I've never heard one do that! I've had engines in my old trucks and how the HH .12 come on the pipe, I love the sound. It's like kickin' in the secondaries on a 4-barrel carb! All of my other boats have never done that.
My Rigger/Mac .21 did that at the last race I was at in Jacksonville Fla. ........ ran like a scolded ape, sounded like it shifted into second gear and picked up aabout 5000 rpm for no reason whatsoever.......... Then I had to rebuild it :
You should see it on a 21 outboard. Mine tends to do it just coming out of the turns as you straighten up - Can be a little bit of a handfull but you don't want to back off the throttle as it's going so fast and the sound is awesome!
Still only the 1 complete O/B tunnel - the yellow F1-11 + N/R.
I have the WOF30 plans & some of the foam & timber - just need the time :
Been racing the rigger alot this year but the tunnel hasn't seen water for a while - no-one racing 'em anymore ??? I'm going to try and give it a run on the weekend if the damned wind will go away. I made a new pipe mount for it as it failed on me at Canberra in the "surf"