As altitude increases, vacuum drops. It takes less throttle to drop near 4" at 5,000 feet than at sea level, and much less at 10,000 feet.
I suggest temporarily (or permanently) mounting a vacuum gauge to help you tune. You want to have a step-up spring that opens at half or less of your 65-70 mph cruise vacuum, and that is 2" or so below the vacuum on a "normal" incline, such as going up an overpass.
But the plot thickens. Higher compression makes for slightly higher vacuum. Since you already are at 5,500 feet, you probably want the high compression anyway. At that altitude you can be at 9.5 or 10-to-1 and still get y with 91 octane. So don’t use a thicker head gasket than the factory did, and don’t swap heads if the combustion chambers are bigger. (You folks at sea level IGNORE THIS!!)
And the plot thickens again. If your Newport did not come from the factory with a 383 Magnum/TNT, and it has a stock, standard (smooth idle) cam now, when you change to a 268 series, the longer cam will make for lower vacuum at idle. (But if you already have a 383 Magnum, it isn’t really that much different.) In addition, if your Newport has “standard” 383, it probably also the stock 12” torque converter. The 383 Magnum had a higher stall 11” converter that makes it easier for the engine to idle in gear (which it needs due to lower vacuum.)
The standard 383 has a cam like a 252 series, the Magnum/TNT has a cam more like the 268 series.
With the 12” torque converter and the 268 cam, the engine won’t want to idle in gear and will fall on its face off of idle. In neutral, the 268 cam will have about 4" less vacuum than the 252, but in gear the 268 could have much less vacuum than the 252. Given you already start with 4-5" lower vacuum than at sea level due to altitude, it doesn't take much to get really low vacuum with a bigger cam.
The best solution is to get an 11” converter with a stall speed of at least 2,200 RPM and not more than 2,800 RPM. This will also make your Newport into the neighborhood burn-out champion.
The next best solution is to go the 3” step-up springs and slightly richer rod/jet combo on the primaries. Which you may have to do anyway with the bigger cam.
The other thing you’ll need to do if you are starting from a standard 383 is to put a timing curve in the distributor similar to the current Mopar Performance electronic distributor and run your base timing at no less than 12 degrees. This IS NOT the same as two light springs. It is the “silver and brown” combination if you are using PN 5153446. The bigger cam likes a quicker timing curve.
As you discovered when the carb went "full rich" as you crossed 9,000 feet, getting the step-up springs right (or power valve if it is a Holley) needs to come first before you can get the jets right.
If you are running a 12" converter and 2.76 or 2.94 rear end (and not planning to change) you might want to consider a 262 series instead. It would work better with the "standard" combination.
The engine came out of a friends '68 Satellite with 105,000 miles on the clock - its the HP with flat top pistons, heads milled .020" to make up for the thicker Felpro blue head gaskets, 11" converter that flashes to 2200 rpm, and highway 2.76 gears (I want my old 3.23's back!). The distributor is the MP kit made by Mallory running 14 degrees initial, 38 total (52 with vaccum advance) all in by 2400 rpm. The cam was pretty well worn, and since I've had great luck putting the XE268 in lots of other engines, I popped another one in this engine. It generally runs really sweet, just has the wrong springs in the carb. I figured out I needed the yellow 4" spring about 5 minutes after I posted last night (I was suffering from intercranial flatulence at the time I posted last night).
I sold my torque monster motor to my buddy to help fund my daughters college education, and took his old Super Commando in the deal. My old motor is currently running 12.5's in his Satellite. The fastest I could get out of it in my much heavier Newport convertible was 13.7 @ 104mph, which was just fast enough to get me banned from the strip because I refused to put in a roll bar)
furious70 wrote ... I let my MAP sensor figure it out
It will be interesting to see what you think of the combo. As you already know, those heads are kind of mismatched to everything else on the engine, and now you're going to make them more mismatched by putting a smaller intake and carb on it.
Heads flow about 250 cfm on the intake, which matches up pretty well with the M-1 intake. I know the RPM manifold will flow a whole lot more than that (nearly 300), but I don't really need that much flow if I'm not revving up past 5K. just trying to see if it will move the torque curve down a little lower and increase the mileage. if I don't like it, it will be a simple job to swap right back. Like I said, I'm bored and need a project (since I don't have any engine rebuild customers over here in ABQ yet). <span class='smallblacktext'>[ Edited Tue Sep 18 2012, 09:54PM ]</span>
Sounds like you are well ahead of me on the 4" springs. Good to know, as I may take the 67 300 convertible out west next summer.
