600 CFM, all I need. Check my math!

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750 cfm isn't "too big" for a 360 engine. That's my simple answer, based on about 45 years of experience. Heck, the stock carb on a 340 was around 800 cfm.
 
Here's an article that might be of interest...

Selecting Right Carbs: How Big Is Too Big?- Car Craft Magazine

In it, they state what I have heard several times and have found to be true as far as using a large carb on a dual plane intake...

"Another important factor influencing carb selection is intake manifold design. With its divided plenum, a typical dual-plane intake provides a stronger signal to the carb than a single-plane manifold. Grant says a 350 Chevy with a dual-plane intake can usually use a 750 carb; if it’s running a single-plane, a 650 carb will probably get the car down the track quicker. One way to crutch a single-plane/large-carb combo suffering from insufficient signal is to use a four-hole carburetor spacer."

I still think you're fine with the 750 and it sounds like you are headed in the right direction with tuning! As far as a gauge, I would not over think it...just get a basic one. Advance auto has one for about $25... Actron part number CP7803. Most of their stores stock it, it's got a large, easy to read dial...and who cares if it's not a $$$ calibrated unit, you are tuning for maximum vacuum anyway.
 
My experience has been it's nearly impossible to over carburetor a SBM. Run it at the track with both carbs and check MPH, then you will have your answer
 
I think people over worry about velocity and I imagine the port and runner would have more influence on it than carb and valve.

Look at efi they don't make a bunch of different cfm models one size does all up to a certain hp.
 
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If you need a vacuum gauge Harbor Freight has some that are reasonably priced.

Keep learning. No on can ever take away wisdom, knowledge and understanding.
 
Got the Scamp back at noon today from the exhaust shop. It sounds GREAT. It feels snappier / peppier. As to be expected.

Now the carburetion tuning begins. I've been reading the Edelbrock manual. The manual is very well written and it sounds like a cinch to adjust. I also purchased an AEM wideband and had bungs installed in the exhaust. So, I am going to try to get the wideband installed this weekend. It will make knowing which way to tune a breeze.
 
7milesout said:
Got the Scamp back at noon today from the exhaust shop. It sounds GREAT. It feels snappier / peppier. As to be expected.

Now the carburetion tuning begins. I've been reading the Edelbrock manual. The manual is very well written and it sounds like a cinch to adjust. I also purchased an AEM wideband and had bungs installed in the exhaust. So, I am going to try to get the wideband installed this weekend. It will make knowing which way to tune a breeze.
Click to expand...


Very good. Once you get the hang of it, you'll be getting paid to help others figure it out.
 
Thanks. In my opinion the manual is written so well that I can "see" what is going on in the carb and how to change / improve it. I already changed the accelerator pump setting.
 
Spent ALL DAY yesterday (Saturday) working on my truck's brakes. Not done either, now I'm going to replace the calipers, the driver's side is stuck.

But today, I got the wideband installed. It went really well. Didn't take too long. You gotta see how I mounted it. I don't love the mounting, but it was simple, easy and it works. I'll get a pic of it to show you guys. The thing is a b---h to start when cold. Before I cranked it, I checked the carb to see what was hooked up to it based on watching Edelbrock videos.

Turns out the vacuum was attached to the driver's side port. Which as I recall is called total vacuum. Which I don't think it correct for this car. But the kicker is, the timed advance port was WIDE OPEN. So ... I switched the vacuum hose to the timed port, and plugged the other side. I would image the choke was being defeated by having this port open. So, after some frustration, I got it cranked and here is what I saw. A little surprising.

With the advance hose switch and the other side plugged, the idle was too low. I noticed this later on in the test drive when it was fully warmed. And went through the lean best idle when I got home.

Cruise Mode - Rich. Not WAAAY WAAAY rich. But rich.
Power Mode - Not too far off, but seemed a little on the lean side.
WOT Mode - Just a hair lean.

Based on this, I think the first thing I'm going to do go with 1 step lighter spring, a 4" (of Hg) spring instead of the stock 5" spring. This will help the vacuum overcome the spring more easily and keep the metering rod further down in the main jet. The effect would be leaning it up, in cruise mode.

Also, just before I got home, I saw that at a near stop, and hitting it, it would bounce into lean (and either stumble or almost stumble) and then go real good. So ... on Friday (pre-wideband) I moved the accelerator pump to the smallest shot. I'm going to wind this post up and move it to the middle shot setting and run it again. I have to do this from a slow roll or it just smokes the right rear tire.

