# Quadrajet secondary air valve and vacuum diaphragm



## Mike O’Neill (Feb 5, 2019)

I have read the various posts that it should take 2 seconds for the diaphragm to extend after being manually compressed and released. Mine is almost instant, so I figured that is the cause of the "bog" and ordered a new one from Ames. New one arrived today, and it is the same way. I recorded a video of it, and used video editing software to precisely measure the time, and it was less than 1/4 second. 
Do I understand correctly that the restricted orifice is in the stem of the diaphragm itself, not in the hose?
I did adjust the spring tension to the recommended 1/2 turn, then went 1/4 and 1/2 turn beyond that - still bogs.


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## BearGFR (Aug 25, 2008)

Yes the orifice restriction is in the diaphragm and not the hose, but that doesn't mean you have to live with that arrangement.

If you want to tune the diaphragm release timing, you can use my trick:

Cut the hose
Get some small brass or steel tubing that's the right size to slip inside the hose and not leak
Cut a handfull short (inch or so) pieces of that tubing
Plug one end of each piece with some quick dry epoxy - like the 5 minute stuff
Drill different size holes through the epoxy -- we're talking VERY small, tiny - wire drill sizes - like 0.020 or so.
Make a handful of these with different sized holes in them - smaller hole = slower release, larger hole = faster release.
Play around with these (insert them in the hose at the point where you cut it) until you get the release timing you want.
If it still doesn't at least make the bog "different" (or completely get rid of it) no matter how fast or slow you allow the air horn to open then the release rate is not the source of your bog problem. By the way, you want that spring to be as LOOSE as you can make it and still have it able to completely close the air valve, but just barely. You can slow down the opening rate by cranking up the tension, but that's the WRONG way to go about it. Cranking up the tension also makes the engine have to pull harder to open it, and in fact may not be able to ever get it fully open. That's not what you want. You want that valve to open as fast as the engine can tolerate it, but no faster. That's why you tune the rate with the diaphragm.

If that turns out not to fix the problem and you've addressed the other possible causes, then my bet is your getting fuel slosh up through the secondary metering rod holes and over into the secondaries. I can tell you what to do about that too, but you'll want to make sure everything else is right first.

Bear


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## Mike O’Neill (Feb 5, 2019)

Thanks for the quick reply. Your post gave me the idea of using some 3/16" brazing rod and drilling it in the lathe. The smallest drill bit I have on hand is .039", which is too big. So I squeezed the drilled rod in a vise until the delay looks like about 2 seconds. The road in front of my house is being paved right now, so I won't venture out to try it immediately. 
Coincidence - my copy of Cliff Ruggles' Quadrajet book just arrived. When I first flipped it open at random, it came to page 110 - Making Choke Pull Off Restrictions.


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## BearGFR (Aug 25, 2008)

Mike O’Neill said:


> ... my copy of Cliff Ruggles' Quadrajet book just arrived. When I first flipped it open at random, it came to page 110 - Making Choke Pull Off Restrictions.


LOL, yeah, that's where I stole the idea from.


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## integrity6987 (May 10, 2017)

Are you guys talking about this...? What's the function??








(Guess to keep the secondary's closed during high vacuum?)

How do I test during cold or hot idle? I don't experience "bog" but usually press the accelerator at slow rate when cruising / or just dump the clutch from 2500! 😁


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## BearGFR (Aug 25, 2008)

Yes, that's it and that's what it does. It holds the secondary air valve (the ones on top, not the throttle plates) closed until manifold vacuum drops. If those big barn doors were allowed to snap open and dump all that air into the engine, that's the source of a huge bog --- a bunch of air all at once with no fuel to go with it. That diaphragm lets them open gradually, giving time for the secondary metering circuits to "see" the air flow and start feeding fuel. Tuning the opening means arriving at an opening rate that is as quick as the engine can tolerate, but no faster. The way to test the opening rate is to disconnect the hose and use one of those hand-held vacuum pump brake bleeders to hold it shut. Release the vacuum and time how long it takes the diaphragm to relax. There's a light spring inside that will push the plunger out whenever there's no vacuum.


