rejetting for higher altitude

[olman]

Hi guys,  running a C9AF-9510-C on a 390 engine.  Holley listing shows it as a 730 cfm. It's running 65 primary jets and 79 secondaries. What should I be running for jetting at 4900-5000 feet.

[machoneman]

Leaner than stock usually, 2 steps let's say. But....I'd run it as is first and see how the plugs look after a few hard pulls. Holleys with stock jetting sometimes run rich even near sea-level. Give her a go first I say!

[thatdarncat]

What's the Holley "List" number for that carb? The Ford Parts book shows C9AF-9510-C as a Ford 2 barrel carb for a '69 Galaxie with 390, and not a Holley. I'm just curious, wanted to verify if you have the original jetting.

Just in general, the Ford Parts book recommends going 2 jet sizes smaller each 5,000' higher in elevation.

[olman]

Got the number right off the choke housing C9AF-9510-C  List# 4609 with 422 under the list# The primary metering block 5673, and sec. block 5671 a true 4150 Holley.

[babybolt]

That's a -U service replacement Holley for a 428CJ.  Should be stamped C9AF-9510-U.

[olman]

The Holley listing for stock is 66 and 79 for jetting.  I'll try going 2 steps leaner from stock and see how my plugs look. Running new wires and cap and Platinum plugs. Pertronix Igniter II and Flamethrower II coil. Use 91 octane with a Moroso octane booster with Chevron gas. Timing is 12 degrees adv. and 36 total at 3200rpm. Bypassed my resister wire too.

[thatdarncat]

Yes, like BabyBolt said, Holley List #4609 should be Ford C9AZ-9510-U, not "C", Ford Universal replacement Holley for any of the original 428 CJ Holleys. You might want to double check, it would be interesting if they stamped your "U" sideways.

Anyway, yes - 66 primary & 79 secondary jetting was factory. The Ford parts book lists that jetting combo for up to 10,000' elevation, however like I said before many of the other carbs they recommend the smaller jets at 5,000'. I think you could try either way to start.

[olman]

Welllll------------ looks like you guys are right and I'm going blind! After seeing your posts I took some white chalk out and rubbed over it and to my surprise it IS a U. It's not stamped as deep on the right side so to me it looked like a C.  Thank you all for clearing this up for me
 A really great bunch of FE people exist on this site.

[Drew Pojedinec]

It's pretty lean to begin with for that size carburetor on the primary.
That 79 for the secondary seems pretty big.  The few CJ carbs of this list number I've seen had smaller, like 72's in the secondary, but they also had power valves back there.

[Tobbemek]

The only reliable answer to your question is what your engine will tell you. As stated earlier by others you just have to try it out.
The size of the main jets are in first hand dependent of the venturi size and booster design and secondly of the engine size and power level.  The OEM Holleys tend to bee on the lean side in the primary calibration for emission purpose in contrast to the universal Holleys that is very often on the rich side.   This carburetor is calibrated of FORD engineers to suit the 428 CJ type engine and is little on the bigger side if your 390 is stockish.  The same carburetor compared on a smaller and a bigger engine tends to need just a little bigger MJ due to a weaker signal in the booster on the smaller engine. so this carburetor might suit your engine on a higher altitude just fine who knows? The engine certainly knows!!   

[CaptCobrajet]

68/80 is a pretty good baseline on the 4609 carb at sea level with a power valve in a healthy 390.  I have worked on several hundred of those carbs in the last 35 years. When we dyno at sea level, and then dyno the same stuff in Ft. Collins CO, it is usually three or four jet numbers.  I say 64/76 is a place to start, and may be right-on.  Those primary boosters typically don't like less than a 64 jet on 400+ inches.