Loving the wideband feedback! And it appears as if all the guidance above is correct. The carb is not too big, it can simply be tuned. However, I wonder if a 600 CFM carb with electric choke (& electric fuel pump) would start better.

I'm probably going to just go ahead and buy the carb tuning kit from Edelbrock on Monday.


7milesout
 
JoesEdge said:
Here's a cool calculator that may help in tuning. Pair this with a wideband A/F gauge and it will guide which way to go as far as jets and rods.

302w.com - Edelbrock Carburetor Jet/Rod Fueling Calculator
Click to expand...

Calibration Kit procured!

I'll check that tool, but really, being an engineer myself and having written manuals, my opinion is the Edelbrock carb manual is really great. I'm confident it is all I will need. Gonna go make a metering rod change. Maybe it will stop raining and I can take it for a test.
 
So you drove over to Summit and got it, cool!
If you don't have to change jets, you can test several combinations in one outing. Takes seconds to change rods and springs once you get the hang of it.
Cross reference the different model's venturi, jets and rods, do a little math, and you will find you can do a lot more than the dots shown. I didn't have an AFR meter, but I had fun messing with mine.
Good luck!
 
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KnuckleDuster said:
So you drove over to Summit and got it, cool!
If you don't have to change jets, you can test several combinations in one outing. Takes seconds to change rods and springs once you get the hang of it.
Cross reference the different model's venturi, jets and rods, do a little math, and you will find you can do a lot more than the dots shown. I didn't have an AFR meter, but I had fun messing with mine.
Good luck!
Click to expand...

I drove (not in the Scamp) and got it. The carb had a big thick gasket (I think it's a heat insulator) and about 4 regular gaskets. On the Edelbrock videos they only use the one gasket supplied. So tonight I removed the carb and all those gaskets. The nuts weren't even tight. I could have loosened them with my fingers. Then I cleaned off the top of the intake. I put on the brand new gasket in the Edelbrock box and put it back together.

I had to get a son to read the tiny number on the metering rods. I did a metering rod change only. It was late when I got done so I didn't even crank it. But I have a couple questions.

  • All those thin gaskets and insulator gasket stacked up must have totaled an inch thick. Was I wrong to remove the heat insulator / spacer gasket?
  • I moved the timing advance hose to the total advance (the lower port) and plugged the timed advance. I read that timed advance was for emissions car. Since the car has no emissions I figure I would be better off with the total advance. Am I correct on that?
  • This is probably a silly question, but I must get clarity. After I cleaned the top of the intake, I put the new gasket on. The new gasket fit the studs correctly but ... the left and right inner sides of the gasket bore closed off some of the opening of the intake. Or think of it like this. The intake square bore was larger than the gasket sqaure bore, on the left and right sides. I got nervous & took the carb back off and compared the new gasket to the 3 or 4 thin gaskets and heat insulator that I took off. They were all the same bore size. So I put it the new gasket and carb back on, and bolted it down. It must be OK I guess. Question: wouldn't it be better if the gasket bore and intake bore were the same size?
Once I did all that it was too late to crank it up. The wife doesn't like it as it is, so cranking it would have only created drama. I don't know what to expect from it except a PITA to crank from cold. And it's very likely to rain tomorrow, so I won't drive it to work. And after work I have a college planning meeting. So I may not get to touch it tomorrow.

7milesout
 
Keep on keeping on. U will get it figured. The thick gasket is a plus because the heat of the engine likes to percolate the fuel out when hot and make re starting a bear. Kim
 
As for my bullets above:

  • I'm going with 1 thin gasket, unless I seem to run into heat problems. I will hang onto that big thick gasket in case.
  • I actually moved it back to ported advance because I didn't understand why manifold advance threw off my idle. Late last night I read about it. And now that I understand the difference between the 2, I'm going back to manifold advance and will reset my idle, and idle AFR.
  • I drove with the gasket as mentioned above, seemingly no issues. It's the same gasket overhang as before, and it is the gasket that came with the carb, so I'm just going to forget about it.
Last night: After changing to the leaner metering rod, it was even richer! So while driving, I was thinking that it was "as if" the 5" spring was lifting the metering rod out of the main jet too easily, and going rich. So I went home and put in the blue 3" (lightest) springs so that the vacuum can more easily hold that rod down in the main jet. Then I drove it again. That made a difference. It was definitely leaner after the spring change. But not nearly lean enough.

