The 5200 is an econo carb, pure and simple. Great up to about 125 to 130 hp, not much more.
It's nothing to loose 20 hp on a 200 cube six if its not been set up on an engine dyno. You must do that first. The ignition gets optimised, then the carb. If you have no vaccum advance, or are running too little static timing without an agressive curve, then you'll loose fuel economy and power.
The compression on most later 200's is down to 7.9:1 or so. They can handle a huge amount of early advance, and then level off to 32-36 degrees by 3000 rpm.
The carb must be jetted with the correct jets. Even if you have got it running well, it may be loosing lots of power from jet sizes which are out.
Once you've done that, you'll get good performance and economy.
There's quite a bit of money tied up in that, but a good dyno operator can lock the ignition, and determine the right amount of advance, and you can have someone re-curve it on a Sun machine.
The second option is to benchmark the igniton system by using the mean best vacuum method the English used in the old days, and others like Ak Miller used it. You need a friend with a stopwatch, a pice of quiet road, better if it is a gentle incline. You wind up the advance by using lots of initial advance untill you get best idle vacum, and keep winding it up until the 30-50 mph( in 2nd or 3rd) increment drops to a minimum. When you hear audible knock you then back off a degree. Repeat until you get the best time without knock.
The Holley is more the carb for a modifed six which is likely to go to 155 hp net or so, with a good set of headers and the right cam. The Holleys faults are the need for clean fuel, proper float level, no leaks, and it isn't a great carb around tight corners like the Holley~Weber is. Often, the base tune-up isn't done with an exhast gas analyser, and its easy to have them running way too rich if the right power valve isn't used. Some guys fiddle with the pump squirter to ensure they have good progression when darting about on the highway. The thing is, seat of the pants isn't good enough to tell if you've got it well tuned.
