Secondaries no help

[rich weyand]

The same engine with the same induction and exhaust systems should use the same mixture. This is how the factory can set up carbs in production for a certain engine and then just bolt them on with no testing.

Your setup is similar enough to mine the carb tune should be about the same, but it was way different. Too different.

I am off the Edelbrock chart with my tune (to the left and down, I think), but it gives me the correct numbers on the AF/R meter, so whether it's off the chart or not, it's correct.

 

[fallguy]

Metering rods swapped. I notice more low end off the line power and more aggressive low end acceleration....ie spins those 35s off the line. However, when the secondaries open it's still too light I think...there should be more...it's better, but it feel that there should be more power with the secondary help???

 

[rich weyand]

Maybe not. Note that the Edelbrock 1405 is a 600 cfm square bore carburetor. That means each barrel is 150 cfm, 300 on the primaries and 300 on the secondaries.

At a volumetric efficiency of 1.0, a 350 at 5000 rpm needs 500 cfm. A normally aspirated engine does not (absent extreme race-tuning measures) achieve a volumetric efficiency of 1.0. For your setup and mine it's probably closer to 85-90%.

Let's figure 90%. That's 450 cfm at 5000 rpm, and 300 cfm at 3500 rpm. In other words, your engine is only drawing enough air through it to need the primaries until 3500 rpm. The vacuum flap on the top of the secondaries won't even open.

Above 3500 rpm it will start to draw on the secondaries, but won't use more than 50% of their capacity even at redline. The secondaries are keeping the engine from flattening out over 3500 rpm, though, so that's good.

The Qjet is a spread bore carb, so the primaries are smaller. The primaries run out sooner, and the secondaries come in at lower rpm. So when you kick a Qjet and the secondaries come in, you notice more of a step in the power delivery.

Glad you are getting what you should get in the bottom register, though!

 

[fallguy]

I have a 1406 but I'm betting not allot of difference...would .098 as secondary jets be more helpful or will I be too rich then?

 

[rich weyand]

1405/1406, same difference. Different stock tune is all. You are actually running something closer to the 1405 tune (performance) now than the 1406 tune (economy).

.098 secondaries will be too rich, and power from the secondaries will drop.

If your engine doesn't flatten out at 3500 rpm, but keeps pulling to the redline, then the secondaries are working fine. Should have thought of this earlier. Sorry.

But at least you got the godawful lean mix out of the primaries!

 

[fallguy]

Well thank you very much. I do appreciate all of the help!

 

[rich weyand]

De nada. I have gotten so much help from various forums, it's nice to help out when I can.

 

[MadOgre]

Maybe not. Note that the Edelbrock 1405 is a 600 cfm square bore carburetor. That means each barrel is 150 cfm, 300 on the primaries and 300 on the secondaries.

At a volumetric efficiency of 1.0, a 350 at 5000 rpm needs 500 cfm. A normally aspirated engine does not (absent extreme race-tuning measures) achieve a volumetric efficiency of 1.0. For your setup and mine it's probably closer to 85-90%.

Let's figure 90%. That's 450 cfm at 5000 rpm, and 300 cfm at 3500 rpm. In other words, your engine is only drawing enough air through it to need the primaries until 3500 rpm. The vacuum flap on the top of the secondaries won't even open.

Above 3500 rpm it will start to draw on the secondaries, but won't use more than 50% of their capacity even at redline. The secondaries are keeping the engine from flattening out over 3500 rpm, though, so that's good.

The Qjet is a spread bore carb, so the primaries are smaller. The primaries run out sooner, and the secondaries come in at lower rpm. So when you kick a Qjet and the secondaries come in, you notice more of a step in the power delivery.

Glad you are getting what you should get in the bottom register, though!

I believe its only the 750cfm Edelbrock carbs that have equal size bores. My 1806 has smaller primary bores and it is 650 cfm just not to the same degree as the quadjet.

 

[rich weyand]

I believe its only the 750cfm Edelbrock carbs that have equal size bores. My 1806 has smaller primary bores and it is 650 cfm just not to the same degree as the quadjet.

Yeah, the 1406 is slightly off square. So maybe he feels the difference above 3000 rpm. Point stands: until the engine needs more than the primaries can supply, it won't use the secondaries, and the "square-bore" nature of the carb and the fact that it is 600 cfm on an engine that only needs at most 450, will mean that the secondaries won't make any difference in the bottom half of the rpm range.

 

[fallguy]

Any reason the suggested metering today switch causes my truck to vapor lock or something?

Since the change, when at operating temp, it dies when I stop or slow way down for a corner...and I wait 30-60sec then it fires right back up and away we go.

I tried a new fuel filter, but no change....

The new suggested rods are all I've changed recently.

 

[350runner]

When this happens Check a plug or two to see if they smell like gas or look fouled.

Sent from the dust in front of you!

 

[rich weyand]

Did you readjust the idle mixture when the metering rods were changed?

What is base timing?

Where is vacuum advance connected, ported or manifold vacuum?

What is idle rpm in Park? In Drive?

 

[fallguy]

Plugs look good.

Manifold vacuum port.

16 timing, 12 vacuum advance.

600-650 rpm idle in gear....800-900 idle

No adjustment of idle mixture screws as it ran just fine and smooth...

Bad distributor control module???

Bad fuel pump???

 

[rich weyand]

Is that vacuum advance 12 camshaft degrees or 12 crankshaft degrees? You can tell by putting the timing light on and watching how far the timing moves when you hook up the vacuum advance.

Adjust the idle mixture screws.

The dying when stopped or decelerating to go around a corner is at idle, with the throttle plate shut and the vacuum at a maximum. That can't be fuel starvation and probably isn't a control module. It's timing and mixture related, because those things change at high vacuum/idle throttle, or it's a transition issue.

But I think it's timing and mixture. Idle is it's own tune. You could be going lean enough for the fire to go out, or you could have too much vacuum advance. Or the vacuum advance could be intermittently sticking retarded and not pulling in.

 

[fallguy]

I think I've solved it. I replaced the distributor control module and test drove for an hour with no issues.

I will report back if that changes.