I have about 300 miles on my rebuilt 427 and sometimes the seconaries dont kick in. My mechanic says the medium cam is holding the valves open longer and not creating as much vacum as the stock setup.
So now were talking new carb & intake.
does this sound right?

 

Find a new mechanic.

A vacuum secondary carb opens the secondaries when the vacuum drops to zero (which it does at WOT). Honestly, cruise vacuum has very little to do with WOT operation. In fact, if you have less vacuum, the secondaries may kick in quicker.

 

When secondaries don't kick in. It has been my experience with older carbs it is related to problems with warn out, bent, or binding shafts and linkages. Look up your carb on the web. Lots of articles on how to adjust the VS.

Your other thing to look at is hotter cams have less vacuum so you have to change the metering rod springs to compensate. With less vacuum the rods would be lifted more running the A/F ratio richer

 

Do you have problems with your headlight vacuum at idle? If you believe what your mechanic says you can always get a mechanical secondary carb.

 

He is right by going to a higher performance cam the valves stay open longer, duration is longer. This has nothing to do with your secondaries not opening up. There sounds to be a carb problem as Gkull says. Fix the problem just don't go bolting on new parts.

 

Is the carb the stock Quadrajet, a Holley, or something else?
I can pretty much guarantee that you dont have to change carbs unless yours is worn out.

 

Consider buying the new carb and intake - then throw it at your mechanic. Seriously, bigger bumpstick often = less vacuum but a new carb and intake?

If you have the correct cfm flowing carb for your set up (tell us what it is) you can just tweak it a little. Hardly ever should you have to buy a new one. However, I do prefer mechanical secondaries - but that's a whole other thread....

There is a lot of reference material available for working on carbs and many people more knowledgeable than me on the subject.

 

That's absolute, complete nonsense.

A true "vacuum secondary carb" (only Holley and BG-based carbs offer vacuum secondary carbs) does not sense manifold vacuum - it is not manifold vacuum that opens the vacuum secondaries. The vacuum is created by venturi vacuum, which is proportional to mass airflow through the primary side of the carb - the vacuum signal is created in the venturi of the carb - not in the manifold. This is completely independent of manifold vacuum, and is completely irrelevant to your cam. (For a complete paper on the subject of vacuum, manifold vacuum, ported vacuum, and venturi vacuum, drop me an e-mail request for my "Vacuum Explained" tech paper - you'll be much smarter than your mechanic after you read it...)

If you have a Q-Jet, you do not have a vacuum secondary carb - the Q-Jet is mechanical. If you're getting full throttle travel (which most C3 Vettes seldom do) and the secondary lockout lever is not engaged due to choke malfunction, the secondaries are working. If the secondaries on a Q-Jet are not working, it's due to one of the following:

1. Inadequate throttle cable travel. Have someone push the pedal to the floor and observe on the driver's side of the carb if the secondary throttle lever and shaft is fully opening. With the pedal on the floor, grab the throttle lever and see if it will move further. If this is the case, either remove your floor mat (typically worth 15 hp on a C3...), or bend your gas pedal linkage arm up to create more throttle travel.

2. Defective choke operation. If the choke is not fully opening, the secondary lockout lever on the passenger side of the carb will not disengage. Verify that the lever is disengaged when the engine is hot.

3. Warped airhorn jamming the secondary airvalve. The 2 rear screws on the top of the Q-Jet will distort the casting when they are tightened too tight. This can cause the secondary airvalve to jam and lock. With your finger and the engine "off," just push the secondary airvalve open and verify smooth operation with no sticking. If it is sticking and jamming against the rear of the casting, you can loosen the 4 airvalve attach screws about 1/4 turn, "tap" the blades forward just a fraction, and tighten them back up for smooth operation.

If these issues are in order, the secondaries on the Q-Jet are operational, and they have nothing at all to do with the level of engine vacuum being produced.
Lars
V8FastCars@msn.com

 

You need to make the drive from Lakewood to Lafayette

 

That sounds like great advice.

 

its a rochester quadra jet that I belive was stock on my 427-390 the intake manifold # ends in 801 so I am pretty sure this is the stock set up.



I checked the throttle cable and rear airvalves for function and they seem to work propery.
My mechanic removed the linkage on the passemger side of the carb and now it it hits every time.
Is this gonna cause a problem?

 

What, exactly, "hits every time".

BigBlockk

Later.....

 

I'm not an expert by an stretch but, of course it going to hit the secondaries every time. The linkage is disconnected!!!

2 things....

1. Get your throttle cable situation fixed. It appears the bracket is out of wack.

2. Get the secondaries hooked back up properly

3. Get your choke working

4. Get it over to Lars in Laffayette....

 

Another question. Where is the secondary air valve rod?

BigBlockk

Later.....

 

 

Vacuum secondaries do not kick in when manifold vacuum drops to zero - manifold vacuum has no affect whatsoever on the operation of a vacuum secondary carb, and lower manifold vacuum will not make them "kick in" quicker. Vacuum secondaries operate off of venturi vacuum, which is proportional to mass airflow through the primary venturi and completely independent of manifold vacuum.

But this is completely irrelevant, because he has a Q-Jet carb and not a vacuum secondary carb - the Q-Jet is mechanical with a mass-flow controlled secondary airvalve.

Your "mechanic" just removed your choke pulloff rod and destroyed the controlled operation of your secondary airvalve is addition to destroying the proper operation of your choke.

 

Lars is absolutely correct and I very poorly stated the operation of a vacuum secondary carb.

The real issue is that:
(1) you have a Q-Jet which is a mechanical secondary carb with a air valve door.
(2) your mechanic removed parts that GM designed for proper carb operation,
(3) you mechanic is an idiot who knows nothing about Q-Jets.

Do NOT go back to this mechanic!! Make the drive from Lakewood to Lafyette and have Lars fix this right. Sorry, if you keep going back to this "so-called" mechanic, you are responsible for your car being screwed up.

 

I have been in touch with Lars and will try to absorb as much knowledge
as I can from him.
I would like to thank everyone who posted for helping me out.
Jeff

 

I took my car to Lars today and now it runs great. Lars is a down to earth good guy and an incredible mechanic. He diagnosed and fixed a few problems with carb & timing in no time. He explained how it all worked in a way that was easy to understand and his bill was more than fair.
Thanks Lars. I had a great time.
Jeff

 

Jeff -
Thanks for stopping by with the car - you have a nice '69 that's fun to drive and work on!

As you noticed during our work, it's important to take a "systems level approach" to the tuning process. By doing this, we were able to identify other contributing factors rather than just trying to solve the apparant immediate symptoms, resulting in some smooth, powerfrul performance and a car that's a ton of fun to drive.

Thanks again for letting me bang a few gears and to remove some of the bad rubber off your rear tires - it was fun meeting you, and I hope you have some great fun with the car this spring and summer. Take care!
Lars