Scottwhite98

7040250 4bbl Quadrajet factory for 1970 Cutlass Supreme (rebuilt). My 350 has stage II heads from J&S machine with the crossovers filled, true dual exhausts (factory top & bottom manifolds), stock 2.56 differential, factory stall, B&M shift kit in the TH-350, Comp-Cams XE256H cam.

When I'm out and put it to the floor, it falls on it's face and takes some time to catch up. After it catches up after several seconds it runs good.

As I read Doug Roe's Rochester Quadrajet book, the secondaries don't lift the metering rods until the secondary air valve opens, and that valve only opens when the vacuum underneath is sufficient to draw it open.

As I understand it, as the footfeed opens the secondary throttle the RPM's are not high enough to to open the secondary air valve which in turn will deliver a richer fuel mix to the intake.

My bet is that the primary metering rods aren't rich enough....or I could be totally wrong. LOL

Scotty

RetroRanger

im no expert but is the secondary air valve opening to soon ? there is a an adjustment for the spring wrap IIRC you loosen the allen head and adj the spring till it just closes the valve and then turn 3/4 turn beyond that, there was another thread recently that I think was similar to yours ill see if i can find it.

edit this is the thread i was thinking of

link to thread

shiftbyear

Get the Cliff Ruggles Book, it's much easier to understand. Your choke diaphragm also controls the opening rate of the secondary air valve (top flap). With your axle ratio, it should be opening very slowly. It might need a new one. Was the carb rebuilt? Was the distributor recurved? There are some videos on you tube dealing with quadrajet secondaries and distributor recurving. Good luck.

Fun71

One sign of the secondary air door being too loose is when you are moving down the road and go WOT the engine bogs for a second or more before starting to pull. When the secondary air door is adjusted correctly, the engine starts to pull almost immediately.

Scottwhite98

It is a HEI recurved distributor.

I don't believe the air valve is opening too soon. With the air cleaner off I have watched it's behavior and it doesn't open until into the higher RPMs. When I've revved it up as high as I care to go without breaking something it only opens about a half an inch.

Carb has a new choke-pull off.

rob1960

Check with Cliff Ruggles ph #1.740.397.2921. He has hi-perf choke pull-offs that release way faster then the generic parts store ones. I redid my buddies '70 W30 "256" carb a couple years ago. I picked up a new pull-off from my local Napa store(they even had it in stock)but it released so slow there was no way it would work so I didn't even try it on the car! I've used Napa carb parts for years & never had any problems but with everything coming from China these days...... This thing was so slow you'd have a birthday by the time it fully released!! It should release in a couple of seconds. I have one of Cliff's price lists that he sent me with an order & it listed hi-perf pull-offs available. I called him up,told him what I had(he can tailor the opening rates to your app.)& he sent me one out(it was around $50) It works great! You can easily check to see if in fact the pull-off is your problem. With the car warmed up,take the pull-off & rod off & plug the vac port for it & drive the car & see what it does. If your spring wrap adjustment on the secondarys is correct & the car pulls fine you'll know that the pull-off is too slow. You said you had a rebuilt carb, make sure all the other adjustments are correct. Like shiftbyear said, Cliffs book is a great resource & so is the carb section in the service manual.

Fun71

Revving in idle won't give you the same results as going WOT under load. The air valve would have to be VERY loose to see it open too quickly at idle.

BlackGold

If it falls on its face, the problem is that the secondary air valve is opening too fast, like everyone except rob1960 told you. If it were opening too slow, the car would simply accelerate more slowly, not fall on its face. Big difference.

Another possibility is that the accelerator pump is not working.

Your cam would appreciate more gear and a higher stall converter. Your current combination makes it even more sensitive to carb adjustments and makes it even more critical that the SAV doesn't open too fast.

Also, is your transmission's kick-down working? If it doesn't downshift when you floor it, then the engine will be at too low an RPM to take advantage of a wide-open throttle and, again, would benefit from the SAV opening slower.

Scottwhite98

Thanks!

Choke Pull Off replacement is from Carbs Unlimited. Reading Doug Roe's book, it seems that on the performance side, getting the air valve open quick is a good thing.

I am confused as to why load is necessary to open the secondaries, since the air valve is just a spring loaded butterfly responding to vacuum. If I rev my engine to 5000 in park the engine ought to be moving the same amount of air as if it were in gear. I guess the only thing load will do is draw out the amount of time the engine is at WOT allowing the choke pull off a few more seconds to open the secondaries.

Won't unhooking the choke pull off in park do the same thing?

joe_padavano

Unless you are hanging onto the engine and looking at the carb while driving, your observations are meaningless. You will not see the secondary air valve open much if at all while revving the engine in PARK.

