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I am about to replace the modulator valve on my C4. The question is.....is there a starting point for the adjusting
screw then so many turns out?
 
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Try using the new modulator "as-is". They are supposed to be "preset".
 

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You can adjust the shift points by turning the screw CW for shifts at a higher RPM or CCW for shifts at lower RPM's.

The adjustment is located inside the vacuum fitting. Usually close enough "out of the box".
 

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Like the others said, try using it right out of the box. If you want to try to adjust the shift points, go 2 turns at a time and see how it runs. The performance will also depend on what you have done to the transmission and engine. On my old 289 w/ C4 and no shift kit, tightening up the modulator worked pretty well, and I was able to get crisper shifts out of it. With the new 331, and a rebuilt C4 with shift kit, tightening the modulator didn't seem to help, in fact, i thought it made it worse. The tranny didn't shift at higher rpm, but it seemed to make it more of a clunky type shift and it took longer. When I backed the modulator it shifted quicker and cleaner.

Dave
 
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