Once I adjusted mine it will hit 12psi floored, at anything above about 2500 rpms. It is not a slow climb. I feel your reluctance to really get into the spring especaily knowing that the snap ring is a pain, but think you really have exhausted all other reasonable avenues. Sorry no magic wand....

By the way I never did do anything to the IP on the 300. Now the 240 is a differant matter, full on tuning plus manifold change.

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1977 240D turbo