From what I can see, the only change was to remove the lower portion of the 750 air box and use a plate to hold on a K&N filter and block air that may bypass the filter. Is that what we are evaluating? If so...
This removes the restriction caused by the area of the stock inlet to the lower air box. This, in effect, shortens the inlet duct moving the atmospheric pressure source closer to the metering jets. In addition, the filter membrane change from the paper type to the K&N, changes the pressure drop across it (likely lower) which also adds to changing the pressure seen at the fuel metering jet exits.
The sum of this change is likely to make the carbs supply less fuel for the incoming air as compared to before the change, or the original configuration.
This would seem to be borne out buy the report of now getting 65 MPG. (Vs what mpg beforehand? I assume lower?? Note lack of supportive data.)
Anyway, with that sort of MPG, I would expect the spark plugs to be mostly devoid of deposits. The concern would be changing operation to a lower altitude, I.E. If normal operation was at 5000 Ft, then traveling to sea level) where an already lean mixture would become even leaner. (possibly with pinging and or detonation, as there is no device to make the engine richer/leaner with altitude changes.)
The exposed air filter is, of course, open to water contamination. As the K&N filter is an oiled membrane, water will wash away the oil and effect its dirt trapping performance. When it does get wet, the filter then needs immediate service to clean and re oil. I simply don't know if water makes a change to it's membrane pressure drop like it does with paper types, by rearranging/compressing the fibers. I would hope not since the K&N is touted as cleanable and reusable.
It is not clear than any engine mods were performed to require an increase in air flow beyond what the original lower air box inlet provided. Certainly a big bore, cam changes, etc. may need and inlet opening beyond what the stock inlet provided.
To remove the instant maintenance attention a wet filter would instigate. I would favor a change to the lower air box inlet, increasing it's inlet area. If the K&N did not provide enough filter area to achieve the desired membrane pressure drop, you could alternately stack two of them and make a chamber collar to allow enclosure of the double stack.
This assumes using the same carb inlet diameter as the stock original carbs. An increase in inlet size there would require a change the inlet couplers. And changing the spacing of the carbs would mean the upper air box probably wouldn't line up anyway.
I have no Cb750 air box spares to test stacking/inlet enlargement idea. I don't know if it would even fit. It's just a working proposal based on my current understanding of induction theory and operation.
Of course, it is also assumed that carb jetting changes be made as appropriate.
Hope this is of some value.
Cheers,
