i got a 350 small block (no not the original motor, and no clue what it got ripped from.) sitting in a 72(I think) chevy cheynne. the problem --
Can't get it to do it while I'm parked, unless I rev it to the high end, (No tach in the truck, so I ain't got any rpms to specify.)
The problem tends to only happen on the highway, going 56mph and up. When applying the throttle, the truck will (Misfire/backfire?) and you feel the loss of power, the truck will seem to lurch (from the loss of momentum I'm assuming) and you hear the carb spit out the gas.
If I stay easy on the throttle, and just try to maintain a steady speed around 60, it'll still do it. While I'm accelerating lightly, just enough throttle to gain speed it'll typically let me get up to 65 mph before it starts spitting again.
-- can smell it in the cab, honestly smells flooded out but i ain't a professional. ...
Things done -
Replaced carb with a freshly rebuilt one, original one was doing it much worse. Coil/ignition wire (comes from the harness to the distributor) was replaced (This also seemed to fix the problem partly.) The wire was broken near the harness, so replaced the thing.
Replaced the spark plug wires, plugs were fine though. Don't have a way to check the compression up here, other than my finger.
Just purchased the timing chain and gears, but I'm very wary of having to actually have to do that yet.

I'm guessing the RPM is in the mid range by the sound of the motor when it does this, it also feels as if after I pass 35-40 mph the truck literally has a decent loss of power (Not sure if that's just the gear ratio or what but it just doesn't feel right? )

Trying to get everything I can in this first post... Usually, I can have it literally floored from 0-40 without it shooting back up a single time, seems almost like clockwork for it to first spit right around 56-65 mph. I didn't get to toy with it too much when I got past 70 mph, but it did seem to actually run slightly smoother when i tried to accelerate (not by much)

Oh, also adjusted the lifters, as they were way too tight. Ran through and re-adjusted them again earlier though rather unnessary.

Also replaced the rotor cap as when I checked under the distributor, the old cap was not actually screwed down at all.

Pretty much the exact steps I've taken in order... The ones that seemed to make the most effect for the better was changing the wire, readjusting the lifters, and the wires themselves. I've tried retarding and advancing, but neither seems to affect it too much... (I am an amatuer but I got a decent round-about knowledge)

I can't think of anything else to add... Any more info, just asked and I'll provide what I can. (Not sure what the carb actually is, and I believe the motor is bone stock)
My uncle believes the cam has went flat, but I only wish to do that as a last resort. Please help, sorry for the long first post, but just tryin to get everything out there.

 

If it's load-related it's usually fuel supply-related.

Won't do it in the driveway just free-revving? It ain't the valvetrain- that's almost purely RPM-related.

Spark CAN have load-related drop-out, but that usually happens with a weak coil or bad wires when you FIRST snap the throttle open and pretty much everywhere beyond that.

I would highly recommend making sure the centrifugal and vacuum advance mechanisms are working properly, however, before you rule out ignition-related issues.

But I'm betting you just aren't pumping enugh fuel to the carb to keep up with demand. Try to put a fuel pressure gague just before the carb you can ACTUALLY READ WHILE DRIVING. I have a remote gague I duct tape to the outside of the windshield (temporarily!) when I go to make a run. If you are outrunning the fuel system it will be OBVIOUS. Fuel pressure will nose-dive as the RPMs come up, followed shortly by the engine falling all over itself.

Clogged fuel filter, bad pump and kinked lines being the usual culprits in that situation.

 

I had this exact same problem with a 76 350 that i transplanted into a 63 truck. It would pop out the carb at higher RPM. But would idle great and run prefect in the lower RPM ranges, The problem turned out to be a wiped exhaust lobe on the cam. I pulled the valve covers and rotated the engine by hand watching the rockers move up and down and noticed that one wasnt moving as much as the others. I removed the rocker and watched the pushrod by itself- not much movement- which meant that the lifter was likely good as it wasnt moving much even unloaded. After making sure that the valve moved freely, i pulled the cam and found two wiped lobes.Replaced the cam with a comp xtreme energy cam- problem solved I figure that the exhaust valve was opening just enough at idle and low rpm to hiss out enough exhaust not to cause a problem, but at higher rpm, pressure would remain in the cylinder after the exhaust stroke, so when the intake valve opened the pressure was released into the intake causing a popping noise and loss of power.

 

KIngsnake's right. I missed that possibility on my original reply.