Isnt a "reduction in higher rpm power" a "stumble"?? Just a thought.
NO. ESPECIALLY "NO" since you've said it happens when "cruising" and "in any gear" You aren't at high RPM when the thing is giving you problems.
"Reduction in power" should be taken to mean that the engine runs good
--but--does not rev as high; and/or does not make as much power at that high RPM. It's not that the engine runs "bad" in any way, just that the power isn't what it could be. And it's a simple matter of capacity: CFM required from a carburetor is MOSTLY (not completely) related to two factors: Engine size (CID) and maximum engine RPM.
Mathematically, it looks like this:
CID X Max RPM / 3456 = carb CFM needed
Note that 3456 is a constant that takes into account the number of cubic inches in a cubic foot; and the fact that these engines are 4-stroke not 2-stroke.
Of course, the underlying assumption is that the carb has been calibrated to supply a proper amount of fuel to go with that air
. So--up to the airflow capacity of the carb--the engine runs good, runs strong, doesn't hesitate, stumble, surge, bog, spit, cough, backfire, etc, etc, etc. Of course, anything that screws up the air/fuel ratio (i.e., wrong jets, wrong metering rods, air valve that flops around like a wounded guppy) will also impact power--and, usually--provide driveability problems as well. So it's the "fuel curve"--the air/fuel ratio provided by the carb under different operating conditions--that is responsible for air/fuel-related driveability problems, not
the capacity of the carb to supply air
Can that mathematical formula be refined to be more accurate? Sure!
Add in a factor to account for the efficiency of the engine to pull in air--which is generally called volumetric efficiency, (VE) but "should" be called mass efficiency. And that is primarily related to camshaft opening and closing events; but secondarily related to inlet and exhaust port shape; ram tuning effects on the intake; exhaust pulse tuning on the exhaust side, and suchlike.
Typical figures would range from .75 to .95; but that's not absolute--there are specialized engines with better than .95 just as there are primitive "dogs" that would be under .75
Now the formula looks like this:
RPM X CID / 3456 X VE = carb CFM needed
Can we get better yet? Yup. Add in a Manifold Correction Factor (MCF) to account for the efficiency of the intake manifold. Two-plane intake manifolds need a bigger carb than a single-plane; and within the two-plane or single-plane design, some manifolds are just plain more efficient. So--to keep is simple--Let's say that intake manifold correction factors range from 1.1 (very efficient) to 1.5 (stock two-plane junk) and that the dividing line between two-plane and single-plane is about 1.3. So a very efficient two plane manifold and a very poor single plane manifold use about the same correction factor.
RPM X CID / 3456 X VE X MCF = carb CFM needed
And that is as refined as I know how to compute it. By the way, this formula came directly off the Edelbrock web site.
For the 383 engine that was the subject of the first post--I wish you had said what the max RPM is. I'll take a WAG and say 6,500 rpm, and that your VE is somewhere around .80.
383 X 6500 = 2,489,500
2,489,500 / 3456 = 720
720 X .80 = 576
576 X 1.3 = 748 carb CFM needed
Your best "baseline" carb is a 750. But, ANY size carb that has a proper fuel curve will give you an engine with no air/fuel related driveability problems. An air valve on the secondaries that flops open before it should will DEFINITELY screw up the fuel curve--and that's where I'd be looking on your existing carb.