Cascade Crew

blue89 · Posted: Wed Apr 08, 2009 8:01 am Post subject: Q-Jet secondary

What is the mechanism that opens the secondaries on a Q-jet? I don't think mine are opening at all. I've pushed up the bi-metallic spring and undone the vacuum solenoid to allow them to open, but still nothing when I go WOT. Could explain why it falls on it's face after 3k RPM. Any help would be great as I have no idea and I can't find a diagram online.

Primaries seem fine, idle is okay but it's backfiring a little. Fuel pressure is at 4psi and I have not verified the timing yet.

Twilightoptics · Posted: Wed Apr 08, 2009 9:02 am Post subject:

Mechanical butterflies on the bottom, suction based butterflies ontop.

blue89 · Posted: Wed Apr 08, 2009 9:15 am Post subject:

The "air valves" on the top open with airflow but the secondaries on the bottom never open with the throttle. I've held the upper butterflies open with my finger (car running) and then went WOT but they never open then either. I guess I can go for a drive with the filter off and see if it opens with a long WOT. They definetly are not mechanically operated.

blue89 · Posted: Wed Apr 08, 2009 11:34 am Post subject:

I think I'm understanding this carb a little better. The secondaries have a spring that hold it against the linkage with the primaries. However, there are multiple things that will keep the secondaries shut. The external choke and the pull off will prevent it from opening.

Sorry Twighlight, it IS a mechanical secondary but it has other means of keeping it shut. I'm going to eliminate these other methods of keeping it shut and see if I can get them to open.

Has anyone had a problem with a sticky throttle plate shaft not allowing secondaries to open?

chevymad · Posted: Wed Apr 08, 2009 5:13 pm Post subject:

Make sure the secondary lockout on the passenger side is working. Theres a small lever that falls down and blocks the secondary butterflies from opening any time the choke is open at all. You want to put the vacuum solenoid back on the upper flap, and set the valve spring too. If it just falls open, all you'll get is a huge bog. Thats where these carbs get that name quadrabog. People misadjusting the spring and vacuum pulloff.

chevymad · Posted: Wed Apr 08, 2009 5:14 pm Post subject:

BTW the upper flap is the air valve, the lower ones on the secondary throttle shaft are the butterflies.

blue89 · Posted: Thu Apr 09, 2009 6:56 am Post subject:

Thanks a lot for you help guys! This really isn't that bad of a carburetor. Seems a bit better than a Holley as far as part throttle metering is concerned. However, I've discovered my problem! The secondaries were sticking to the throut. It wasn't a terrible stick but just enough that the springs couldn't overcome it. So I took the air valves off and cleaned everything up. Works way better now! I'll drive it tonight and see how it improves things.

I saw the secondary lockout. You sure that needs to be there? I think I'm having problems with the external choke not opening up fast enough right now. After about 10 minutes of monkeying with the throttle and letting it idle it's still not warm. Could be because it's 650lbs of iron, dry? So I'm thinking of zip tie'ing the choke in the warm position and letting the pull back valve do it's thing. Think that will cause a bog?

Alphius · Posted: Thu Apr 09, 2009 7:13 am Post subject:

chevymad · Posted: Thu Apr 09, 2009 6:02 pm Post subject:

You can remove the little swinging thing that locks the secondarys if you want. But leave the vacuum pull-off. The pull-off not only pulls the choke open, but its also controls the vacuum secondaries. Keeps it from bogging. Theres alot of tricks you can do to these carbs. Make sure your secondary throttle plates open a full 90*. Gm changed cfm ratings on these simply by changing the secondary stops so that they limited the opening.

Also there are some tweeks you can do to the air valve. Pontiac versions came from teh factory tweeked. Theres a couple windows you can cut into the valve, allowing vacuum to reach the boosters before the valve opens. Helps the transition a bunch. Some people rough up part of the air valve too. .I forget what that does. You also want to adjust the air valve spring so that its just tight enough to prevent bog. That will allow the air valve to open more, and sooner.

One neat thing you can do with one of these. Because the secondaries are so much larger then the primaries. 2.25" vs 1.5" You can tune the primaries to the lean side for mileage. Then simply over come the leanness by setting the secondaries rich. Secondary metering rods are very easy to change too. One little screw removes the hanger and both rods, without ever getting into the carb. I think you can still get new rods from edelbrock. But its also easy to take some out of junk carbs.

blue89 · Posted: Fri Apr 10, 2009 6:59 pm Post subject:

Thanks for the advice. I got the o2 sensor in today and discovered that it's running at .122-.240 volts at idle and WOT, respectively. Pretty sure that means I need more fuel Very Happy

So this would be handled by those long metering rods?

chevymad · Posted: Fri Apr 10, 2009 7:55 pm Post subject:

Get the idle mixture set first. Then see how your cruise is. You can make that easier by locking out the 4barrels til you have that set. Theres a set of primary metering rods & jets inside the carb to set that. There's also a spring that operates those rods which you can change to set how fast the fuel curve comes in. If the primaries are anywhere close.. I'd probably leave them be.

Now after you have that done, move on to WOT afr. Check it, then change those rods as needed to get it where you want.