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Tbuckets
New User
Jul 3, 2011, 4:41 PM
Post #1 of 4
(4304 views)
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Low Vacume Reading
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Hope you can help;I am new to posting so please bear with me.
The engine is a 60's Chevy Sb 327 ci,everything appears stock - no radical cam; except it has aluminum intake and edelbrock four barrel carb, 150 psi so their is no high compression there, rebuilt in a few years past- just ran compression test all within 2 lbs of 150 psi. Engine tested at 21 inch vacume 3 months ago before replacing intake manifold; after that I have had 13 inch vacume but the signal is dead steady -no movement. Have used two guages with same result. Normally I would say I missed on the intake install (I have done about 25 of these-never a problem-I take a long time and are very attentive to detail.). At the time after the intake install and before running the vacume test I was turning over the motor with remote starter standing in front of engine and heard a pronounced PING or maybe a lite Bang at the moment the stater engaged; noise not from starter but from front of engine. I originally thought I had a Hydro (gasoline) lock up (I have a riding mower which has done it twice and it sounded alot like that; but after puling plugs no gas was found; so noise was not a hydro lock- cause remains unknown and it has not occured again; but since finding the low vacume reading I am thinking maybe the chain jumped a cog on the timing chain; broke a tooth, or ... Since the guage says steady low reading is Late valve timing, late ignition timing ( It has been set at 10 BTDC),or vacume leak-carb-intake; (carbports all plugged up as well as manifold vacume ports- all been checked several times- carb is even plugged on secondary side as well as buterfly closed- carb to intake gasket checked and replaced, oil squirted around base with no change in reading or speed). I am thinking if it is the intake leaking it has to be on the inside- lifter valley area. I removed the valve cover breathers and pcv valve and plugged those with no change. Engine cranks and runs fine (I haven't driven it since problem started) runs smooth, revs fine.
I am thinking it is either the intake or the timing chain (late valve timing- would that be the result if it jumped a cog? I could probably figure that out in time but I am getting old and my head hurts thinking about it. I am sure some expert out their knows for certain.) BUT I hope some one will know a trick to eliminate one or the other. Interesrting that on the guage and manuals it says the low steady vacume reading is late ignition, vacume leak, or late valve timing and describes each on how to fix EXCEPTthe late valve timing- It tells nothing on that; I don't think chain strectch would come into play within 75 miles from the reading of 21 and the low reading now of vacume; do you?
I sure would appreciate any help anyone could give; I think this website is a great idea. For my experiance as a mechanic, I am not ASE certified or anything but have done my own mechanical work (old school -not fuel injection stuff) since changed my first cam in 1970 on a 65 Mustang; put a Lemans cam in 289 with 318 duration (advertised of cource) out of Muscle Parts Catalog( still have the two books).
Any help greatly appreciated...
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cardc1
Novice
Jul 4, 2011, 10:21 AM
Post #2 of 4
(4286 views)
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Re: Low Vacume Reading
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Chain is soo easy to get to on this engine thats the first thing i would have checked with a vacuum reading that low.
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Discretesignals
Ultimate Carjunky
/ Moderator
Jul 8, 2011, 8:54 PM
Post #3 of 4
(4264 views)
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Re: Low Vacume Reading
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From reading your post, it is kind of odd that this all started after you had replaced the intake manifold. Chain jumping would not only affect the manifold pressure, but the compression in the cylinders also. You might be able to see a vacuum leak under the manifold by plugging the valve cover breather and plugging the PCV hose. Connect a hose and vacuum gauge to the dipstick tube. If your engine is sealed good, you might be able to see vacuum build up if your intake is leaking underneath.
When watching the gauge, don't let pressure build up too much (over 3psi) unless you like blowing oil out the rear main or any other weak gasket.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Jul 8, 2011, 9:59 PM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Jul 9, 2011, 4:27 AM
Post #4 of 4
(4254 views)
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Re: Low Vacume Reading
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Tbuckets: Welcome but easy with the run on sentences like that - awful hard to read and follow.
Check valve timing when you see low vacuum like that - yes a chain can jump. Simple enough to just set to TDC and turn crank bolt both ways to turn engine watching distributor rotor and how many degrees it turns before distributor rotor turns. 5 degrees is enough. More is trouble.
A wild vacuum leak could probably do that low vacuum thing too like a brake booster or hose to it?
T
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