Carb tuning question for altitude and temperature change.

[nikwho]

Hello all,
I live in Flagstaff, AZ at 7,000, where its pretty cool most of the time. I'm getting ready to take my '59 Apache down to Phoenix for Dino's Git Down Chevy Only truck show in November (13th & 14th). I'm running a Quick Fuel 650 on my 400 HP small block 383. Why a 650, you ask? Because I live and run my truck at 7,000' ASL. Never put the truck on a dyno. It's a mild 383. Anyway, looking for carb baseline tuning suggestions. Im running the Quick Fuel Brawler 650. Stock primary jet size is 70. I've changed that to 66. Secondary jets were 74. I changed to 70's. SHOULD I just go back to stock for Phoenix in November? There are a lot of variables. Just looking for a good safe starting point. Haven't spent a LOT of tuning time with my current jets, just figured that it'd be a safe starting point. Leaning towards going to recommended stock jet sizes for my trip to Phoenix and back. Driving the truck, BTW, not trailering it.

Thanks,
Nik

[geezer#99]

Going back to stock will make you rich at higher elevations. That’s assuming it was rich with 70’s.
How do you test. Got a afr set up or reading plugs?

You’ll be spending most of the trip at lower elevations. Peace of mind says go back to stock.
I’ve travelled from 3400 up to 6000 and down to 1000 and back with no jet or timing changes.

Have you done any timing changes?

[nikwho]

I should have added that ill be traveling to 1,100' ASL. It likes a little more timing up here. Thinking of that, I'll need to back it off. I think that Im at 16° initial and about 38° "all in". Its a vaccum/mechanical advance HEI. Should be 70°-80° air temp. Average online said 76° for November.

[nikwho]

No AFR readings, though my new exhaust system has bugs welded in already. Im going that route soon. Might need to for that trip! Like I said, limited test and tuning time on the new engine. Ive pulled plugs and they looked good, but it was on mains, secondaries and idle circuit after pulling them. I haven't gotten out and made wide open throttle or half throttle runs, then shut immediately down and checked them.. not yet, anyway.. Went from NGK UR5 plugs to UR4 plugs. I figured being a moderately stock engine, there was no need to move to a cooler plug from stock. And I BELIEVE that the UR4 NGK plugs are considered a "standard" plug. I've got both, though.

I'll pull a little timing out of it, to get it at 34°-36° total timing. That'd give me around 12° or so initial timing.

[geezer#99]

Aftermarket carbs are usually set up for use at sea level. Up to a thousand feet not much change.
Jet it back to stock but leave your timing alone. Easy to adjust if it pings.

[nikwho]

My new World Products heads managed to get put together and installed with no seals in place, which caused some weirdness that took a minute to figure out, so I'll have to pull all of my rockers, retainers, etc, pull springs, install seals and reassembled the intake/distributor, etc. and readjust the valves. Once that's done, I'll re-time it with a little less.

[Dead Parrot]

Many years ago, I did the opposite. Went from the about 1000ft of central OK to about 9000ft in Central CO. Swapped rods in my Eddy 1405 to go 2 steps leaner once I got to a stopping point about at about 5000ft. On that carb, a rod change is simple 10 minute task and doesn't risk damaging any gaskets. Was really nice having a good running truck that didn't leave a trail of black smoke. The rods were some I already had in a tuning kit and according to the tuning chart would get me fairly close. Reversed the swap on the way back.

Another thing to check is the octane rating on your gas at 7000ft. IIRC, the standard level gas at 9000ft was 84 rating instead of the normal 87 in central OK. My mostly stock 350 didn't notice but your 383 might.

[nikwho]

Good points,
Due to my compression ratio and timing, I run only premium pump gas. However, in my area, I have not seen any fuel lower than 87 octane. At times I do miss the simplicity and quick adjustability of an Edelbrock carb! I don't have the same option to change rods on my Quick Fuel carb. It's a Holley, for all intents and purposes. I will likely just put it back to stock settings, or perhaps one jet size leaner than stock, and trim my idle adjustment screws once I drop down in elevation.