So a couple weeks ago I drove my car down to Englishtown,NJ (50ft) from my home in Mahwah,NJ (350ft), about a 2 hour drive down the highway. When I went to race on the 1/4 mile at Englishtown, the car pulled normal in first then just died and sputtered in second. My 1/8 and 1/4 mile mph were the same. Also the car didn't run right for the rest of the night in any gear really. It ran as if somebody decided to play with the timing. On the way home, the further north I drove, higher elevation, the better the car ran. I wouldn't think that only a 300ft change in elevation could make the car run worse, but I was only running #65 jets in the primaries and #68 secondaries. The next day I changed the primaries to #69 and the car actually ran better and was more responsive. I thought I found part of my problem.
Fast forward to this past weekend, I drove from my home to Atlantic City, about 2.5hrs on the highway and again the more South I drove the worse the car ran, to the point that if I really tried to accelerate at all the car misfired and stuttered the whole time. It would however run right at a cruise rpm with little load. If I had to accelerate I would need to drop down a gear to avoid the stuttering. After the car show was over, about 5hrs, I restarted the car and it ran just as poorly. On my ride home it seemed to run better the more north I went, but it still was stuttering a bit when I got home. To top it off I only got about 8-9 highway mpg the whole time, I usually get 15 highway mpg.
Any ideas?
The motor is a 73 400, with a '70 383 4-bbl intake, 670cfm Holley Street Avenger, no vacuum advance, a mild cam, 727, and 2.45 rear gear.

WAG's: plugs/wires/vac leak

Sounds like weak ignition parts.
3000' is a lot and will be noticable. You need to change jetting and timing to tune for a 3000' change, no doubt for optimal performance.

Thanks, it is a 300ft change not 3000ft.
I would like to think my ignition system is up to par. Everything is about 3000 miles old. I have an MSD Streetfire box, stock distributor, firecore wires, and NGK plugs. My only thought would be that when I replaced the distributor, it was hard to get the reluctor gap tight enough and can't be gapped to the proper gap without making the slot longer.

Thanks, this distributor ran fine with the stock electronic box, but when I switched to the MSD, the box would not recognize it. So I went to gap the reluctor and the tightest I could get it was .010" while the specs should be .008". At .010", the MSD did recognize the distributor. I won't be home again for a month but I will try elongating the slot and making the gap closer when I do.

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Your problem is in the distributor, you shouldn't have to elongate the slot to get the proper gap .

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John is right, you should not have to alter the slot. Is your reluctor wore out? Also this measurement should be done with a brass feeler gauge otherwise you can damage the pickup coil. It may be that your pickup coil is on it's way out, I had the same symptoms when mine started to go and it got progressively worse until it quit.

This distributor was a rebuilt unit I think by Cardone, I had to get a rebuilt one since my old one got damaged by a fire, and the rebuilt one probably had the gap problem from the beginning I just didn't notice until I changed ignitions.
I used a brass gauge to set the gap as best I could. I will try replacing the pickup assembly next time I am home.

You might want to check the pickup before you replace it, that may not be the issue. If you have a volt meter there are a couple of tests to determine if it's up to par.

Just a little update. I checked the powervalve and it is fine. I also back the timing off about five degrees to about 10* and it is running even worse.

Also I noticed while driving up a hill that the engine was surging. It would run smooth then rough all the way up.

Disconnect the MSD and go with standard electronic Ignition or put in a point dist if the timing chain isn't bad I would do that as I have had my share of MSD boxes get goofy after they heat soak due to Long Drive

After 26 years the Direct Connection orange box finally started to get wonky.

It got replaced with a $40 MSD 6A ebay special that came with the bottom plate falling off. I make drives of 600 or more miles at a time and so far the 6A hasn't had a hiccup.

Maybe I just have good luck with electronics but I've never had a failure that kept me from making it home. I wouldn't turn down another 6A, that's for sure.

R.