The K0 was known for this, here's the causes you might be facing, 40 years later:
1. The jets were too rich to begin with, the emulsifier tubes too restricted, and the plugs too cold of a heat range. Taken together, then driven in town or commuting work, the bikes used a set of plugs every 500-1000 miles.
The fix for these original problems were:
The jets were reduced from 120 to 115.
The emulsifier tubes were drilled out to .039" on the upper 8 holes.
The spark plugs were changed from D8E to D8ES-L (Heat range approximately 7.5, extended tips). The ND plug X24-ES-U is far better than even this plug, which is unavailable today (except in an "R" version, will get to that later).
2. If the pipes have been changed to HM341 baffled types, the stock jetting MUST be changed to match the jetting found in the 657 series carbs: i.e., 105 or 110 mainjets, drop the needle one notch.
3. The spark advancer came on too quickly in the K0 engine, which caused a sooting problem in the range of 1800-2500 RPM (this was changed on the later model K1, and again on the K3). Run the engine above this range (3k minimum), or cut one-half turn off the spark advancer's coils to slow the spark and generate a little more low-end heat from the retarded spark. This helps. It also takes some of the suddenness out of the departure from a dead stop, civilizing it a bit in traffic.
4. Run a lower octane fuel. Today's fuels are engineered for a different flame rate from the fuels of the K0 era, so premium in those engines can cause a lot of fouling. Run midrange fuel on the hiway, regular in heavy traffic, to raise the specific heat and help clean the plugs. You'll probably also find that Shell's nitrogen formulation will help keep those plugs cleaner longer.
Spark plugs: the fouling problem was so bad on these bikes that Honda asked NGK for help in 1971. The result, in January 1972 was the D8ES-L plug, which had an extended tip and heatrange halfway between the D7E (too hot for hiway) and the D8E (too cold for heavy traffic). The extended tip helped burn off the soot during hard acceleartion (like freeway onramps), so it helped a lot. Then, in the summer of 1972, Nippon Denso (NGK's parent company) brought out their "U" series plugs, which have a smaller center electrode with an extended tip and a U-shaped ground electrode which maintains the proper gap much longer than on a standard ground strap. These instantly became a bit hit on these bikes, as they finally fit the all-around driving profile and stayed much cleaner, much longer. I use those exclusively, to this day, and put them in every bike I tune: everyone notices it immediately. Today, I have to find these plugs on line, but worth every cent, IMO. In about 1996, the D8ES-L plug was discontinued. Today, the DR8ES-L plug is again available (it probably fits some modern bike), but using these with stock coils means you should also change your sparkplug caps to new NGK non-resistor caps, if you have stock coils. The plugs have enough resistance to run that way, without the extra cap resistance.
The sandcast K0 had about 40 degrees of spark advance timing, while the diecast dropped this to about 32 degrees. The lesser one ran the plugs a bit warmer from the retarded timing, which also helped. Later model 750s had even less advance, especially those with the HM341 baffled pipes. This helped even more. By the K4, with 105 main jets, baffled pipes, 28 degree spark advancer, a little more static timing at idle, and the owner's manual recommendation of Regular Gas (instead of premium) by Honda, the plugs stayed pretty clean.
But, we won't talk about the loss of HP from these changes...
