I have a 71 Centurion Convert 455 and was told that my secondaries are not working(opening) on my Rochester Q-Jet. I am not sure what this means. The car run and drives but I have a feeling that I'm missing a lot of power when I "floor it". Should I get it rebuilt? Does anyone know the number so I can replace it with a rebuilt one? I think when the carb is working nice that it will make a big difference. Any input would be appreciated. Thanks Todd
7041540 is the model number on it. A carb rebuilt might be a good idea. They're probably just stuck shut from corrosion or possibly some grime that has sealed them shut. This is a good novice project you can take on yourself if you are careful and take notes on how to reassemble it.:TU:
Make sure the choke is opening completely and all of the linkages on the passenger side of the carb are free. The secondaries won't open unless the choke is open all the way.
I agree with Dave. As I previously mentioned, rebuilding a Qjet, though fairly simple, is not frequently necessary. As Dave described, the choke must be FULLY open. There are linkages on the pass side which will prevent their opening if not. I hjave seen other problems cause this as well. You might try opening the air valves by hand, with the engine off. Also note that the metering rods move with the air valve. They are in the center, between the air valves. When released, the air valves should close completely,following your finger. If this happens, you might also try opening the secondary throttles by hand, again, with the engine off. Operating the throttle by hand is the way to go. When the primaries are fully open, so should the secondary throttle blades. Note: the air valves and the throttles are independent. The throttles are in the throttle body, at the bottom of the carb. The air valves, in the air horn, are operated by air movement. They are separately operating systems. You could easily fix the problem by spraying the external linkages with carb spray. I have frequently seen choke problems be the cause of secondary problems. Give this system the hairy eyeball before messing with other stuff, especially if the idle is finicky, or slow to drop to hot idle. Ray
You'll also want to check that the secondary vaccum pulloff is working. There will be a small rod connected to the secondaries that terminates at a small vaccum actuated plunger. Make sure it had a vaccum source and is working correctly.
Q-jet Secondarys One way to tell if the secondaries are opening is by the SOUND. Flip the air cleaner top (dish) upside down or even leave off the air cleaner (temporary measure only) for a test drive. When the Engine is fully warmed up---- Floor It! you should here a distinctive Moan or Wail. CAUTION, This Sound is Addictive! Especially in combination with G-force acceleration and the aroma of Unburned HydroCarbons!!! By the way the "choke pull-off is a device that restricts the opening of the Secondaries, a pollution Control Device, I unhook or REMOVE mine. AND that pesky linkage on the passenger side, between the choke & the secondaries,---A pair of pliers will bend or break that Sucker right off. Simplicate and Add Lightness! I live in the South, When the choke doesn't work correctly, I disconnect IT too. 2 or 3 accelerator pump strokes to the floor will richen the mixture enough to fire off. Then a little footwork with the throtte for 10 or 20 seconds and you are ready to Blast-Off and find some Ricer Kids to Humiliate, hey WOW, I GOTTA GO drive a BIG BLOCK with a Q-jet!!!!!!!!!!!
Re: Q-jet Secondarys Roger, You may be able to get away with that if you have a numerically high rear gear, and loose converter, but that choke pull off also slows down the air valves, so they don't flop in too quickly. For most street cars, eliminating that pull-off and linkage is the way to Bog city:laugh: It's like using a Holley double pumper on a stock motor with tight converter, and highway gears. The motor can't use the additional airflow and fuel, and low speed performance suffers. Greg Gessler has several different pull-offs he uses to tune the opening rate of the air valves on his F.A.S.T. 72 GS Stage1. He uses different ones according to track conditions, and available traction. My point is in most cases, eliminating that stuff is not a good idea. Just my .02
secondairies True, True, but the air valve opening is also controlled by the spring on the air valve. The spring can be tightened to eliminate excessive Bog. Also I'm not drag racing my car and am not trying to get every last 1/100th out of it. Just try it, it is free, & easily reversible. The main thing is to drive & enjoy these cars!
Something to consider here. The choke pull off is used to slow the initial opening. If eliminated, the engine will rev, then choke on the additional fuel and bog. Tightening the spring will only cause lost performance. Believe me, if GM could have saved the money for the pull off, it would have. The two work together; eliminating one or the other WILL adversely affect performance. If the secondary throttles respond to WOT, the choke is operating correctly. If you can push open the secondary air valves with the engine off, and they return when finger pressure is released, they are working. They should not be difficult to open; light finger pressure should be sufficient. Be sure that when you open the secondary air valves that the metering rods also respond. This is a simple mechanical linkage not requiring a degree in rocket science to check out or fix. Do it right or expect other problems to ensue. Ray
