If you adjust the wastegate, and it has no effect then you need to check alda, the lines from the manifold to the alda, overboost protection, etc. Stock it is normally limited at 12psi.

I removed the alda and everything that goes with it when I had a 300. Boost was around 9psi, and I needed to adjust the wastegate to get the correct 12 psi. the springs weaken over time, so it is not unusual for it to be lower then it should.

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1977 240D turbo