Tonight I'm going to change to manifold vacuum. Don't know if I will get to drive it though. But my theory is that in cruise mode, there should be more timing. The additional timing would help burn the fuel more completely. My theory tells me running manifold vacuum will also drive it toward the lean side. So, before I change rods again, or jets, I need to test it on manifold vacuum.

Wrote all the above earlier. Now it's late. I did change to manifold vacuum. It seemed to have no affect on cuise AFR. Still very rich. Power mode seems close.

I tuned the idle to be 13.5 - 14.0. But that seems too lean. It seems very lopey. Alright at idle but when I put it in gear it seems to almost want to stall. Edelbrock suggests 13.0 at idle so I will tune for that. And go another step leaner for cruise mode.


7milesout
 
7milesout said:
As for my bullets above:

  • I'm going with 1 thin gasket, unless I seem to run into heat problems. I will hang onto that big thick gasket in case.
  • I actually moved it back to ported advance because I didn't understand why manifold advance threw off my idle. Late last night I read about it. And now that I understand the difference between the 2, I'm going back to manifold advance and will reset my idle, and idle AFR.
  • I drove with the gasket as mentioned above, seemingly no issues. It's the same gasket overhang as before, and it is the gasket that came with the carb, so I'm just going to forget about it.
Last night: After changing to the leaner metering rod, it was even richer! So while driving, I was thinking that it was "as if" the 5" spring was lifting the metering rod out of the main jet too easily, and going rich. So I went home and put in the blue 3" (lightest) springs so that the vacuum can more easily hold that rod down in the main jet. Then I drove it again. That made a difference. It was definitely leaner after the spring change. But not nearly lean enough.

Tonight I'm going to change to manifold vacuum. Don't know if I will get to drive it though. But my theory is that in cruise mode, there should be more timing. The additional timing would help burn the fuel more completely. My theory tells me running manifold vacuum will also drive it toward the lean side. So, before I change rods again, or jets, I need to test it on manifold vacuum.

Wrote all the above earlier. Now it's late. I did change to manifold vacuum. It seemed to have no affect on cuise AFR. Still very rich. Power mode seems close.

I tuned the idle to be 13.5 - 14.0. But that seems too lean. It seems very lopey. Alright at idle but when I put it in gear it seems to almost want to stall. Edelbrock suggests 13.0 at idle so I will tune for that. And go another step leaner for cruise mode.


7milesout
Click to expand...



You're doing well. Pretty soon you'll be tuning Holleys like you been doing it all your life.
The metering rods are kind of like the power valve. The springs you swapped are like the opening point of the power valve. The power valve channel restricters are the same as th small end of the metering Rod. A jet is a jet.

It's easier to change emulsion and air bleeds on a Holley.

See, your almost there. Keep it up.
 
yellow rose said:
You're doing well. Pretty soon you'll be tuning Holleys like you been doing it all your life.
The metering rods are kind of like the power valve. The springs you swapped are like the opening point of the power valve. The power valve channel restricters are the same as th small end of the metering Rod. A jet is a jet.

It's easier to change emulsion and air bleeds on a Holley.

See, your almost there. Keep it up.
Click to expand...
(Doing the hand over head thing here). Whew! You went over my head with all that. I don't think I will (or can) change air bleeds on an Edelbrock.

I've got time after work today (I think). Gonna go deep tonight. Gonna pull the airhorn off and replace the main jets. Now ... I gotta go figure out which main jet to go with.

It's not simple, because Power Mode is not defined. Meaning, carb idle has a definite throttle location. WOT does as well. Cruising and Power are not defined. But cruise mode certainly would be a steady speed on flat level ground. But a steady speed at what rpm? Maybe I should shoot for my target AFR on cruise mode at 3 points ... 1) 1,500 rpm, 2) 2,000 rpm, and 3) 2,500 rpm.

But Power Mode ... how do I know I'm in power mode? Dunno? Best I'm guessing is that I should be able to discern when the secondaries kick in via the AFR gauge. And Power Mode will be some acceleration less than that.

From what I can tell, my Power Mode is now pretty darn close. But my cruise mode is still way too lean. Back to the 1407 calibration chart in the manual!


7milesout
 
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