Jet Performance Q-jet tuning

[Mertz]

I have a 72 Chevy 350 that I put on an alum EGR intake and a Jet Stage 2 Q-jet. When I first installed it it idled fine but fell flat on its face and died when I floored it taking off so I made few changes. I changed the secondary rods to DAs and the hanger to a G. I read a G and DRs are a very good combination but didn't have the DR but DA is leaner than the ones that were in it (AU). It still fell flat on its face but I don't think the choke was all the way open locking out the secondaries. After driving a bit it was performing better but not great so I changed the vacuum can to one that is a lot quicker. That didn't work and created a bog at the transition. I also tried disconnecting the squirt pump and there was not really a lot of difference. Could have a bad pump but it seems to pump fine and I can see a mist of gas coming out of the horn. I also set the AV spring to 7/8 turn. I don't know what the primary rod and jets are. Somebody from Jet could probably tell me. I no longer have the little afterfire when I let off the gas I had with the other Q-jet I had on it. I don't know what to do next. I suppose a new squirter. I have a old stye performer coming that I will install incase there is a problem with the stock manifold. I didn't find any oil in the intake so I might not have a problem there. I am getting 19 inches of vacuum at idle but it drops to 0 when I get on it. At crusie it will drop below 10. The Edlebrock cruised at 16. Coasting I get 21 to 22.

[Jim_PA]

Forget about working on the secondaries for now. In fact, I'd recommend either locking them out, or just don't push the pedal to the floor (just 1/2 - 3/4 throttle so you stay out of the secondaries...)

When you say it died, did it completely cut out, or just feel sluggish?

Which hole do you have the accelerator pump lever in? Outer hole or inner hole?

Did you set the idle mixture screws with a vacuum gauge?

It's either the accelerator pump, or the primary circuit is too lean.

Try turning out the APT screw one turn and see if it helps.

[Wrenchbender Ret]

Try raising the float level a little.

[Mertz]

It completely died and took a few seconds to get going again. I left the pedal to the floor to see how long it would take to come back. It seem like a long time.

Accelerator rod is on the inside hole.

Idle mixture screw set for maximum vacuum. They are 3 turns out.

I pulled the pump and it seemed fine so I put it back in. I will try the APT out one turn.

I was playing with the secondaries because I thought it was opening to early and causing the bog. I now think it is in the primary system. The stage 2 has modified idle circuits so I thinnk that is ok. No restrictions.

[Mertz]

I talked to the Jet tech guy who was very helpful. He told me the jets in a stage 2 are 74 and the rods are 41. He also said I could turn out the APT up to 3 turns. So now I believe I have been on the rich side. I did several tests this evening. I turned out the APT a total of 7 turns (The tech guy said base was 5 so I turned it all the way in then out 7) I also adjusted the idle mixture screws down to where it would just start to stumble and then opened them up a bit. It rev'd great in the driveway and idled fine at 900. I was off the idle speed screw before making the adjustments and now I had to turn it in to stay at 900. Vacuum was a great 20. I took it for a test drive and the off the line is much better. It no longer dies when I floor it. It takes off fine and accelerates fine until I get up to 45 which is when the secondaries start to kick in. There is a bog at that point but when I get above 50 to 55 it takes off again. I can hear the sound of the secondaries working. I also tried it with the pcv valve disconnected the port plugged and it might have been a little better but not very noticeable.

I have the secondary air valve spring at 7/8 turn with G hanger and DA rod. I might try putting the AU rod back in it or go to a even leaner rod. I also turned out the APT one more turn but didn't have time to test tonight.

I am making progress. I think I need to get a better transition into the secondaries and I might have it. A little better of the line wouldn't hurt either.

[Jim_PA]

7/8 of a turn... are you counting from "zero tension" on the spring, OR 7/8 of a turn after you've wound the spring just enough to close the flaps by itself???


Take off all spring tension, slowly tighten the spring until it just closes the air valve, then go another 1/2 turn.

What I'm saying is... 7/8 of a turn from the point that the spring tension is just enough to close the flaps is a lot of tension, and is probably not allowing the air flap to open enough.

7/8 of a turn from zero spring tension might be OK.

I would go back to the rods and hanger they gave you as a baseline if your air flap spring wind-up is set properly.

[Jim_PA]

Oh, and for the primary side....

A good test is to disconnect the accelerator pump completely... cruise around at low speed, and just slightly blip the throttle. There should be only a slight stumble. If there's a HUGE stumble, you are way lean... if there's no stumble at all, it's too rich.

After that test, if all succeeds, you can try the outer hole on the ACC pump lever just to see how it feels.

[Mertz]

I set the spring with the carb off the truck. I got it to where it was just touching and then turned it in 7/8 of a turn. There is a good shot from the pump and I have it on the inside hole. I will try going back to the originial secondary hanger and rods. There was no letter on the hanger but the rods are AU.

I tried disconnecting the pump but not with this set up. I know I was running rich and now I shuld be much leaner and will try it again.

[Mertz]

I have made a few corrections since the last post. I changed the jets back to the 73s and put in 43 rods. I put a long spring in the APT and set the APT by marking the depth of the jet shoulder with a black marker on the rod just above the first step. I can see it in the jet and can adjust it until the black mark is equal with the top of the jet. I have the secondary tension spring at 1/2 turn. I runs great off the line and above 50 mph but a little weak at 40 to 50. I am going to lighten up the spring and see if that works. I have the AU secondary rods in it but have a DL slotted rod I might try. I think I am really close except for the slight bog and it heats up after I have gotten on it.