I have a really heavy, 1987, 26 foot motorhome running a carbureted (vacuum secondary) 460, three speed automatic with dual rear tires. Idles nice, no indication something is wrong with the lower end of the motor. There are a couple things wrong:
The vacuum secondaries do not open on the motor. Carb has been rebuilt twice and looked over by a couple of pros, and they open when the carb is on the bench with air being sent through the primaries. There is ample vacuum at the manifold (measured at 20-25 from a vacuum port) and there are no obstructions preventing the secondaries from opening when it's on the manifold. The exhaust was cut off to see if there was an obstruction, but there was no difference in performance. Even if the secondaries are opened manually, they won't stay open on their own.
Loss of power on the highway under load, and an increase in engine temp. Normal operating temp is 200. I've had it get up to 235 before being able to coast it back down to something more manageable. There is no automatic kickdown, I always have to drop it into second manually to keep up speed to 45-50 mph. Even on a slight incline, keeping the speed around 55-60, the engine temp creeps up.
Kind of at wits end trying to figure out what the problem(s) is(are).
Anyone?

 

Hard one, I am sort of leaning toward maybe an intake leak, letting air in and causing a lean condition that will raise operating temps, also contributing to maybe not adequate air velocity maintained to hold secondaries open, since air might be sneaking in from elsewhere, next suggestion is something I have seen before, a stuck secondary float, and when the secondaries open, the air flows until it leans out, then it gets hotter from lean condition, then air velocity can't be maintained since engine is not building power to accelerate rpm, This can be diagnosed by putting a stop gap in the linkage, like a screw, zip tie, wire, anything that will hold the secondaries open. If you see problem get worse, or temp climbs or symptoms of lean condition, correct sooner than later. Next is remove air cleaner and open seondaries manually while looking down throat with light. you should see gas coming from the venturies, in the secondaries, both of them, if you don't see a nice mist being drawn from the venturies, look or the problem. Those motocraft carbs like to do that sometimes.

 

 

Power problem solved. Yes, 3 years later.
While I'm sure everything I did helped a little, it wasn't until I fiddled with the vacuum advance pot on the distributor that did the trick. The pot was set to advance timing beginning at 14in.Hg, while the motor idles in drive at 17inHg. The timing didn't advance until I was well into the pedal. I used a 1/8 allen wrench to turn out the pot, and I got it to about 15in.Hg. Significantly better throttle response. I plan on tweaking it closer to 17in.Hg to get the best drivability.
Over the past 3 years, I up-jetted the carb to #63s (Holley said heavy trucks with the 460 used them), put in an 8.5in.Hg power valve, replaced the distributor (digging out the old one that was stuck in the block with a chisel - in December), cap and plugs. I got the secondaries to open up after all this, but going from barely limping along to WOT was a bit much. I really noticed the difference with the vacuum advance.
I was told by a pro that my original radiator still had good flow. The water pump was replaced, as were the hoses with non-collapsing ones, a new thermostat, and everything was flushed. I put in a new 2-row aluminum radiator this Spring. I really haven't had the time to test it, but I trust it more than the old radiator with "good flow." I'm wondering if this, coupled with the resolved timing issue keeps the heat in check.