Paul -
That's the airhorn gasket, and it won't have any effect on vacuum passage connections between the throttle plate and the float bowl.

Lars

 

Mooser & Paul -
The passenger side nipple is always full manifold vacuum. When the nipple was not used, the hole was not drilled, and there was a "boss' on the casting to assist with alignment of the choke pulloff plate, which otherwise aligned on the vacuum nipple. Here are a couple of shots of a carb without the nipple, showing the boss aligning the choke pulloff plate:





Carb drilled for the nipple had the long nipple installed as shown below:



That nipple connects directly to a hole in the bottom of the float bowl shown by the tip of my pen:



With the gasket in place, you can see the gasket hole that aligns with the float bowl hole:



Here's the gasket laying on the throttle plate, seen from the top. Pen points to the vacuum hole:



With the gasket off the throttle plate, you can see where the vacuum hole connects to the throttle plate and how manifold vacuum gets there:



You can trace out your own vacuum passages (which should be identical to that shown above) using some WD40 and the stupid little red plastic tube that comes with the bottle - this will squirt through the passages so you can trace it out to see where the problem is.

Lars

 

Thanks Lars.
I'll try to chase that today and see if I can figure out what is blocking the passage.Based on what I have and the location of the hole indicated in your 4th picture I should be able to feel the intersection somewhere down around the 5 o-clock position.

Since it's not leaking I'm assuming the gasket is wrong and blocking that port at the base-plate.

I adjusted the secondary spring just sitting on the bench to touch+7/8 turns and then got looking as to why there are so "sticky" and ended up pulling the flaps off (those screws are a PIA, guess the last thread must get staked or just buggered up) and taking the rod out, it's bent slightly and binding in the mounting holes.
Have to try to straighten that out today a little better and get it to move freely.

I knew I should have just slapped a SEP field around this thing while it was on the bench

Anyway, I'll see if I can find some indication of that cross-hole.
Thanks again
Mooser

 

It is ......:o.

But I really do appreciate Lars' pics.

 

The "spec" on the airvalve spring is 3/4 turn windup - that will work well on almost any Chevy application, producing very good secondary response without any bog or hesitation. Only go up to 7/8 windup if you have a slight stumble going into the secondaries.

It's common for the shaft to have a little bind, and it's also common for the airvalves to rub and catch against the rear of the airhorn opening as the casting gets distorted from over-tightening of the 2 rear airhorn screws. It's important to eliminate all this binding, just as you're doing, in order for the secondaries to operate correctly. Good job on the work.

Lars

 

Ok, blued the shaft and checked, it was rubbing hard when in the closed to slightly open position right in the center area



Indicated it and used a small version of the Lars tuning hammer to straighten the shaft in both axis.


Now it rotates perfectly smooth/loose. put the flaps back on and set the spring to 3/4 turns. check for clearance, everything moves beautiful. Zero binding anywhere. Flap actually "flaps" for the first time in ??? years.


Checked the primary side and moved the flap slightly since it was just barely touching on the pass.side of the casting. (no real affect)

All thing being equal I didn't have to remove/replace any gaskets so technically I didn't take it apart.

(although whoever thought up that link down to the choke lever bit was quite a comedian)

Got everything buttoned back together, manifold changed, distributor back in, coolant topped up, and it actually started again! (you can see I've got one of those bazillion volt coils so I can just drape the plug wires near the plugs and it will work)


As for the mystery hole in the side.
I went in with a paperclip with a 90deg bend in the end and a shart point, and dragged from the bottom to the top and back again and worked my way all around the entire hole just barely turning each time. Unless the intersecting hole is smaller than .04, I can't find any trace of an intersecting hole like the one shown.
I don't want to take it apart right now (as you can tell, there will be many oddities once inside) but when the car comes off the road I'll be digging in farther to see whats going on here.

Maybe this was made during a transition in design that was cast for the port but then built without it?

Thanks for all of the help guys. Much appreciated.
I'm off to reset the timing, mixtures, idle, choke, and everything else again.

Mooser

 

Very nice work indeed! Did you find an intake manifold vacuum source? I have not. Despite my mistake above I believe it is the gasket.

But I did 50 miles today with the Wife on country roads and even with ported vacuum the 1974 hums. I am reluctant to fool with the carb as I do not have Lars' skills. Not even close. So I'll drive as is. Sorry to show my Q-Jet ignorance in your thread.

But I wish you well. You seem to be on your way to success!

 

Thanks Paul, it's coming along nicely.

For now I've "tee"d into a manifold line for the advance, seems to be working fine (Did this on the old manifold back at the start of the season, dropped the temp a little also)

I didn't want to get into the carb at this time, (forget about skill, I can barely spell Lars) but knock on carb-cleaner soaked wooden bench tops, it seems to be working as designed, for once in the last whatever years.

I got about 27 feet on the car today as I backed it out to clean up the floor-dri underneath.

I need to re-torque the manifold and I've got to shorten one of the heater hoses a little (pipette is in a different place on this manifold)

The blind port is bothering me already though.
All the best
Mooser

 

I'm quite envious of your abilities and toolset, Mooser. You're a lars-in-training.

 

I agree.

 

I'm in no way even remotely fit to carry Lars' old beer caps.

 

If you could just help keep track of where I put my beer bottle, we'd make a great team!
Lars

 

Lars,
I just want to make sure I fully understand what your telling me here so I can make sure I'm not misinterpreting it or getting confused since this is a pretty new area for me.
Your saying that it's possible to put beer down?
Just checking
Mooser

 

No, I'm saying that a beer should never be put down, and when you screw up and put it down, things go very wrong. That's when you need the help of good friends to help you get through the tough times of the loss of a loved beer. People who care quickly put a fresh cold beer back in your hand and lovingly admonish you to not put it down again.

