Why did Rochester build it this way? What could the advantage possibly have been? Was it to prevent bog?

Isn't the whole point of the air valve to open gradually to prevent bog anyway?

 

Who knows for sure. But, I know that a low compression 150HP 4500 RPM 305 doesn't need a full 750 CFM and all that fuel. Were you asking how to fix it? All you need to do is grind down the stop.

 

Oh, no, I've read about how to fix it. I just wondered why they would do this. The design predates CAFE and emisions and all that other nonsense, so there should have been a real reason.
I mean, if you were to stuff a sock in your mouth before running a marathon, there must be a damn good reason for it, right?

By the way, how far do I grind down the stop? Do I want the air valve to be perpendicular to the bore? This wouldn't cause a bog, would it? Since the air valve is sucked open by the motor, it will only be opened the optimum amount, right?

Thanks,
Homer

P.S. I drive a 150hp, low compression 350. But I'm sure I don't need this yet either.

 

Grind it down till its between 85 and 90 degress. Make sure not to go any farther than that because 1 your not going to be gettting any extra flow and 2 I think if it goes more than 90 deg it can bind itself.

 

Ok, I'll do that, seems simple enough.

I was looking at the carb earlier, and it seems almost like it was designed to be opened all the way. Even the secondary cam seems designed to give extra enrichment for a wider open air valve.

Did Rochester get paid off by Holley to shoot themselves in the foot powerwise?

 

I would stay closer to 85 degrees. And yes, over 90 degrees and the air valve can stick open. Just like a throttle body on a FI car, going 90 or farther just creates turbulence. Grinding the tab will take a while, so grind a little, check it, grind some more, check it, you get the idea.

And a well tuned QJet can make just as much power as a Holley of equal CFM. Rochester designed the QJet for economy before performance - but you can change that. The Holley is performance only (sometimes ), and NEVER economy.

 

usually if the flaps were going to get stuck open the vacum will pull it closed

 

Why is the Qjet so unpopular for performance? Lack of parts? It's too darn complicated? Distribution problems between front and rear cylinders?

 

Bingo! In stock form they didn't perform too well, JUST LIKE A HOLLEY. A Holley is relatively simple to tune, mostly just swapping jets. Bigger hole more fuel - any dope understands that concept. But trying to tune a QJet is more complicated, because they are VERY sensitive to engine vacuum. Try to explain the function of secondary metering rods to a Holley guy and they get a confused look on their face.

 

Wait, Holleys don't have metering rods? How do the jets work then?

 

The jets operate straight in a Holley. Extra enrichment is provided via a power valve, which operates separately from the jets (but flows through the same boosters).

I got spanked a couple of weeks ago by a '67 Nova stock class 327 q-jet that ran 12.2's here, 11.7's at Topeka (.9 under the index). Don't try to tell me q-jets can't run and can't be made to run consistent.

 

I'm very glad to hear from many different sources that the Qjet can run well. I have a Qjet and I'd rather run that than swap something else in.

I have another question, however. I noticed the other day that the secondary throttle blades don't open all the way. Is this normal? Should I attempt to correct this?

https://www.thirdgen.org/techbb2/sho...hreadid=132375