Q-jet question

[Justin Fleming]

I have a 86 k5 with a q-jet that has vaccum secondary. I have been wondering for a long time if the secondary is really opening at wide-open throtle. From what I see the blades open for sure due to the mechanics and linkages, whats in question is the top butter flys that allow it to breath. From what I can see they are not opening. I put a long piece of vaccum line to the back of the canister that that operates the linkages. This cannister holds vaccum and also pulls the mechinism towards the fire wall. There are two linkages attached to this mechinism, however when I have the vaccum canister at full stroke none of the linkages are moved and if they did it would seem that they would be moving them the wrong way. The mechinism that is attached to the canister is sloted, hence thats why the linkages never moved. If the linkages did move it appears that it would hold the top butterflys closed. So I wonder the following:

1. am I missing something
2. was this put together wrong by someone during a rebuild
3. is there a different way to test what I am trying to prove

It has always been my experience that when a four bar opens you can really hear the transition in noise even if the truck does not have major horse power I would assume the same transition in niose would occur. When I mat it there is zero difference which is why I think the secondary is not breathing and the metering rods are not being lifted to allow for more fuel.

thanks

 

[Justin Fleming]

I have a 86 k5 with a q-jet that has vaccum secondary. I have been wondering for a long time if the secondary is really opening at wide-open throtle. From what I see the blades open for sure due to the mechanics and linkages, whats in question is the top butter flys that allow it to breath. From what I can see they are not opening. I put a long piece of vaccum line to the back of the canister that that operates the linkages. This cannister holds vaccum and also pulls the mechinism towards the fire wall. There are two linkages attached to this mechinism, however when I have the vaccum canister at full stroke none of the linkages are moved and if they did it would seem that they would be moving them the wrong way. The mechinism that is attached to the canister is sloted, hence thats why the linkages never moved. If the linkages did move it appears that it would hold the top butterflys closed. So I wonder the following:

1. am I missing something
2. was this put together wrong by someone during a rebuild
3. is there a different way to test what I am trying to prove

It has always been my experience that when a four bar opens you can really hear the transition in noise even if the truck does not have major horse power I would assume the same transition in niose would occur. When I mat it there is zero difference which is why I think the secondary is not breathing and the metering rods are not being lifted to allow for more fuel.

thanks

There has to be someone who understands what I am explaining........

 

[Hogback Fabrication]

We're gonna need pictures to see what you mean.

 

[skelly1]

You should be able to see them open with a quick WOT at idle (with hood open, hand on throttle mechanism). Watch your face if this thing backfires. I do recall the difference in engine noise when they do open. I also remember that if they don't open you will get some pinging going on because it is going to be running lean under load. Also, if you post some pics that would help too.

 

[dyeager535]

I think I got it. You are describing the choke pulloff and/or vacuum break, which also holds the secondaries shut.

I'm fairly certain yours is operating correctly, here's what happens.

At idle or steady cruise, you've got fairly high vacuum. Obviously, with high vacuum barely open throttle) you don't need more power. So the vacuum applied to that diaphragm keeps the secondaries from opening even if the lower plates (mechanical portion) are. If you mash the pedal, obviously vacuum drops to nothing, which means the diaphragm releases, letting the secondary air valves open. Popular misconception with Q-jets, high vacuum keeps the secondaries shut, *airflow* opens them and keeps them open.

Very unreliable to test this when not moving, since with no load, the engine doesn't need the additional air, and typically you won't see anything but a fraction of movement.

 

[Justin Fleming]

I think I got it. You are describing the choke pulloff and/or vacuum break, which also holds the secondaries shut.

I'm fairly certain yours is operating correctly, here's what happens.

At idle or steady cruise, you've got fairly high vacuum. Obviously, with high vacuum barely open throttle) you don't need more power. So the vacuum applied to that diaphragm keeps the secondaries from opening even if the lower plates (mechanical portion) are. If you mash the pedal, obviously vacuum drops to nothing, which means the diaphragm releases, letting the secondary air valves open. Popular misconception with Q-jets, high vacuum keeps the secondaries shut, *airflow* opens them and keeps them open.

Very unreliable to test this when not moving, since with no load, the engine doesn't need the additional air, and typically you won't see anything but a fraction of movement.

I think you are on to what I was talking about......I this is the choke pull off and not what I thought it did.....

So what allows the secondary top butter flys to open? These are directly connected to the metering rods...which I beleive allows more fuel into the system when called for........Do these butter flys open based on pure suction when your at wot?

I guess I am a littel confused and will have to take the air cleaner off again and study it some more....

 

[dyeager535]

You've got it I believe.

GM had a whole bunch of vacuum setups on Q-jets, but typically you'll see one or two vacuum cannisters, sometimes they combined the operation of one for choke and secondary.

In any case, yes, it's purely engine demand that opens the secondaries. With the truck running, try pushing the air valve open. (not too hard, just to see what I'm talking about) Then, turn the truck off, and try again. No vacuum, they open easy. Notice that the air valve is spring loaded, that's all the engine acts against when under "heavy" throttle.

It doesn't take anywhere near WOT to drop engine vacuum to almost nothing, put a vacuum gauge on a manifold vacuum port, and go drive around. You'll see why driving with a steady foot saves a ton of gas.