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My C-4 was rebuilt with Art Carr guts about 5 years ago. The trans sat for that many years. I installed the other day with new motor. I noticed that if I ease up the rpm scale to above 2000, the 2-3 shift will rev. before it engages. This is fairly predictable. If I ease up to about 3K rpm, it will rev about 500 rpm between shift. Now, I 3/4 throttled this monster and the 2-3 shift at 4K rpm didn't skip a beat. Any idea what could be causing this?
 

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Sounds strange but I believe it. I never had that problem so I wouldn't know what the cure is.
 

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Since the trans sat for so long without being used maybe the intermediate servo is sticking.The problem might go away after some more miles are put on it and the parts/seals loosen up. It probably wouldn't hurt to pull the intermediate servo and clean it up and make sure it moves freely then reinstall it with some new seals and give it a try.
 

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Sounds to me like a seal on the intermediate servo. Possible the pressure is low from sonething getting hard from setting all those years? I've seen them do it but never had one do it.
 

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What you have by the sounds of it ,is a slight flare on the 2-3 change,The intermediate band releases quicker than the direct clutch applies.More throttle = more line pressure ,hence the shifts ore ok.I have found that the 'A' servo has a tendency to do this more than the others,though it could be just a coincidence.The servo is on the passenger side in your case and has a letter cast in the side cover.Some servo's are better than others.
There are some aftermarket billet servo's that claim to stop the 2-3 flare ,but i have heard some guys still have the prob.
The best fix is as suggested above and start checking the basics and check the front band adjustment..
 

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The 'H' servo is one of the popular ones for high performance use.I would start with what the other guys recommend above as a starting point and check the servo seals and check the band adjustment.
 
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