How much does 1 jet size affect A/F?

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For better tuning you would need to install a 2nd AFR gauge in the other exhaust.
I've tuned (tried to) a dual carbed 500" stroker motor like this and the difference in mixture from both carb was severe.

good Dutch by the way :)
 
67cuda360 said:
No I'm not really involved at all except for seeing them drive around, or run into road blocks for the stages the've set up :) I think it runs from Sept. 14th - 19th this year.
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You do know there was a new model Challenger in a couple of the recent runnings? And a class that you car would fit into? I wanna be your groupie when you decide to run! There are a couple Chevvies that need to be trounced....
 
nm9stheham said:
You do know there was a new model Challenger in a couple of the recent runnings? And a class that you car would fit into? I wanna be your groupie when you decide to run! There are a couple Chevvies that need to be trounced....
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Ha ha, I always wanted a groupie LOL, thing is, If I'd run, I'd end up getting more and more competitive, and that just gets me in trouble LOL
 
I've just finished making the jet and rod changes but it's dark out now, so I'll have to wait till tomorrow (if weather permits) to take it for a spin. I seem to have overtightened the banjo fuel fitting again (yep happened before) and the plastic gasket seems to have been compressed too much, a little bit of a fuel drip while running.

Does anyone know if I can just get a plastic gasket from a hardware store or are those not resistant to fuel? I'll call a performance place around here in the morning to see if they have one but in case I'm SOL it would be nice to have an alternative.
 
You might find a phenolic ring from the store in a gasket assortment; probably better than plastic. And no kidding, look at parts for hose and dishwasher inlet seals, and oil ring assortments are common too in a really good hardware store; might be something close enough there for temp use. 4-5 psi is not much.
 
Okay, I was able to sort out the small fuel leak. Took the car out for another run with the following settings:

Primaries:
rod 6542, which is 1 step lean on cruise and only 0.5 step lean on power section of the rod, with jet .092 (this combo is #16 on the carb tuning guide reference chart)

Secondaries:
.089 jet (which is 3 steps lean)

Here are the results (based on 5 runs)
cruise A/F 14.5
Part throttle A/F 15.5 - 16.5 (16.5 if going uphill at part-throttle)
Full throttle A/F 12.5

So my Part throttle is way to lean, and I think it is because my engine has such strong vacuum at cruise (20" at cruise) that on part throttle (11"-13" of vacuum) the step-up spring is still way to weak to move the primary rod into the power section of the rod, what do you guys think?

I'm currently running the purple step-up spring (rated at 7"), so I'll switch to the strongest one i have but that's still only rated at 8".
 
Is it correct to think that the uphill part throttle is more throttle than at cruise? OK, I just looed at the vacuum numbers again and yes. That IS a lot of vaccum at cruise and part-throttle-uphill. (Curious to know why...??) Are both at speed so that wind resistance is significant? (> 45 mph).

If so, then, yeah, I would expect the AF to move toward WOT levels at part-throttle, uphill. But again, why such high vacuum numbers? Is this intake vacuum?

Are the power (step-up) spring levels rated to manifold or to throat vacuum (like ported)? (I have not found that info for sure yet....) That's important to know, so as to know in which direction to move the spring number....From looking at the catalog pix, the shorter springs (weaker?) look to be the lower number.
 
yes uphill throttle is more than throttle at cruise. It take quite a bit of pedal to get into the 8-9" vacuum range, more so than what I consider to be part-throttle :) yes these tests were at speed, going around 45 - 60mph. (i'm doing my run's on and off the highway section here, it's a nice stretch with some uphill sections)

The vacuum gauge (part of the instrument cluster and double checked against a handheld vacuum gauge) is on the manifold side of the carb, below the throttle blades, so this is full manifold vacuum.

When I look at edelbrocks tuning manual they state that if you are experiencing a lean bog while accelerating to go to a stiffer spring (rated higher) to have the rod start moving up faster (out of the lean section of the rod).

"Are the power (step-up) spring levels rated to manifold or to throat vacuum (like ported)?"
Interesting question, I wonder if someone can shine some light on that :)
 
Step-up springs like 50% of manifold vacuum in gear. Ball park, mine likes 2HG less than that. Read back through that Trudyduster's thread you responded in, all of these edelbrock/quadrajet tuning issues have been touched on. Read the quadrajet tuning 101 PDF By Lars, just because it says 'quadrajet', doesn't mean the tuning principles don't apply to your carb too.
 
I'm reading through the PDF you mentioned, interesting stuff.
I'll try the method you mentioned "disconnected plunger" tomorrow and see what it does.

Quick question, and I might be over-thinking this, but could it be possible that at cruise (20" vacuum) I'm actually running off the idle circuit of the carb (which at idle is set at around 13 A/F) and when I move into part-throttle (11"-15" of vacuum) it opens up the primary throttle blades and goes lean since I've leaned out the cruise step of the rod?

I'd set the idle mixture screws for best vacuum and best idle RPM when I sorted out the timing (last year). Is A/F 13 too rich for Idle or about right?
 
