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#1
05-21-2020, 09:25 AM
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Erratic vacuum - new 4oo
The guys over at Barr's Restoration fired up my 67 400 HO and even after some preliminary tuning, the vacuum gauge is bouncing like crazy. They checked for nay leaks or cracked hoses, and everything seems to be buttoned up. This is a brand new motor, though some of the assembly is 'old'. The heads (670) were done by Gessler about 10 yr ago, and they were bagged up and stored. The short block was done in 2015 and the whole motor was put together and bagged. The mechanic thinks it might be valve train related. They are still looking into it, I just thought I'd ask some of the folks here.
I have a short video of it, but am not sure how to upload that.
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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#2
05-21-2020, 09:32 AM
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I tried uploading the video here.
__________________
Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#3
05-21-2020, 10:10 AM
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Make compression tests, valves may be sticking.
Or on the opposite, worn or loose valve guides. Also check for weak or broken valve springs. |
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#4
05-21-2020, 12:32 PM
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Sounds like a tight valve as far as adjustment, that it's not fully seating. If they're hydraulics, and the valve train is adjustable, I'd be looking to run the adjustment again with the engine running.
Was the engine properly broken in, running at 2000 RPM for 15-20 minutes? If not a flat lobe or one that the pushrod isn't turning at idle denotes a problem. And as has been said above, a tight valve guide, or broken valve spring, are other causes of your symptoms. A defective valve job can cause the same symptom, bent valve, burnt valve. A compression test, or a leak down test, will narrow it to the cylinder causing the problem. The tech said valve train problem, and I'd agree with that assessment. Fresh engine I'd be looking at misadjustment of the valves if they are adjustable first. From watching the video there appears to be a miss from watching the air cleaner stud vibrate. Idle it too high, slowing it down would most likely show a dead/weak cylinder.
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Brad Yost 1973 T/A (SOLD) 2005 GTO 1984 Grand Prix 100% Pontiacs in my driveway!!! What's in your driveway? If you don't take some of the RACETRACK home with you, Ya got cheated    
Last edited by Sirrotica; 05-21-2020 at 12:45 PM. |
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#5
05-21-2020, 12:51 PM
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Who did the valve adjustment? Ask them their procedure.
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 466 Mike Voycey shortblock, 310cfm SD KRE heads, SD "OF 2.0 cam", torker 2 373 gears 3200 Continental Convertor best et 10.679/127.5/1.533 60ft 308 gears best et 10.76/125.64/1.5471 |
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#6
05-21-2020, 01:32 PM
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Wow...looks to me like an intake valve open issue to cause the vac to drop instantaneously.
George
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"...out to my ol'55, I pulled away slowly, feeling so holy, god knows i was feeling alive"....written by Tom Wait from the Eagles' Live From The Forum |
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#7
05-21-2020, 04:19 PM
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Either an intake valve is not closing completely or it's sticking open.
Quote:
Been there and done that with a BBC in my friend's truck. Two intake valves and one exhaust valve mis-adjusted by the assembly 'mechanic'. Ran a little rough, vacuum fluttered, slight pop in the exhaust (from the open exhaust). Thankfully once we got the valves adjusted correctly, the vacuum was steady and the engine smoothed out and ran like it should. Good luck! |
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#8
05-21-2020, 08:03 PM
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Get them to try drizzling a little marvel mystery oil down carb to lube valve guides.
Don’t laugh. |
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#9
05-21-2020, 08:05 PM
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And now I’ve seen video, definitely check valve adhjustnent
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#10
05-21-2020, 10:02 PM
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Appreciate the feedback. This is a hydraulic roller cam with 7/16 studs, so there is some adjustability to play with. I traced out to the engine builder a little while back to ask about lash settings. Unfortunately, he did not return my text/email.
Sent from my iPhone using Tapatalk
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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone. The frogs take up where they left off. |
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#11
05-22-2020, 06:04 AM
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Have the guy put .005" lash into all the valve adjustments and then see if the issue is still there at idle.
Also as commented on a compression test needs to be done with all the plugs yanked out.
__________________
Wernher Von Braun warned before his retirement from NASA back in 1972, that the next world war would be against the ETs! And he was not talking about 1/8 or 1/4 mile ETs! 1) 1940s 100% silver 4 cup tea server set. Two dry rotted 14 x 10 Micky Thompson slicks. 1) un-mailed in gift coupon from a 1972 box of corn flakes. Two pairs of brown leather flip flops, never seen more then 2 mph. Education is what your left with once you forget things! |
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#12
05-25-2020, 02:08 AM
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just to be on the super safe side, try another gauge? try a different manifold port on carb if possible. ??? just an idea. prob not but..
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69 Gto, 390 posi gears,th400 w/jim hand converter/406 pontiac/#64 HEADS/ 10:1 compression/ 190 PSI with/ TRW 160 thou domes / hooker headers 1 7/8, PRW 1.5 rockers, 405 Crower Springs, Holley 750 vac with proform upgrade, Performer RPM on points / 284 H Single Pattern Crane |
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#13
05-25-2020, 07:21 AM
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The hollow bolt that your taking a reading from is atop a single runner your going to get a spike every time that cylinder hits
Move gauge to front of carb to read the plenum more accurately avoiding a ported source
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A man who falls for everything stands for nothing. |
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