checking tillison pop off pressure

[doug tenney]

I know people have used plastic needles successfully in the past, but they've never worked for me.

David may be right. This carb may have been modified for alcohol.

Compared to running on gasoline, an engine running on alcohol requires a lot more fuel That;s why you can get away with an inlet needle that doesn't seal completely. Still not good.
Just buy a rebuild kit that includes the standard rubber-tipped needle.