I don't think you are overthinking this; that's part of learning this stuff. (Took me a lot of that type of thing to get at all comfortable on Webers...) Gosh, I gotta wonder why the vacuum numbers..... is this a 273? Yes your theory makes sense...I just can't tell you.
 
No it's a 360 below are the engine specs.

I'll play around with it a bit more tomorrow and see what she does.
Tinker tinker tinker LOL.

engine specs:
360 LA engine, not sure what cam (told it was RV cam) with solid lifters, RPM airgap intake, stock manifolds with 2.25 inch dual exhaust with H pipe. Cleaned up J heads (slight port and polish), flat top pistons and a 650 AVS thunderseries carb backed by a 727 transmission with stage 2 transgo shift kit and 8.25 rear with 3.23 suregrip.
 
I've just tried the disconnect of the pumpshot method to see if the primary is rich or lean (as PoisonDart74 suggested).

Car at idle (around 650 - 700 rpm), engine warm, A/F reading of around 13, blipped the throttle with the pumpshot connected, revved nice and free, no bog or hesitation.

Disconnected the pumpshot, blipped the throttle, revved nice and free no bog or hesitation, was reaaally close to identical. I then placed my hand over the primaries to choke the carb. Engine wanted to die.

So this would lead me to believe the primaries are too rich (currently setup at one step lean). I also tried to see if the pumpshot was working by depressing it a bit (at idle no revving) and the engine wanted to choke so the pump is working.

My concern is, if I go further lean on the primaries, I'm going to have an even leaner part throttle than I already have right?
 
Your primaries are a jet/rod or two rich, remember you want 'a slight hesitation' when the accelerator pump is disconnected, then take it around the block with the accelerator pump disconnected to verify. And yeah, part throttle will be leaner, but it's a step in the right direction. I'd think that it's not going to be a direct ratio to your cruise, try a step leaner to get your off-idle right and go from there. One move at a time. It's a balancing act.

As far as what number your trying to shoot for on the gauge, and how the carb acts, I'd go with what makes the carb and engine the happiest every time.

I'd suggest going in the steps provided on page 9 'tuning for beer'. Start at the front, move to the back kinda thing..


After everything is set-up, and technically tight in the ballpark, THEN fine tune it with the gauge. If your still getting something as wide as 2-3 jet sizes, and deciding on power valves and having lean and right conditions to the point you can find them with low-tech means, you've got to get that ironed out first.

Glad ol' Lars could shine some light here, I love that ****in' paper... Got mine tuned like a damn drum.
 
You are sure this test works on a 650 AVS correct? I mean the Quadrajet has a idle circuit that works together with the primaries IIRC, but the 650 AVS has a totally separate system, keeping the idle circuit separate from the primaries. So could that change the way the test works...?

I'll give your suggestions a shot, but I can't understand how going leaner on the primaries will positively affect my already too lean (16 A/F) part-throttle situation...?

Ha ha, sometimes it feels like I'm getting deeper and deeeeper into the forest LOL
 
What feeds the idle circuit? Get that sorted so you can move from idle, off-idle, part throttle cruise, and WOT.

As far as the disconnect the accelerator pump shot, and test primaries, that's part of the primary circuit.


Is this lean situation driving down the boulevard, or sitting around? I'd tune to a boulevard cruise way before sitting around puttering revving in a parking lot.
 
To answer the question on if it works on the AVS or not, I can't say, I don't have one. What I do know is all the edelc*ck carbs work on a similar principle to the squarebores.

So, the answer is, it isn't gonna hurt to try, the big picture, general principles are the same. I'm sure you could even tune a Holley in a similar fashion, though the plumbing fixtures might change. Lars has a write-up on those too.

http://www.corvette-restoration.com/resources/how_to_articles.htm
 
PoisonDart74 said:
What feeds the idle circuit? Get that sorted so you can move from idle, off-idle, part throttle cruise, and WOT.

As far as the disconnect the accelerator pump shot, and test primaries, that's part of the primary circuit.


Is this lean situation driving down the boulevard, or sitting around? I'd tune to a boulevard cruise way before sitting around puttering revving in a parking lot.
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The idle circuit is run by adjusting the two idle mixture screws on the front, this is what the car runs on at idle (no load) the primary mixture is totally out of the equation at this point, that's why I was wondering if the test you mentioned would work.

When I place my hand over the primaries, the engine wants to die but that's only because at idle the idle mixture screws are set to 13 A/F so when I cut of the air it goes too rich and wants to die.

My lean situation is at part-throttle. I run about 14.5 A/F at just cruising (55 mph at about 10% throttle) when I depress the throttle a bit (lets say 30% throttle) the mixture goes way lean (16.5 A/F) so I'm a little concerned to lean it out even more on the primary side.

I don't think I'm even getting into the power mode of the rod at 30% throttle and 11-15" vacuum, that's why I was hoping your test would show I was running lean on the primary side, not rich LOL
 
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