibration is always somewhat close.Just because vacuum is same doesnt mean the carbs are synced usually have to sync them somehow. also sometimes have to lessen jet size if plugs are too cold .
 
Ps anybody want to look down a vs carb to see when it opens on a dyno during a pull? Sometimes they dont open until 4000 + rpm
 
j par said:
I think the big difference in this analogy is this isn't a boat! LOL
The cars a tad bit snappier and the RPM's don't take long to move upwards...
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Maybe, but boats have more load on them more of the time which is what @PRH is getting at. Cars are only producing major power (and thus major airflow) while accelerating.

If you just snap the throttle open and free-rev it, the air demand is not the same as when you mash the gas with a load on the engine. An accelerating engine (no load) will ramp-up air demand more quickly as revs pile on fast and the air column speeds up quickly. An engine at low revs but with lots of load will not, because it's not accelerating and the higher cylinder pressures will increase internal EGR/reversion there can be problems with mixture not reaching the cylinders - AKA "bog". The air doors not opening aren't necessarily a cause but a symptom. Once the engine has more air demand it'll probably help pick up and take off.

In the case of the 409 with the drilled holes, that may have been the result of the primaries being too small to adequately fill the engine and so the air demand remained low at lower revs and wasn't helping to open the air doors. You may be running into some of that, but it's hard to say without the engine being on a dyno (high load) or without knowing exactly what your symptom is (which is why the pros lament not being able to BE there) because a 'bog' is pretty vauge. Putting holes in the air doors may help, may not, worth a try either way though. It may even turn out that the air doors aren't in play with the 'bog' but are an entirely separate issue that would show up more at the track or on the dyno than when mash the gas from slow roll.

The more things that get tried, the more evidence there will be for a particular solution, so keep tweaking at it. It'll be interesting no matter what.
 
joec said:
they work like a vacuum secondary . If the secondary air valve opens to quick it bogs if to slow its lazy. the nice thing about the primary rods is you can change the jet size without changing the actual jets just by metering rod size.
if the secondary air flap opens too quick it bogs because there isnt enough fuel to cover up the hole created by the throttle plate opening because it doesnt have a secondary acc pump.How many holleys or avs or afb have you had to change a large amount of jet if there wasnt something wrong with the motor?Point is the calibration is always somewhat close.Just because vacuum is same doesnt mean the carbs are synced usually have to sync them somehow. also sometimes have to lessen jet size if plugs are too cold .
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:thumbsup:
 
PRH said:
Relating to the original post about the carb test on the 409.....
Post #18

Tuning the new Demon carbs | 348 and 409 Engines
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That was a cool discussion that I hadn't seen before. I appreciate you posting it up... Although it did sound like a bunch of us going at our opinions...

PRH said:
Never mind........I should have known better :BangHead:
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take it easy man I was at work and noticed your boat comparison and thought it was a pretty good chuckle. I didn't mean to ruffle your Guru feathers..
Anytime you think you're Guru'ness is in question you get all bent out of shape LOL...
 
joec said:
they work like a vacuum secondary . If the secondary air valve opens to quick it bogs if to slow its lazy. the nice thing about the primary rods is you can change the jet size without changing the actual jets just by metering rod size.
if the secondary air flap opens too quick it bogs because there isnt enough fuel to cover up the hole created by the throttle plate opening because it doesnt have a secondary acc pump.How many holleys or avs or afb have you had to change a large amount of jet if there wasnt something wrong with the motor?Point is the calibration is always somewhat close.Just because vacuum is same doesnt mean the carbs are synced usually have to sync them somehow. also sometimes have to lessen jet size if plugs are too cold .
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Don't confuse a bog with being choked out. And they do not operate the same.

One uses Venturi vacuum to pull the secondary throttle blades open.

The other one mechanically opens the secondary throttle blades. Nothing vacuum about that. The air valve is opened from air flow. Not the butterflies.

Two different systems.
 
yellow rose said:
Don't confuse a bog with being choked out. And they do not operate the same.

One uses Venturi vacuum to pull the secondary throttle blades open.

The other one mechanically opens the secondary throttle blades. Nothing vacuum about that. The air valve is opened from air flow. Not the butterflies.

Two different systems.
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I would call it more hesitation cough than bog...
 
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