View Full Version : Secondaries not opening, '79 CDV w/425.

05-03-10, 03:28 AM
Been noticing the last couple time I've been on the freeway that my secondaries haven't been opening when I go WOT. I had the same problem a couple years back, & it turned out to be a vacuum leak & that was that. Now, my old mechanic did that, & I don't have a vacuum diagram to reference... so, could anybody tip me off on what hose it might be? Or what else could be causing the problem?

Thanks, let me know if there's any more info I can give.


05-03-10, 05:18 PM
Likely a stickey or mis adjusted carb. Bruce Roe

05-03-10, 08:39 PM
i got them off ebay, as soon as i receive them i will post them.

05-03-10, 09:03 PM
Likely a stickey or mis adjusted carb. Bruce Roe
That's what I would think. but I had it rebuilt less than four thousand miles ago. It used to work great until recently, that's why I was wondering if it might be a vacuum issue again.

05-03-10, 11:17 PM
If the choke isn't set right, secondaries can't open. Bruce

05-04-10, 12:59 AM
But... I haven't adjusted the choke since they were working?

05-04-10, 09:09 AM
If you have a MightyVac or similar vacuum pump, check the vacuum motors on the carb. I think the motor on the back of the carb controls the secondaries to keep them from opening at lower engine temperatures. The front one controls your choke kick down. The vacuum motors have a set leak down rate and are supposed to hold vacuum for 5 seconds. Provided my memory serves me correctly. It's covered in the service manual.

05-04-10, 05:11 PM
Holley 4bbl vacuum secondaries or Rochester QJ ?

05-04-10, 05:18 PM
Rochester Q-Jet. Sorry, thought it was implied. :)

05-04-10, 05:28 PM
Is the choke opening all the way when it's warmed up? If the choke isn't absolutely all the way open the secondaries won't open.

05-04-10, 06:07 PM
I'm an Olds 455 builder, not Cadillac 425................

Yeah, that: the choke exercises a mechanical lock on the secondary air valve. The secondaries (butterflies) are mechanical and open with gas pedal depression; the air valve - on top of the secondary bores - opens as airflow through the carb increases. It's an automatic carb-sizing system: the carb will NEVER flow more air than the engine needs at any given rpm/load. DO NOT attempt to mess with the secondary air valve opening tension. Screw that up and you'll have an incredible off-idle and midrange WOT bog.

With the engine OFF, block the throttle wide open and manually open the choke. Observe the choke mechanical lockout operation. With a finger, open the secondary air valve - you'll see the secondary butterflies down in there, already wide open - and as you open the air valve feel for binding or hard spots. It should open and shut easily with no hangups. Also, note the air valve cam and hanger which lifts the secondary metering rods out of the jets as airflow into the engine increases. Elegant mixture control. (If there's any binding, clean out the axles and linkages with choke cleaner spray.)

The vacuum cans at the right and rear (?) of the carb linkage are choke pulloff and air valve damping. Check the small connectors and maybe a Y connector in the vacuum lines.

If your choke is hot air - manifold heated - then either the hot air stove and pipes or the choke cap bimetallic spring may be faulty and not heating up enough to fully open the choke blade.

Find a HP Publications 8x11 paperback by Doug Roe - Rochester carburetors. More than you can ever memorize concerning every model of QJ.