Is 8-9 psi manif. vacuum normal in an almost stock 350 with the smallest thumpr cam? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 06-11-2012, 08:35 PM
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Is 8-9 psi manif. vacuum normal in an almost stock 350 with the smallest thumpr cam?

So, it's basically a stock 350 bottom end with slightly above stock aluminium heads, rpm air-gap intake and a holley 650 double pumper. Timing is 18 base, and 18 mechanical with vacuum hooked to ported so it doesn't affect anything at an idle.

I have the smallest thumpr cam (i didn't know anything about cams when I agreed to go with thumpr) and I'm only getting about 8-9 psi manifold vacuum. Doesn't that seem a bit low?

I saw some minor oily leakage from the rtv seal on the front of my manifold and just got done re-doing the intake with new gaskets and everything. Still 8-9 psi.

These are the specs on my carb and all I've changed is I lowered the primary jets to 68, and I drilled small holes in my primary butterflies so I could idle without having double my entire transfer slot exposed.

The funny thing is, it runs beautifully. Other than hard starts because of the 18 base advanced, rocking back and forth at low rpms because of my 2.73 highway rear end (soon to be 4.11), and a little stinky at times, it runs great, I don't even have problems with my power breaks. I mean, my freaking power brakes don't ever run out of vacuum with 8-9 psi?? Maybe my gauge is broke? I JUST bought it 2 weeks ago.

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Last edited by Zerocyde; 06-11-2012 at 08:38 PM. Reason: spelling
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Old 06-11-2012, 11:51 PM
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Quote:
Originally Posted by Zerocyde
So, it's basically a stock 350 bottom end with slightly above stock aluminium heads, rpm air-gap intake and a holley 650 double pumper. Timing is 18 base, and 18 mechanical with vacuum hooked to ported so it doesn't affect anything at an idle.

I have the smallest thumpr cam (i didn't know anything about cams when I agreed to go with thumpr) and I'm only getting about 8-9 psi manifold vacuum. Doesn't that seem a bit low?

I saw some minor oily leakage from the rtv seal on the front of my manifold and just got done re-doing the intake with new gaskets and everything. Still 8-9 psi.

These are the specs on my carb and all I've changed is I lowered the primary jets to 68, and I drilled small holes in my primary butterflies so I could idle without having double my entire transfer slot exposed.

The funny thing is, it runs beautifully. Other than hard starts because of the 18 base advanced, rocking back and forth at low rpms because of my 2.73 highway rear end (soon to be 4.11), and a little stinky at times, it runs great, I don't even have problems with my power breaks. I mean, my freaking power brakes don't ever run out of vacuum with 8-9 psi?? Maybe my gauge is broke? I JUST bought it 2 weeks ago.
Power brakes like at least 14 in/Hg, but if the brakes are working OK then either the booster somehow doesn't need that much vacuum or the gauge is off.

I would usually say you needed more timing, but 18 degrees BTDC should be about right for the small thumpr cam. But if you want to try a quick and easy test, simply hook the vacuum advance to manifold vacuum (plug the ported vacuum port), readjust the idle speed, and see if the vacuum comes up, and if so (it should) how much it comes up.

Alternatively, you can try more initial and less mechanical, but aftermarket distributors like the MSD do not allow for much less than about 18 degrees of mechanical advance using stock bushings. If you're using a GM HEI there are threads on how to limit the mechanical advance if you do a search.

You didn't need to drill the butterflies, that carb has adjustable secondaries to get idle bypass air.
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Old 06-12-2012, 12:40 AM
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Quote:
Originally Posted by cobalt327
You didn't need to drill the butterflies, that carb has adjustable secondaries to get idle bypass air.
I couldn't get a straight answer anywhere on exactly how to do that. But I figured the butterflies on it are just held on with a screw so I could swap replacements on once I found out the proper way.
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Old 06-12-2012, 03:01 AM
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Quote:
Originally Posted by Zerocyde
...I figured the butterflies on it are just held on with a screw so I could swap replacements on once I found out the proper way.
True. Not to get ahead too far, but one thing about removing the butterflies- be sure to grind the ends of the screws off level w/the shaft because the ends are peened or staked to prevent them from loosening and falling into the engine. If the ends aren't ground off the chances are real good one or more of them will snap off, leaving you w/a big headache. Use loctite and/or restake the new screws on reassembly.
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