Went through the Great Divide Basin (off-road) then Tetons and Yellowstone (on-road) in 2011 and 2009 in a soft-top 74 Trailduster. I re-jetted the Holley for altitude when I got to eastern Wyoming. It did quite well, went as high as 10,500', no problems. I haven't done altitude changes with an Edelbrock yet, so your experience helps!
The biggest challenge I found was that some gas had ethanol and some didn't, so it would run better or worse depending on what the last station sold me, then change again at the next station. Seems the higher in altitude you go, the more ethanol screws up driveability.
There is plenty of trunk space in the 300, and it gets 16-17 mpg, so why not? I'd bet if I went back to stock rods, jets, and the stock 4" springs, once I got near 5,000 feet elevation, it would be just about right. As long as I don't off-road it like I did the Trailduster, she should be fine!
As promised, I did the swap today and was very pleased with the results after some trial and error. Ibolted up the little Eddy #1405 (600 cfm) first, but it ran completely flat at only 4800rpm, almost like switching off the ignition ... needless to say I was not impressed!
I put the Eddy #1407 (750 cfm) back on and with the smallest jets and largest rods in the kit (.104 primary jets with .075x.052 rods, yellow 4'' springs and .101 secondaries) I was able to get it tuned in at 14.5" vacuum, which is pretty good considering the 5500 ft altitude and XE268 camshaft. The idle screws ended up one complete turn out from bottoming at best lean idle, which is barely lean enough. (Note: if your idle screws can only be turned out 3/4" turn from bottoming at best lean idle, or you can't kill the engine by screwing them all the way in, then you should put in the next smaller jet. Conversely, if you have to turn them out much more than 2 1/2 turns, you're too lean and should put in the next bigger jet))
The engine has much better throttle response off idle and no bogging when kicked down like it did with the RPM manifold. Normal driving is very smooth and torquey (and a bit quieter), plus it pulled really well up to 5800 rpm before it started to run out of steam, which is a lot faster than I intend to wind it up. She smoked the tires easily, something I had to try really hard to do with the RPM manifold and highway gears. It will be interesting to see if the fuel mileage improves by a measurable amount... and now some pics:
<span class='smallblacktext'>[ Edited Sat Sep 22 2012, 11:47PM ]</span>
That's why I put the 750 back on it!!!! Went for a long ride yesterday after installing a new turn signal switch-the new combo is sweet..... <span class='smallblacktext'>[ Edited Sun Sep 30 2012, 09:41AM ]</span>
Good write up. I do think for a single carb a 750 is a better choice. But a 600 jetted up will do better. Kind of like running a 500 vs 600 on a small block. You get better throttle response form the 500. Your 600 set up is far from close to the 1405 set up though. You are still about 8% leaner. I went through a lot of this when setting up my carbs a while ago. You are only about 2% richer right now. I did find the same as you though that making it richer made the car run better. I am currently running 100 jets with 70/47 rods in both carbs. I ran a 68/42 rod when running on just one carb. Getting a jet kit is the only way to go in my opinion. it is cheaper in the long run and lets you keep changing till you have what works best for you. Make sure you keep a record of what you have in it and also what the results were. makes it easier to fine tune. I just went through 3 days to get my carb in my ranchero totally dialed in. One thing I found for both of my cars is. Leaner on the main circuit makes for better low end throttle response. Richer is stronger in the mid range. To give you an idea how to use the chart. if you have a 100 jet and 70 metering rod. You get 30/53 fuel curve. 100-70=30 and 100-47=53 with that info. You can richen up one point at a time. For my ranchero with a .095 jet 75/47 was to lean and 73/47 was to rich figured out to be 20/48 or 22/48. I needed the middle. I ended up with .083 jets and 65/37 rod. ending with a 21/49 over all if that makes sense to you. One step leaner and one step richer. Once you figure the math out. Every thing gets to be very easy. For me. Changing springs made very little difference. I also run the middle hole for the accelerator pump. You shouldn't need different with a 600 carb in my opinion if it is jetted right. I am no expert though. I just have a lot of time invested in tuning the edelbrocks
<span class='smallblacktext'>[ Edited Wed Oct 31 2012, 07:14PM ]</span>