 

Well said, Lars!


Keep the shiny side up!
Scott

 

So I got everything buttoned back up, reset the timing (not perfectly happy there, have to re-look at it again but it's a solid 36, goes a little odd with the vacuum advance back on) then set the idle mixtures

They will kill the motor if I bring them in and at about 3-1/2 now they stop having any affect on the vacuum gage (was about 4-1/2 before, not sure why it changed)
It's only pulling just around 15" and it isn't very stable (bounces from around 14-1/2 to about 16+ and sort of stumbling) Tach on the timing light shows the idle moving around also 600-750-550-700 etc. Car does have a healthy cam but should be a little more stable than this.

Anyway, left them there (seems like the timing is a little behind, would have liked 17-ish) and set the idle (screw) for 600, can only get it to idle decent at about 675-700, set the stop solenoid for the same. in park it's at about 900-ish

Went for a little test drive and apparently my secondaries haven't been working right for quite a while, BIG difference according to my a$$dyno and I doubt much if any of it comes from the different intake.

Idle crept way up so that it was about 1000 in gear, adjusted it back down again and went for another drive, crept up again a little. I've guessing that since I had all the linkages about and flipped the carb over a zillion times it needed a run to settle into where it wants to be but the idle system is kind of funky, once it's off idle is seems to be quite happy.

The lifters need to be set again (a little ticky still) and that may help settle the vacuum bounce down some.

Carb still need mucho work but these few fixes made a noticeable difference for now.

Probably should go around and look for a vac-leak too.
Thanks for all the help guys, info and encouragement always appreciated.
Mooser

 

Apparently I've got the carb from a snow-plow



The hole only goes in about .075" and is blind, doesn't connect to that side hole at all. Both holes just dead-end.

Also, as you may guess, I ended up caving in and have the entire carb apart on the bench (well some bits are outside in carb-cleaner)

I spent most of the morning blowing air though passageways and tripping floats and pistons seeing how some of this works.

Also found out that two of the air-tubes (thin brass tubes coming down from the top) were laying in the carb


The piston that the primary rods are attached to sits on a spring but is completely stuck inside the cylinder and had to be pried out with way more force than a person would expect (after soaking and all kinds of staring at) while heeding the warning of not bending the two little arms.

Since there is a spring, one can only guess it should actually move freely


And that I have a rudimentary understanding of how most of this sort of functions (not why but how)
Kind of surprised that the primary rods don't close the jets, just seem to change the orifice size. After looking at how the fuel is pulled (pushed I guess) by the engine it makes sense (spent some time trying to simulate it with an air-gun, failed miserably)

There's one passage I couldn't get any air movement through on the passenger side, I'll try again after cleaning but it should be part of the mixture screw passage, I'm guessing I dislodged something (crud) when I took the screws out since it was responsive on the car

The current mystery is exactly what this little lever is for


Doesn't seem to really move, has a slight spring like quality when I push it. Hopefully when I put the bottom on I can see where it might push something but right now it just seems to sit in the little slot in the body that feeds to the top.


So much for ignoring the carb until winter, hopefully I can get all the pieces back in with minimal leftovers

Mooser

Update
Looks to be an adjustment screw underneath that lever, it's capped off on the outside so must be some factory setting. Appears to limit the piston as to how far down it goes and therefor how far down the primary needles will go into the jets.
Found it in Lars paper, adjusts the power piston, covered by metal plug, screws usually seized, leave it alone.

I checked the piston size (.3406-.3408) and the bore (.339- pin goes easy, .340- pin goes very snug / stuck) so it was close enough that I just burnished it with a little diamond paste and a dowel spun to .339+) piston moved nice and smooth now, I can see a few high spots on both sides of the bore where it was stuck.

Since there is no movement in that lever and nothing else touches this piston, the hole at the bottom of the cylinder (where it touches that lever) must see full manifold vac and that would pull the piston down and lower the needles into the jets and that would reduce the size of the opening for the fuel at idle. As throttle increases the vacuum goes away and the spring pushes the piston up and the small diameter of the needles is in the jet and more fuel is allowed through ????
(I should buy that book from Cliff actually should have bought it BEFORE doing this)

 

Well it's back together (again)
Everything cleaned, all holes checked one at a time
All surfaces checked and seem nice and flat according to a straightedge
Power piston (or so I believe that's what it is) set somewhere higher than .025 and lower then .030 and moves freely (here is where it was stuck, pretty much most of the way down)


Float set to .375 or (just a hair under)
Float bowl is full of gas right now and sitting up above a piece of paper to see if anything leaks
Mixture screws set at about 3-3/4 turns, close to where they used to be so it should at least be in the ballpark
Everything moves freely, nicely, properly (even the fast idle lever but it seems pretty bent, twisted, abused, better start looking for a new one of those)
So I'll put it back on tomorrow (hopefully) when I get a new filter (forgot that one) and see if it's better or worse, but at least it's slightly cleaner on the outside (lots cleaner on the inside) and a few screwy things are fixed

This morning


Tonight


It's like I did nothing all day
Mooser

 

Everyone says it's bad to have the needle retaining clip thru the hole(s) in the float...? No...?


-ALF out....
P.S. very cool stuff goin on in here....

 

Yup, the needle clip is installed incorrectly. It should be installed around the rear, rounded feature of the float arm.

Also, the primary rod retaining spring is incorrectly installed, mangled, and destroyed. Either remove it, or replace it with one that's correctly installed.

Lars