Punch it for power!

This question is concerning the Q Jet secondary air valve. Most of the older carbs have a little plastic air valve and later carbs have a metal one.
I have heard that some of us are eliminating this secondary air valve altogether, and simply adjusting the top secondary butterfly spring a little tighter.

I wonder if the butterflies end up being partially closed at the top end of the higher gears. I wonder because I am finding that I need to tighten the secondary spring fairly tight to totally eliminate the hesitation when I first punch it. Is the tighter butterfly spring setting hurting high RPM airflow?

 

I would say yes. Rochester spent the extra money for that little PITA diaphragm for a reason. If you have a plastic one, you can enlarge the hole for a faster opening, just don't over-do it.

I am also surprised when using just a spring that you aren't running very rich as it over-draws from the secondary enrichment wells.

Buy Doug Roe's book. Anybody fooling with a Q-Jet should have it. the Q-Jet is a great carb, once you learn to work with it, and not against all of the designed-in features. Roe goes a long way into describing the whys and wherefores of all those features.

 

Have run the QJ without that vacuum operated dashpot for years. As long as you get the windup spring adjusted right, there will be no problems. The air valve will stay closed and the spring will only be over-ridden with enough vacuum pulling past the open mechanical secondary blades. No problem with over enrichment through the secondary tubes.
And as Adam said you can drill the orifice to a larger dia. in the dashpot and still retain than system and link. Its just if you go too large, you'll open the secondary air valve too early and will end up buying another dashpot and trying again. I just found it easier and cheaper to tighten the spring.
And the Roe book is a must for anyone wanting to tinker with their QJ. Best all around carb in my view if built and adjusted correctly. :TU:

 

Thanks, Guys!

Your two views certainly lend some insight to the topic. I have the Doug Roe book, but it is a VERY OLD COPY! It is copyrighted 1973 (!)

What confuses me about that book is all of the very intricate, and complex modifications that he describes. How many people actually do these things?

I think that most street / strip cars can get away with only jetting and secondary air valve adjustments. Maybe I am wrong about that.

Lastly: Here is my thinking on the secondary air valve. I will try tightening the spring up a little more, but I am concerned about going too tight in an effort to dial out all of the hesitation. Of course, the air valve is there to slow the opening of the secondaries. Without it, we have to increase the tension (resistance) of the top butterflies to get rid of the hesitation. On a normal carb, the spring tension is there to meter how much air the engine gets in proportion to a given amount of pressure drop (vacuum) below the butterflies. By running without the vacuum break, we are changing the purpose of the spring tension. I wonder if we end up robbing our engines of some CFM via the spring being so much tighter.

Rock on,

 

It's hard to argue with Dave's results, but me personally, I'd put the vacuum break back on. Do not forget that it also pulls open the choke when the engine starts. I always enlarged the hole to start with using a 0.5mm cleaning pin for a mechanical pencil.

If you get the hole too big, put a little epoxy in there and drill it back at the smaller size.

Dave is right, it's one of the best all-around carbs there is. Even if we think differently on how to make it work the best.

 

Andrew,

I used to be a QJ only proponent for about 99% of our typical street type buildups. Then along came alum. cyl. heads that flowed in the 300+ range with the camshafts to compliment them and much more power. The QJ does have its limits, especially in those applications, but there comes a point when a good Holley or derivative of, just flat out overtakes the abilities of a QJ no matter how modded it is.
Many years ago, after buying the Roe book I took a 72 QJ from a 455 and tinkered with that thing with just about every mod in the Roe book. I then compared that carb back to back (along with several other carbs) in an all day thrash at a local track. I didn't take into account how the air had changed during the day. But our tracks are all at sea level and it was a pretty mild day. The QJ was just barely outpaced by a 4781 Holley in both et and mph. The 4781 is an 850 carb and this one had the air horn milled off. Both carbs were optimized as far as jetting and tuning, as I had run both carbs a lot. One thing the QJ did WAY better was use less fuel, a lot less. Which made the QJ the much better street carb. And being that close in performance really reinforced my opinion of them.
I don't think doing all the mods would be necessary, it just depends on how much tinkering you want to do and what you expect to get out of it.

 

Thanks Dave,

That's great info.
I did wonder how close a modded Q Jet can come to a Holley double pumper, and how many mods might be necessary.

I tightened up the secondary spring again. It's fairly tight now, after this, being the 4th time I've adjusted it. I believe that in the long run, I'll end up back with a vac break diaphram. For now, I'l just mess with what I have. Hey, it's an excuse to floor it a lot!

 

Before reattacking the air valve again, there are several things that can cause the same problem. They include the accelerator pump, and the feed tubes for the secondary barrels. If the feed tubes are not properly sized for the application, the result will be a heasiation on the second or third time the secondaries are opened. Also, if the accelerator pump is not operating properly, the result can be confused as a hesitation on opening. Another thing to consider is the air temperature. If the incoming air is not properly heated, the result will be a lean-out on opening. Other things are also in question, but let's start there. Ray

 

I can use some help on this topic also.

I have a QJet off a 72 350, and its pretty messed up. The intake (which i believe is stock) has an automatic choke, not manual or electric.

But the accelerator pump is leaky, the arm that pushes it constantly falls off, and the secondaries dont open to my knowledge.

I was getting ready to buy an edelbrock intake kit (b4b intake, 750 performer carb, and air cleaner)

What are your guys opinions on all this stuff and also how will it perform in comparison to a messed up qjet.

Also, i am replacing a leaking fuel pump, but with the leaky stock one i got on a good day with the qjet 8 MPG on a 430, will this increase or decrease with a new carb?

Thanks

 

Q-Jet! :TU:

Instead of throwing all that money at Edelbrock, you'd be WAY better off to send your messed-up Q-jet to a guy like John Osborn, and having him rebuild and set it up for your exact car/application. Worst case scenario will run you maybe $400, where the B4B and Edelbrock and air cleaner will run you $500 or better, and won't run nearly as well.

(Keep in mind that there's a lot of argument about how good a B4B or Performer intake really is over stock, and the Edelbrock carb is just a rebadged Carter AFV, which is basically a smaller-secondary version of a Q-jet.)

I had a Qjet that I'd modded and ran really well, but the airhorn leaked because it was warped, and it needed throttle shaft bushings. I sent it to John Osborn (along with a long list of info he requested), and what I got back is a carb that runs noticeably better, and gets better mileage, for just under $300 shipping included. I HIGHLY recommend his work. :TU:

 

How can i contact him?

 

Just had him do one for me. Can't put it on though, till the weather gets better. John Osborne Performance
P.O. Box 65
Rineyville, KY 40162

Call him at: 1-270-737-4467
I hope this helps.
Regards, John. :TU:

 

another good one is jim mcgowan .he owns the quad shop in rockford il.