600 CFM vs 750 CFM...

[yellow rose]

Exactly!
As far custom metering rod shop, it might be you. Sand paper will take the metering rods down, and I think you can get creative on where they need to be filed down. I've done this before more than once with great success

Yep. I prefer a micrometer but you can use a caliper to measure the metering rods. Just sand them down to where you need them.

Don't forget...there are 3 different accelerator pump nozzles for those. I forget that they came with but I think it's the middle one.

You'd be surprised what a bigger nozzle will do for you. Sometimes they want a big hit of fuel, very quick. The smaller nozzle gives a smaller amount of fuel for longer.

 

[ir3333]

i think i may have mis read your post.I thought you were fighting an off idle stumble with both carbs.
Light acceleration can go lean after the initial pump shot though.

 

[7milesout]

Guys,

I think I have reached the limit of my tuning. A quick summary. I wanted at 1406, for the economy tune it comes with, but purchased a 1405 to retain the simplicity of a manual choke. The first thing I did was change the jets and rods to the stock 1406 settings.

Upon driving, that was too rich during cruise, and too lean during acceleration (not WOT). What I have tuned to now is at the limit of what I’m going to get without sanding metering rods.

Here’s what I did last night, all in about an hour, taking my time. I pulled the top off the carb and swapped the primary and secondary main jets. The primaries were 0.098” and the secondaries were 0.095”. That is now reversed. Primaries 0.095”, secondaries 0.098. The reason for that move was 2 fold. 1) I needed to go to 0.095 primaries but had no other 0.095’s except what were in the secondaries. And, 2) when at WOT on this previous set-up, the AFR’s were perfect. It would start off in the 12.5 AFR zone and approach 12.2 AFR. And it felt stronger than I had ever felt it before. So, swapping the jets should produce the exact same AFR’s at WOT.

The next thing I did was to put the top back on the carb and swap metering rods to 75/37’s. In effect, this put me at #14 in the 1406 calibration chart. Richer than the stock setting of a 1406 for acceleration, but leaner on cruise. Next I swapped the vacuum back to full advance, turned the idle mixture screw in about a half turn each, then put it all back together, and set the pump shot to the middle setting. I didn’t get to crank it as it was getting late, and the rain from Irma was moving in. All in all, I’m happy with the Edelbrock carb. No leaking, quick and easy to tune, and runs good in every condition except the 1.5 second lean spike (which I will continue to try to figure out). I suspect it will run and drive well, but may still not be lean enough under cruise. The only thing I have left (aside from sanding metering rods) is to change springs. I’m on the strongest spring, so going down in springs should allow the vacuum to hold the metering rod down in the jet more and more, as the spring sizes are reduced.

I’ve made a spreadsheet that tabulates richer/leaner characteristics with all the main jets and rods that I have accessible to me. This set up I am on now, is as lean as I can go on cruise, without going any leaner while accelerating. And I do NOT want to be any leaner while accelerating. I’ll report back on this thread about how it drives. And I’m going to run my fuel mileage test route again and see where it is at.

Question: In the 1405, there are these metal plates that are press fit into the bowls nearly on top of the secondary main jets. They had to be pulled out to remove the secondary main jet. It looks like a baffle. I can only guess it prevents too much fuel from flowing away from the front under acceleration. Does anyone know what the purpose of those metal plates are?


7milesout

 

[rumblefish360]

You got it!

 

[roccodart440]

That was my 1st thought. But I was curious
about the overall effects. And throttle stopping would leave you with all the negatives of a large carb and none of the benefits.

I've yet to find the negatives of a larger carb on a mopar. Albeit I only run holley's but I always run a 750 on my SB's and my BB went from a 750 to an 850 to a 950 and loved each bump. It suffered no negatives. No on a BB chevy I have seen negatives from a larger carb, even when tuned properly. According to my engine builder it has to do with the heads.

 

[7milesout]

roccodart - I don't know the Holley carbs, but I have to believe what I will say below holds true. The 600 CFM Edelbrock has smaller primary venturies and smaller primary bores. In all cases given one engine (say for instance my LA 360), the air velocity through the primaries will be greater. That increased air flow VELOCITY (not increased air flow, just VELOCITY) will have the potential to better atomize the available fuel.

Assuming both vehicles are tuned perfectly, the 600 would tend to crank up easier, and be better at low rpm (street manners). At what rpm the transition happens to where the 750 is better would be difficult to say. And I believe the transition would be so gradual, it would not be something a driver could say, "exactly 3275 rpm, the 750 is better." My meaning on this paragraph is, the lower the rpm, the more advantageous the 600 CFM would be (again, on one given engine).

For sure at WOT a bigger carb will have an advantage for bigger engines. On my Scamp, it seems to always shift by or before 5,000 rpm. And I would be "killing it" if I were to cross the line at 5,000 rpm in 3rd gear (not gonna happen). So ... I will never get to the point I need more airflow than a 600 CFM will deliver. The 600 CFM has the same secondary as the 750 CFM.

Quite honestly, with the 600 CFM, it feels like it is pulling harder at WOT. I had the 750 pulling 12.2 - 12.5 AFR's, and now I have the 600 pulling the same AFR's. With the 600 it "feels" stronger. The only reason I can contribute to that is the increased airflow through the primary side is atomizing the fuel better, and yielding more power.