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## Mike O’Neill (Feb 5, 2019)

My tiny drill set arrived and I made a restriction out of 3/16" brazing rod. It is .210" long and has a .016" hole through full length. I placed it in the hose near the diaphragm end. The spring return time is longer but still well under 2 seconds. Both with the diaphragm that is on the carb and the new one from Ames. They must have strong springs in them. I even tried the piece of brazing rod in the hose before drilling it, to make sure it didn't leak around the rod - it didn't.
The car still bogs badly on a standing start with pedal floored, maybe just a little better than before. It bogs again when it shifts into second (TH400), but much less. No bog when shifting into third. If I floor it at 35 mph or faster, there is no bog. Part throttle acceleration is smooth. Accelerator pump is working.
My boat (built in 1987) has a Mercruiser 260 HP, 350 c.i.d Chevy engine with Quadrajet. I checked the spring return time on it - also well under 2 seconds. Come to think of it, it has always had a slight "bog", but I never paid it much mind due to how I use the boat.


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## PontiacJim (Dec 29, 2012)

If it bogs from a standing start, I would not necessarily be looking at the secondaries.

Have you ever put a vacuum gauge on the engine to make sure you don't have a vacuum leak causing the stumble?

First, what is your idle RPM?

Second, have you adjusted your idle mixture screws? How many turns out are they from seated? 

Is the accelerator pump rod in the hole closest to the carb?

Have you looked down the carb throat (engine not running) and observed how well the accelerator nozzles are supplying fuel into the carb? You may need a new accelerator pump if you do not see a nice long strong stream of gas being shot into the primaries.

Do you know that your float is set correctly and not too low or high?

It could be fuel slosh as BearGFR has stated. Fuel can slosh out the holes that the secondary rods go through at the top of the carb - those things connected to the secondary hangers. Hard acceleration can force the gas back and out the holes and cause a stumble on hard accelerations.

Timing can be critical. What is the initial timing set at? What is your total mechanical timing? Is your vacuum advance hooked up? Ported or manifold vacuum?


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## BearGFR (Aug 25, 2008)

Mike O’Neill said:


> My tiny drill set arrived and I made a restriction out of 3/16" brazing rod. It is .210" long and has a .016" hole through full length. I placed it in the hose near the diaphragm end. The spring return time is longer but still well under 2 seconds. Both with the diaphragm that is on the carb and the new one from Ames. They must have strong springs in them. I even tried the piece of brazing rod in the hose before drilling it, to make sure it didn't leak around the rod - it didn't.
> The car still bogs badly on a standing start with pedal floored, maybe just a little better than before. It bogs again when it shifts into second (TH400), but much less. No bog when shifting into third. If I floor it at 35 mph or faster, there is no bog. Part throttle acceleration is smooth. Accelerator pump is working.
> My boat (built in 1987) has a Mercruiser 260 HP, 350 c.i.d Chevy engine with Quadrajet. I checked the spring return time on it - also well under 2 seconds. Come to think of it, it has always had a slight "bog", but I never paid it much mind due to how I use the boat.


Bogging again on the 1-2 shift is a little odd. By then your secondaries ought to be fully online. If the 1-2 shift is really firm and hits "hard", the problem might be fuel slosh. What happens is fuel rushes to the rear of the bowl, comes up through the secondary metering rod holes, and spills over into the secondaries where it causes a rich bog. Try this: pull the top, put a strip of black electrical tape over the secondary rod holes in the gasket. Put it back together then carefully puncture the tape when you put the secondary rods back on. What you're doing is making a seal around the rods so fuel can't spill over. It may take a few tries to get it right. Work the rods enough so that they don't bind. See what that does for you. That tape won't last forever, so if that cures it you can make a more permanent seal out of a very thin piece of metal, like a soda can.
You've got to make sure everything else is right first though. This fix cured a bog in my 69 that I chased for a good long while.

Bear


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