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Discussion Starter #1
Hello all. I have scoured the search function but am still having trouble.

I installed my new 406sbc last week in my '69 and I have been struggling to to find an engine vibration in the 1000-1100rpm range. Anything below or above this range(up to 4k) smooths out pretty good but for that 100rpm range it shakes the whole dash and steering wheel pretty bad. (It makes idling down the street pretty embarrassing)

Prior to assembly, I had the rotating assembly balanced (external balance) at a highly reputable machine shop and I have already eliminated the drivetrain as the culprit by observing the same vibration with the torque converter disconnected from the flywheel. (Happens in Park, Neutral)
I have tried adjusting the timing and I am currently running total advance @38 (14 initial + 24 mech) all-in at 3000rpm. I am pulling good vacuum at 17" idle (~800rpm) so I do not suspect a vacuum leak. The engine runs and sounds great but the plugs read a little lean(White, no brown). I am using energy suspension poly engine mounts and admit that the engine sits slightly higher in the frame mount groove on the passenger side; but would this make that much of a difference?

Any ideas would be greatly appreciated.

My setup:
406sbc (10.8:1 compression)
Scat 9000 ext balance crank
5.7" Scat forged rods "stroker clearanced" (yeah right)
Externally balanced (8" balancer, sfi 168 tooth flexplate)
Comp Cams Hyd Roller 280HR (280 dur, 224 @.050, .525 i/e, 110 lobe ctr)
Edelbrock Performer EPS intake (2701)
Edelbrock Performer RPM heads 170cc, 70cc (60719)
Edelbrock Performer 750cfm Carb (1411)
 

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I have the same engine mounts and have not observed any problems like yours. Mine were tough to line up too, but once I got the bolts through they worked fine.
 

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Discussion Starter #4 (Edited)
Could it be misfiring or a lean spot..
Thank you very much for your feedback. :)

I do not suspect a misfire; could be, but how would I diagnose it in that 100rpm range? As for the lean spot, I am at the point in my idle mixture screws where if I go any more in the rich direction it begins to stumble. Is there a trick to isolating a lean spot? Do I need to jet up my primaries? I have read that the 1411 comes lean stock and I haven't changed the jets yet.
I seem to remember in my last 406 (Scored #7 cylinder, R.I.P., see decell smoke post) that it had a similar shake to it. I re-used the balancer, flexplate, and carb but I had the balanced components inspected at the machine shop prior to balancing.
Sorry to ramble, but I am hoping to spark any additional leads.
 

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I had a similar problem with vibration that would get so bad that the bolts holding the tranny would loosen on the drivers side causing the block and tranny to "slap" each other if I wasn't constantly checking/tightening them. Turned out to be a wrong flex plate put on after a rebuild.
 

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Discussion Starter #6
Found it!

Well after narrowing it down to the balancer and flexplate, we pulled the tranny last night and found that the flexplate was put on wrong!! (Tranny had to come out anyway to make way for the new 700r4). It is a new crank and apparently it didn't come with an alignment pin installed.(didn't even notice) I was busy assembling other components when my brother-in-law was putting it on and didn't even think to make sure it had the alignment pin in there. Well, lesson learned. At least it was something 'simple'. ;)
Still needs a running test, but 99% sure this case is closed. :) Thanks everyone for your input.
 

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I NEVER picked up on why the dowel pin, but I think it is b/c we get in a hurry or tired or whatever and stab it on and flat miss the holes IF it ain't there!!

And I've caught myself a couple a time's too...................

Ever try to use flex plate crank flange bolts w/ a flywheel?? Another trip ta the Parts Store by me, He, He!!!!

pdq67
 

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Discussion Starter #10
Darn Brother -in -Law... he's probably a nice guy so take it out on his sister.:thumbsup:
LOL yeah he is a real nice guy and I am pretty sure that I am not going to tell him. He and I busted our butts getting the front subframe/brakelines reattached, the long block assembled, dropped in, and the engine/ front clip all re-assembled in a single weekend.( I should post the pics..) I know he would feel real bad if I told him that he messed up. He is an oldsmobile guy and I'm pretty sure that olds' flexplates only go on one way(without an alignment pin); so maybe that gets him a pass. ;)
 
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