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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.
Gary

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Old 05-29-2011, 05:31 PM
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Sounds like your figuring it out, what cam do you have in it that the vac is so low. Some run a vacumn cannister for power brake vacumn, maybe you could incorporate that in the stsyem and run off that. Must have some wicked overlap. How long have you had this prob, did it just start, maaybe if things were good before it is actually trans related not vacumn. Just a thought.
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Old 05-29-2011, 05:58 PM
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vacuum modulator with red and black stripe are for low vacuum application.

harsh engagement into gear can be many things:

High RPM idle with tight converter.

Cushion plates removed from forward drum

PR spring from shift kit does raise line pressure.

Do not run vacuum line from a storage tank to the trans modulator. You need direct manifold vacuum
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Old 05-30-2011, 09:17 PM
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Truck Specs

CAM: 450 lift, 224 duration, 206 lobe centre.
Heads: high output 416 heads unshrouded with large valves

As Crosley noted, needs to run off of direct manifold vacuum but you can switch to mechanical modulator.
Also, it has been this way since the engine/tranny was put in and started for the first time so it is not new.

Thanks for the info Crosley on the red/black stripe unit, I will get that one and try it. Do you have a range that it works with? (to see if I meet its criteria).

The RPM is set and stable at 800 with choke off. You do NOT want to shift with the choke on as it is a heck of a kick in the pants.
Can't speak for the tranny internals specifics as it was that way when I bought it. It does have a stage 2 shift in as per the seller and was rebuilt.

Are there potential problems running an all rubber line to the modulator (as stock is mostly metal with 1-2" of rubber on the end)? I bought the proper vacuum line. I was lazy and did not want to spend the time bending a metal tube but will if it is deemed a good idea.

Thanks
Gary
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Old 05-31-2011, 07:17 AM
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question

Crosley, just a clarification question, are you saying the modulator with a red AND black stripe are low vacuum? Or are you saying the ones with EITHER a red or black stripe?

(Not sure if they make then with 2 color stripes)

Thanks
Gary
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Old 05-31-2011, 07:42 PM
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Just cn't figure out why your vacumn is so low, that isn't a major amount of lift so should look into why vac is so low first and that will help the problem if you find something to raise it a bit, just thinking.
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Old 06-01-2011, 10:07 PM
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Quote:
Originally Posted by ggevaert
Crosley, just a clarification question, are you saying the modulator with a red AND black stripe are low vacuum? Or are you saying the ones with EITHER a red or black stripe?

(Not sure if they make then with 2 color stripes)

Thanks
Gary

The large diaphragm Modulator w/Red & Black stripe is for engines with Low Vac signal.

What is your Cam's part#? I agree^^^, shouldn't have low Vac unless this a Low lift circle track cam etc.. "106*" Lobe Seperation angle Cam?
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