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Bad vibs - chasing an engine vibration

Magnum fan

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Hello Mopar gurus,

Some quick info on the car as equipped: just purchased from original owner, 1978 Magnum GT 400 ci, edlebrock carb and edlebrock intake, mild cam (sorry that's all the elderly OO could tell me about the cam, no paperwork or specs on it). I have had it 2 weeks, love it. Just working out a few bugs...

I'm chasing an engine vibration, it's there at idle and cruising speeds, although more so just above idle when you start accelerating slowly. Once you get above 1200 rpm it vibrates less (or I should say I notice it less) it's still there...
Searched thru all previous posts to get ideas, here is what I have done...

1. Found one plug wire with a break, replaced all wires and plugs (gapped at .035). Vibration persisted. All plugs are firing as evidenced with a timing light.
2. Found vacuum advance line from carb to was split at both ends 1/4 inch (however advance was still working). Replaced. Didn't improve anything, vibration still there. I didn't get an opportunity to buy some carb cleaner to check for other vac leaks, I'll do that later this week.
3. Timing: Noticed 2 schools of thought regarding whether the distributer advance vacuum line should be plugged into ported or manifold vacuum. Line was originally on ported, switched to manifold vacuum, then after reading all the timing posts I could find, set initial base timing (with advance disconnected and plugged at carb base) to 18 degrees, all in timing is 36 degrees. The car was originally at 8 degrees, and admittedly accelerates better, but still shakes. By the way I also previously timed the car 18/36 while it was on ported vacuum, didn't seem quite as zippy on acceleration and the vibe were still there, so I went to manifold vac and again set the timing to 18/36.
4. Using a vacuum gauge and rpm meter, adjusted the carb idle mix screws carefully, according to previous posts guidance and edlebrock manual to maximize vacuum. I'm reading 17 lbs, steady, with good throttle response. In the end the carbs seemed setup OK and my settings were within an 1/8 of a turn of where they were originally.

Anyways, that's where I'm at. I have no experience with the 400 ci engine, my dad had some 360 chryslers in the 70' s and they were smooth engines. Right now I'm wondering:
A. If this could be a harmonic dampener problem, but it looks OK ( how can I diagnose that?)
B. or if the mild mystery cam could be all or part of the bad vibes.
C. or if a torque converter problem .

Could use some advice from the experts...

Thanks
Al (Magnum fan)
Update, at some point in the past other engine work (rear main seal replaced) was done, no idea if pistons, rings, valves are original. Given the OO saved all receipts since new, and I have no recepits for any of that type of work done it would seem they are original.
 
distributor cap cracked? or a engine mount broke?
 
Hello slimt,
Distributor cap looks OK, no cracks visible, no apparent carbon tracks, rotor looks good. Haven't inspected the motor mounts, will do later this week. Thank you for the tips...
Al
 
No problem..:) I have had simular issues.. just winging ideas from this end,, only other thing I can think of is a flex plate.. all the bolts in place? even a bad pully will throw off a smooth motor.
 
No problem.. only other thing I can think of is a flex plate.. all the bolts in place? even a bad pully will throw off a smooth motor.
Good points, I'll operate the engine without the accessories (ac, power steering etc) and the alternator and see what happens. Won't be able to do that until late in the week, I'll update y'all later...thanks again...
 
Hello hemi-itis,
Yes, does it in neutral too. Vibes are more pronounced at lower rims (shakes the door panels), settles out a bit above 1200 rpm, but still there...
 
Harmonic balancer?
 
Last edited:
Hello pp1rt and slimt,
I marked the harmonic balancer with a white pen, to see if it's broke loose, I'll drive it tomorrow, is that a good approach? The 400, from what I read is externally balanced...
 
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