This sounds just like vince g"s problem a little farther down...mine is doing the same thing basically. If I take it up at maybe half throttle, it will go right up to 6K easily, and smooth....but if I get into it, it breaks up badly at about 4400 rpm...If I keep my foot in it, it will just keep breaking up. Fuel pressure stays up at 5 # (normally at 6 or 6.5) It almost sounds like Im hitting a rev-limiter, but I dont have one!! Its a 454, (.030) Holley 800 DP, 72/80 jetting, no rear power valve, new MSD pro billet dist, new Accel wires, TH400 tranny, timing is 12 initial, 34 total all in at 2800 with the vacuum adv. disconnected and plugged, or 44 degrees total with vacuum hooked up. Advance is hooked to ported vacuum, not full time. Plugs look brown, but not light...sorta deep tan colored after hard run. Im sure it could be ignition related, but being that its all new, Im hoping not...jetting maybe?? It runs really great at lower speeds, or at less than full throttle. Help!!! What to I start with?? :mad: John

check the voltage to the ignition system
I can almost bet it's there
what type of plugs are you running with the msd
i know a/c delco rapid fires work really nice together with the msd,,
if the msd box has an improper ground or not enough current it will break up badly
i had the same problem in my 66 with the 454

Thanks Rick....I'll check the voltage to make sure something didnt come loose, and the ground too.I run R44TS plugs. I changed the wires last night, but havnt taken it out for a test yet. I had the Accel 300 wires on it, but I also know a lot of people have had major problems with them. I put a set of Taylor 11 ml. wires back on from last year, I know they worked well before. I'll try one thing at a time and keep at it. John

Well, after changing over completely to the stock HEI (which works very well), I still had the same problem. Turns out that the secondarys on the carb were way lean, it was leaning out when they opened. I rejetted my 850 Holley DP, set all the levels everything, and it runs like a scalded ape....no problems.. Hope this info can help someone else. Now, put the MSD back in, or leave it alone?? HHmmmmm....... J

Johnny O what size pri. + sec. jetting did you end up going to with to fix the problem.

Dino

Dino, I sent you mail. J