I have a bad misfire that occurs at any rpm under full throttle. In first gear I can floor it and it wont misfire, but the tires break loose and hook around 3000rpm. Then about 5500rpms it starts to misfire and tach goes wild. Sometimes it will go past and I can rev it to 6400rpms, but most of the time it will just stop at 5500 at which I shift into 2nd the rpms drop to about 3000. It starts to misfire again and the tach needle jumps from 3000 to 6000rpm. I replaced the msd ignition box, new plugs, coil, rotor,cap. I checked the boots and wires, which they seem ok(8.5mm msd wires). I even replaced the valve springs. Does it seem like it could be the fuel pump? Im tired of just replacing parts, but it seems like I replaced most of them anyway. Anyone have insight on this?

 

I know that you checked your wires, but how old are they??? I had almost the same problem with my 440 and I changed the wires and all was fine. I know what it is like to just dump money into some unknown problem. If you check your plug wires with a dwell meter before you replace them, you might find that they are ok. However, if you have any problems with your plug wires (no matter how good they look, I have gotten bad ones out of the box!) incorrect dwell will throw your spark curve off and you will misfire. And yes it would be most prominent at WOT. Good luck

 

Its the spark plugs most likely. I had that problem on my 71 bee, Spun and then hooked up in first gear with no misfire, then you shift into second and it starts misfireing. I replaced mine and it works great.

 

Without seeing it first hand I cannot say with absolute faith. It could easily be the carb, fuel pump, fuel filter or the fuel line. Start with the cheapest and easiest by inspecting the fuel lines for any kinks and then replace the fuel filter. I've had filter problems causing surging and even kill the car. Another possible cause could be with cars equipped with K&N air filters that have been over oiled restricting air flow causing similar problems in the higher RPM's.

 

I had that problem with my 340 duster. turned out to be a bad coil. Had misfire on my 71 Dodge pick up that was a bad set of points. Lots of stuff to go wrong there.

 

Personally, I would take it to the next level. Plugs, wires, cap, rotor, coil.

 

I would go after the wires first.

 

You have a head scratcher there Daman..................In reading through the replies I'd say it could be any of the things suggested, and yes it does suck just replacing expensive parts and guessing.
Finding the root cause of failure does take some extensive digging though, So, with that in mind, try the "dark test".
Do a night drive. Then with the car fully warmed up, pull off the road someplace without a bunch of street lights and lift the hood, find and blip throttle a few times while looking around at the spark plug wires, plug boots, distributor cap, and everything else I might have forgotten to mention here in the high voltage side of the ignition system.
If it's jumping spark or has any external voltage leakage - you will be able to see it as an arc.
Some guys say to use a spray bottle of water to help locate weak ignition wires and insulators by misting water around the plug wires. Be carefull with that, but it is a valid technique.
IF you still don't find anything, I would pull the distributor and the oil pump drive at that point and check all of the shaft bushings.
The oil pump drive shaft has a bronze (or aluminum) shaft support bushing just below the cam gear that drives it, and they can push the distributor shaft around when they're worn.
Somehow your getting spark scatter at those particular RPM's, it's just a matter of locating the root cause in this case.
Another errant thought here on something to check..............doing a timing chain slack test, although I'm not sure exactly what car your working on, I can say from experiance that a big block with a performance cam won't always show late valve timing symptoms untill the chain is whupped to the point of almost falling off, so do a slack test if you have'nt already.
Good luck on that,
~Bob~