I was running up my latest plane; a Wild Hare 28% Extra with a Stihl 62cc that I converted. With the cowl off, I couldn't get the mixture to go too rich. So I wasn't sure it was going rich enough. I could just keep opening the needle and nothing would happen. There is no room for a velocity stack in the cowl, so I figured the prop blast was blowing away the reversion, if not actually sucking fuel out of the throat of the carb by venturi effect when the prop blast went by the open carb throat. So, I thought, "Well, when the cowl is on, the prop blast won't hit the carb directly", so I installed the cowling.

Running it with the cowling on, it right away went too rich. Even with the high speed needle closed down to 1/2 open, it was still pretty rich. I thought "Well, we know what causes that!" So, I took the cowl back off and ran a static pressure line from the carb regulator cover back into the fuselage so that the carb wouldn't receive cowl pressurized air. Back on with the cowl.

Now, unbelievably, the engine wouldn't go rich enough again. Hard to believe but enough prop blast was getting to the carb to affect it. So since I had noticed when running it with the cowl off that when I placed my hand in front of the carb that it would richen up, I decided to make a baffle to do the same thing with the cowling on.

The pics explain it. Now it tunes perfectly with the cowl on. I wonder how many of the "Mixture Changes in Flight" problems we all have diagnosed as being the static line, might also have had a problem like mine; the airflow past the carb throat disturbing carburetion.....

AV8TOR