I don't see the exhaust hot air valve causing a hesitation. There can be other indicators for timing too high such as running rough. It can cause a rough idle if it gets too high. I tried my 170 around 20 and it idled too rough. I never drove it so I don't know if spark knock was present, but I didn't like how it idled at 20 so I backed it down to 12 and it idles great and is zippy as can be.