A 600 CFM could be placed on the biggest big block around, and just around on the street it would drive like a dream. But on the drag strip, it would be slower.


7milesout

 

[rumblefish360]

7milesout, IMO, and experience you are correct. I don't know about "cranking up" though. (You mean starting the engine right?) I don't see a difference there myself. Also secondary size. The Venturi part may be different IDK. I'm just to lazy at this point to go measure.
(Concrete work today...)

The point where a 750 is better than a 600 is 100% engine combo dependent.
On a lowly stocker, mild performance engine, it would only be at the top end.
5 - 10 HP.

If the engine was more of a street brute or strip runner, the difference would be immediate as per air needs.

*I think* engine masters something of the sort. In one episode where they swapped stock Edelbrock heads for ported versions, in the Dyno graph when comparing the two, you can see where the ported heads start to really take off. While the carb remained the same, you can see how the ported heads consumed more air and made more power. It is kind of the same thing. More head equals more air flow which equals more power just like a carb would, IF the engine can consume it.

The carb was MORE than enough for the stock head.

My mostly stock 5.9 has a Edelbrock 600. B body, AG intake, Hooker headers @ 1-3/4 - 2-1/2 exhaust pipe, electronic distributor/chrome box. Stock broom stock cam. 3.55's.

My solid cammed 360 uses a 750 and could probably use more. 4 spd, 4.10's, etc...

 

[7milesout]

I don't know about "cranking up" though. (You mean starting the engine right?) I don't see a difference there myself.

Yes, I meant during cranking. I say this all with a footnote that I have no scientific evidence. But it is based on the principle that the air is moving faster through the primary side of the carburetor on the 600, and is even more critical at slow cranking rpm. And therefore would again be delivering better atomized fuel. I can say for a fact the 600 carb cranks up easier on my car. But it could be that it started out in a much better state of tune than the 750 did.

I think we're seeing things eye to eye. I'm curious about your 5.9. Have you had it on the strip? And if so, what E.T.'s did you run? The previous owner claims he ran under 14 seconds on this car years ago. I think he is simply mistaken. I think he ran under 15 seconds.

I say that because the best I could get it the only time I ran it was 15.44. And it was carbureting poorly and exhausting even worse. As good as it is now, I have my doubts it would hit 14.5. It's going to take a LSD at minimum from where I am right now, to hit 13.999999999 seconds. Based on that, there's no way he ran less than 14 seconds. And I saw notes from the glove box he made where he says, "spun" (due to the open diff) "bogged," (due to too rich) etc. So I know it didn't run well back then either.

7milesout

 

[318willrun]

Yes, I meant during cranking. I say this all with a footnote that I have no scientific evidence. But it is based on the principle that the air is moving faster through the primary side of the carburetor on the 600, and is even more critical at slow cranking rpm. And therefore would again be delivering better atomized fuel. I can say for a fact the 600 carb cranks up easier on my car. But it could be that it started out in a much better state of tune than the 750 did.

I think we're seeing things eye to eye. I'm curious about your 5.9. Have you had it on the strip? And if so, what E.T.'s did you run? The previous owner claims he ran under 14 seconds on this car years ago. I think he is simply mistaken. I think he ran under 15 seconds.

I say that because the best I could get it the only time I ran it was 15.44. And it was carbureting poorly and exhausting even worse. As good as it is now, I have my doubts it would hit 14.5. It's going to take a LSD at minimum from where I am right now, to hit 13.999999999 seconds. Based on that, there's no way he ran less than 14 seconds. And I saw notes from the glove box he made where he says, "spun" (due to the open diff) "bogged," (due to too rich) etc. So I know it didn't run well back then either.

7milesout

this post confuses me. If I read it correctly, your saying your 360 4bbl is doubtful to run 14's? What car, what gears? If it's an A-body, regardless of gears, there looks to be other issues....

 

[rumblefish360]

I have not had my 5.9 to the strip.

 

[7milesout]

this post confuses me. If I read it correctly, your saying your 360 4bbl is doubtful to run 14's? What car, what gears? If it's an A-body, regardless of gears, there looks to be other issues....

No, I'm saying it is doubtful it will go quicker than 14.0 seconds. Sorry, I got in a hurry right there at the end of the work day.

72 Scamp, 3.55. If it had a limited slip diff, it might hit 13.9. With the current open diff, no way.

 

[318willrun]

No, I'm saying it is doubtful it will go quicker than 14.0 seconds. Sorry, I got in a hurry right there at the end of the work day.

72 Scamp, 3.55. If it had a limited slip diff, it might hit 13.9. With the current open diff, no way.

I'm not putting a feather in my hat, but I'm giving you a measuring stick. My first time out with my '76 Duster (not a factory 360) a couple of weeks ago, I just ran a 14.29 with 2.45 open rear end, stock converter, stock exhaust manifolds, in an almost complete stock 360 (just added a 4bbl and a .427 lift 204*@50 cam). My cam is barely over a 360 2bbl cam. I plan to go high 13's by only messing with the carb, as it is very lean right now. I'm going to put another carb on that will perform better, and I hope to go 13.99

• I think you have alot more potential, is my point. I guess i don't know if you have headers, or what cam, but it sounds like you may have alot more in it.

 

[7milesout]

The latest mains and rods I put in it ... well I'm WAY too lean now. So, it's back to the tuning tables and scratching my head. I'll get it figured out.