RE: How sensitive is intake mani length on a little 4 stroke?
The $5 words for tuning an intake are Helmholtz resonance.
Using the basic formulas you can tune the intake ( make the intake tract resonate) to certain frequency. If the tuned freq matches the engine rpm you get a positive, supercharger effect and gain power. Operating out out of the freq range you get a 0 increase or an actual decrease in power.