To clarify, it isn't vacuum that opens the secondary air valve, it's mass air flow. The air valve and attached secondary metering rods are actually a mechanical MAF that regulates the mixture based on mass air flow. This is one reason why I say the Qjet is the most advanced 4bbl carb ever designed.

As noted above, the vacuum diaphragm on the air valves also controls opening rate based on throttle position. There is an orifice in the inlet to the diaphragm that limits how fast air can enter the diaphragm housing and allow the air valve to open. It's really more of a dashpot than a vacuum actuator. If this vacuum housing is original, the diaphragm may be bad, again allowing the air valves to open too quickly.

oddball

While an engine is an air pump, the total air flow through the engine is not purely a function of RPM and/or throttle opening.

BlackGold

NO!!!
Getting the air valve to open at the proper speed for your combination is a good thing.

If you want it to open quick, just disconnect the choke pull-off and see how you like it. Heck, just wire the air valve wide open so that it's ready and waiting for you to floor it. (Hint: it'll be worse than the fall-on-its-face behavior you already have.)

You have highway gears and a tight converter. Your engine does not leap from idle to 4000 RPM in an instant, so neither should your secondary air valve.

Here's an experiment for you: Wire your secondary air valve shut so it can't open. (Many Q-jets have a little lock-out lever which does this whenever the choke is cold.) Take it for a drive and I bet you'll have the best throttle response you've ever felt. It'll only be lacking at high RPMs.

oddball

I think the books say faster is better because the typical secondary/choke dashpot is horribly slow. Like several seconds slow.

So should the spring tension or the dashpot rate be the primary controller of the secondary air valve opening rate?

BlackGold

In my experience, replacement vacuum breaks are faster than they should be. I'm not sure if this is intentional in the making of one-part-fits-all, or if it's because they no longer use a precision-drilled orifice like the original ones do. Instead, they seem to just crimp the end of the tube that the hose slides onto and hope the resulting hole is the right size.

Remember that the engine vacuum seen by the secondary air valve once the secondary throttle plates are open also helps pull on the vacuum break. So it opens faster on the engine than it does in your hand.

My view on tuning the secondary air valve is to set the wrap spring pretty much as lightly as you can and still have it close the valve, then control the rate of opening with the vacuum break. Other people may like to also control the rate by tightening the spring. I'm not going to say that's wrong, but it adds another variable, making it harder to repeat results. Having said that, if you can't find a vacuum break that's slow enough, by all means tighten the spring a little.

joe_padavano

I've usually done it with the spring, but either way works fine. I will point out that it's usually a good idea to wipe a little grease on the pin that the hook in air valve spring slides on. Friction there will make opening more erratic.

GEARMAN69

One other thing that makes for a great working secondary transition is opening up the holes slightly and adding a small slot to help allow some fuel to pull thru in advanve of the flap opening to aleviate a lean bog transition. The muscle car era Pontiac carbs had large slots cut in and a larger hole than the run of the mill carbs that have no slot and a smaller hole. The mod is easy can be done with a dremel bit of a small round or even triangular file and does not have to be as larghr as the OEM ones pictured. These mods are in Ruggles book BTW.. I did it to my 77 carburetor and also used to swap the notched OEM flaps from early carbs to newer ones or modify it like my current carb. I will be doing this mod to my 70 350 olds carb as well. Currently it is on my sons truck and has a fantastic primary side after swapping to 71 jet and 44 rods still lacks on transition needs this mod.

The Olds carb has no slots and the small hole.

Copying this below from a page where the mod was done.

"Secondary System
I ordered Edelbrock "CE" secondary rods and installed them.

I drilled out the secondary discharge holes from stock 0.029" to about 0.033". Don't be fooled like I was initially; these discharge holes have a large ID visible, but deeper in these holes the ID is smaller, and this is what you want to modify...the holes are in the main body casting of the carb; I'm pointing to these holes in the photo below.

In addition, I also added small cutouts (see Ruggle's book page 106) to the air flaps. This helps atomize the fuel when it is initially discharged from the secondary circuit and the flaps arent' quite opened up yet. I also did a minor adjustment to the air valve spring for quicker opening. "
http://www.squidsfabshop.com/gtotrain/gtotrain.php

Octania

is the choke pulloff link in place?
It also serves to limit the SPEED at which the 2' air valve opens.


It must be in place and not modified [hole drilled out]

distributorguy

Back to basics before getting too deep into carb adjustments - try advancing the distributor 5 degrees and see if the dead spot fades. Then try another 5 degrees. Your motor will likely take as much timing as you can throw at it before the starter kicks back! Always get the timing perfect before playing with fuel, and don't assume that some literary magic timing number is right for your engine. Mine is at roughly 25 BTDC at idle, 40 degrees total. Runs exceptionally well, no pinging, engine never exceeds thermostat temp.