What power valve do you have?
For most of the part, a 500 Holley on a 200 will have poor gas velocity, and poor vacuum. The 2v has massive intake ports which are over sized on a 250, let alone a nice little 200!
The power valve is an auxilary richeing device which is equal to going up 8 main jet sizes on both jets. Most V8 Holley 500's have 10.5 or 8.5 power valves. In its normal 283 to 460 environment, there is heaps and heaps of vaccum around to ensure the power valve hardly ever opens.
On a 200 six, there is less vacum. You have lost half the cubic inches Holley designed the carb for. When the part throttle vacuum is below about 8.5 inches of mercury ("Hg) on a vacum gage, the 8.5 power valve will drop extra fuel into the intake manifold. Up to 10 teaspoon fulls a minute.
Lets look at a stock #4412 500 cfm Holley on a 200 2V, and another in a 351 2V Mustang. Power vave is the stock 8.5. Ambling around town, keeping up with the Accords, the six cyl car will be at about 8.5"Hg often. A 8.5 power valve will be close to opening. The 351 2v with a 500 would be reading 10.5" hg. If you have a 4.5 power valve, it will drop fuel in later, and make it more economical. Most of the time, it will be very lean, but when you stomp on it, it'll richen out like a race car. 90% of the time, it will run with no power valve at all.
The solution?
Easy. Go for a power valve which opens late (low number), and very lean main jetting (low number). Any holes in the fuel delivery can be taken up by bigger pump jets squirters. 31's or 34's instead of 25's or 28's.
The key to making the 200 run leaner is starting with a very small main jets, and a very rich power valve. A set of 63's with a 4.5 power valve will do better for economy than a set of 68's with an 8.5 power valve.
