engine vacuum ??

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75 fantasy

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« on: July 17, 2013, 10:56:31 AM »
back story....
i bought a boat from an old friend for its gm 350 motor, pulled the motor and sold the boat. friend said motor was just rebuilt before he bought it and said it ran great for two seasons and was only selling it because he was moving.
my problem is that after installling a new gm master, booster and disc brake conversion my brake pedal does not come back up fully(slight drag and lights stay on) have read that it could be low vacuum(min 17hg) or bad booster.

so my engine question is how much vacuum should a stock gm 350 have at idle and at 3000 rpms (from full manifold vacuum port)
currently at idle it has 14 hg
currently at 3000 rpms it has 20 hg   

thanks
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ka0tyk

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« Reply #1 on: July 17, 2013, 11:02:52 AM »
you should only need ~15 or so to properly operate the booster. 

things like cam and timing can affect the vacuum.

mobboss

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« Reply #2 on: July 17, 2013, 11:20:55 AM »
It sounds to me that you have not hooked up the vacuum line correctly, or you have a leak, you could also not have bleed the booster right.
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GT Jets

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« Reply #3 on: July 17, 2013, 11:29:49 AM »
14" to me says lumpy ass cam.

You may need to install a vacuum reservoir if that is the case.

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« Reply #4 on: July 17, 2013, 12:44:02 PM »
It sounds to me that you have not hooked up the vacuum line correctly, or you have a leak, you could also not have bleed the booster right.

i have checked and rechecked the vacuum lines - all good, checked the booster - all good on no leaks or fade.
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75 fantasy

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« Reply #5 on: July 17, 2013, 12:45:53 PM »
14" to me says lumpy ass cam.

You may need to install a vacuum reservoir if that is the case.

Sent from my SGH-T989 using SoCalJetBoats mobile app

it does sound a little lumpy at idle and it has a hot rod style booster - btw i got rid of the proportional valve to eliminate it as a cause.
note: the po before my friend had a windage tray installed so not sure why??
« Last Edit: July 17, 2013, 12:51:49 PM by 75 fantasy »
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75 fantasy

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« Reply #6 on: July 19, 2013, 09:57:13 AM »
GT Jets
So it's normal to for vacuum to raise with the rpms? (I would think yes but I'm not an engine guy) if its possible to explain in short why do larger cams have less vacuum?
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