This is for the engine builder/tuning experts only please. First some history and crucial data. The block is a 74 400, just rebuilt by a good local speed shop. Stroked it to 461 and bored .035. Heads are 1968 [URL=http://www.gtoforum.com/usertag.php?do=list&action=hash&hash=1]#1 6[/URL] with bowl work to take them from 72cc to 79cc. Heads also ported. Pistons were selected to give me 10:1 CR. Carb is an Edelbrock 750 that was on the old motor and ran fine. Dist, coil, plug wires, etc are all new "ready to run" (and not on a resistor wire). I am familiar with finding TDC, and setting up the dist on #1 . This was all double checked. FP is new and high volume. Cam is a Comp XE274. Rockers are Harland Sharp 1.6:1. I filled the crank with Brad Penn break in oil and primed the oil pump, and had plenty of pressure on the mech guage. So here's the issue... I went to break in the cam this weekend, and it wouldn't fire up on the first try. So, I gave it a little more timing and it fired right up. I took her up to 2500 rpm and held it there. Eight minutes into the break in it stumbled to a halt. The oil press and temp were good. I restarted it and it only ran 3 or 4 minutes this time before it stumbled out. I played with the timing a bit (30 to 36 degrees @ 3000 rpm), but no matter what, I can only get it to run 2 to 4 minutes at 2500 rpm or above. I only took it up to 3000 rpm, but when I try to drop her down to 2200 rpm it becomes erratic and dies. It feels like it would run fine at over 3000 rpm, but I won't try that until it finishes the break in. I thought that maybe my new FP was to blame, or maybe the fuel line I ran was too close to the exh manifold, so I rigged up a simple gravity fed tank directly to the carb. This had the exact same results, which tells me it's not the FP or the fuel vaporizing. So, if the carb ran great on the old motor, and I bypassed the FP, where do I look next? What would cause it to run at high rpm but stumble at anything under 2500? I already disconnected all the vacuum hoses and capped them off, so it's not a vacuum leak there. I also checked for vacuum leaks at the carb base and intake manifold by spraying, again, no leaks. I'm not a professional, but a pretty good shadetree mechanic, but this has me stumped. Could the module in the new dist be breaking down? Could the new coil deliver enough spark for higher rpm but not under 2500? Could the cam have failed in the first 8 minutes, and if it did would it run strong above 2500 rpm? BTW, even after it cooled off completely, I could not run it below 2500 rpm. BTW, because the old motor ran on 89 octane, I added octane boost to the tank to handle the 10:1 on the new motor. What the heck am I missing here? It seems like timing or a massive vac leak, but if I advance it or retard it anymore than I have, it won't start at all, and I've ruled out the vac leak. I'm looking for advice from people that have seen this before or know just what to look for, or how to troubleshoot. Please, no comments about a Holley carb, fuel injection, or a roller cam.