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Old 05-29-2011, 12:04 PM
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Low Vacuum at idle - harsh shift

Any thoughts on low vacuum at idle and associated initial harsh shift into drive or reverse?

I am looking at getting a mechanical vacuum modulator as that seems to bypass that problem but want to review options first.

I'm a tranny newby but I'm a quick learner so any help/tips/hints are appreciated.

My engine produces 11" of vacuum at idle at the carb. All the research I have done indicate the harsh initial shifts are because of the low vacuum causing the modulator to think it's WOT so all line pressures are at full. All other shifts are fine.

I eliminated a possible vacuum modulator problem by purchasing a new one and testing it with a vauum pump.
I purchased a new red-stripe one but I am having problems finding info on it's range or, for that matter, for any of the modulators. I read somewhere that the red stripe on is 12-15" so that was the closes to my reading and that is why I bought it. I have not yet played with the adjustment screw inside the nipple but plan is back it all the way out bringing it as close to 11 as possible.

I am also running a rubber line from the carb to the vacuum modulator so on the list is replacing it with a copper 1/4" line with 1-2" of rubber on the end. Not sure if that is a big impact on this but it is noted as the proper way to do it.

Any help is appreciated.
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