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V-8 Engine Tech

Rhythmic vibrations from 383

Mopar Big Block Talk

by AndrewP » Tue Jul 10, 2018 1:52 pm

Hi gents,

I've bitten the bullet and ordered a set of the Accel 9001 leads whilst eBay had a 10% off sale.
My question is does anyone have a pic of how the plug leads are orientated (general consensus seems to be pointing down) and the run of the cables back to the dizzy?
A few of you have mentioned pointing the boots down, but I'd appreciate ppls input into what's worked for them. I'd rather not destroy these leads on the manifold..

Cheers

Andrews
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by slotcarone » Wed Jul 11, 2018 12:50 am

On my MKIII I have mostly 90 degree boots and a few straight. The 90 boots are pointing down away from the manifold. There is plenty of room below most of the plugs. :)
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by Dave 1141 » Wed Jul 11, 2018 8:20 am

Hi Andrew,
Re: positioning of HT leads, here's a few pics. This is how mine was when purchased so probably not the original layout. And yes, my coil has been moved to under the fuses.

Cheers

Dave
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by Richie » Wed Jul 11, 2018 9:08 am

At the risk of hijacking the thread, though it may not be the case, the pics you've posted there Dave 1141 appear to show a 'blank' radiator cap on the expansion tank - this should be a pressure cap.

I also note that the vacuum advance is disconnected too...

Hoop the new leads sort your vibration Andrew.
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by Dave 1141 » Wed Jul 11, 2018 11:34 am

Thanks Richie. It's actually a 16 psi cap but looks blank thanks to my rubbish photography. And no vacuum advance as I use a Rev-n-nator electronic ignition.

cheers,

Dave
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by AndrewP » Sat Jul 14, 2018 2:49 am

Thank you gents, all fitted and can happily report a nice smooth engine now.

Interesting as the vibration didn't appear at idle (assuming a cylinder was missing/down) and only really manifested itself between 1500 - 2500 RPM. I'm wondering if my old (newish) leads had a break somewhere in them causing issues at higher RPM?

Thanks again..just waiting for the rain to stop to take the car for a drive. Winter here :(
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by AndrewP » Sun Aug 26, 2018 10:43 am

Well...the first signs of the winter ending have allowed me to get into the garage. My success story earlier was a little premature as I was still experiencing the vibrations as described in the first post.

Unfortunately the new leads didn't really fix the vibrations (but as it turns out, I needed to fix the old ones) so...I bit the bullet and decided to swap out the Mity mounts and fit standard rubber mounts.

I can report that the mount change has made the difference between the 'harmonics' being transmitted to the occupant (me!) at certain revs.

As these were fairly stock standard rubber mounts I ended up having to modify them a little as both sides came with:

- too long a mounting bolt meaning the bolt fouled the block before they could be slid in. I allowed 2 or 3 remaining threads (above the nut) then cut them down.
- too long a locating dowel meaning they didnt locate on the chassis plate correctly. 5mm off allowed them to seat correctly

Overall I havent been impressed with the Mity mounts with some bad manufacturing defects (brackets not square, and the nut welded in wonky etc.). Something I dont expect for a set of mounts that cost nearly 5 times as much as a standard set.
Longevity will be the tell, although for the cost of a standard set, I can modify as needs be and I think I can replace in less than 1 hour if I get a good run.

Finally, did check that the torque converter/ring gear has no weights, so looks like all the vibrations I was experiencing were due to the Mity mounts being ill fitting and too hard.

Hope this helps others.
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by Steve Payne » Sun Aug 26, 2018 8:08 pm

There seems to be a lot of rubbish mounts out there at all prices. The nut is so wonky on some I chop it off and just use it like a nut and bolt.

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by AndrewP » Tue Sep 04, 2018 10:24 am

<sigh> ...ok..part deux and I'm open to all suggestions as to things to try.

So I spoke too soon, the vibration is still there and it's driving me mad
In summary, sitting in neutral, not moving, raise the revs to approx 1300-1400 and the rhythmic vibrations start.
I've replaced the leads and plugs, new distributor cap and rotor arm, nothing majorly visibly or audibly wrong with the engine.
It does feel 'lumpy' though. Not as smooth as a V8 should sound.

Back tracking a bit, recent changes have been an addition of an Edelbrock performer intake manifold..is it possible that an air leak could cause the rough running?
Pulling the plugs, all were dark brown but one was noticeably wet with what I think was fuel.
Any reason why one plug would be wet when it's fed from a common manifold?
Last time I did a compression check nothing showed up obviously wrong...is it worth checking cam lift etc?
The cam doesn't rattle on startup too much so I haven't considered it a problem..
Finally I have checked the flex plate and converter and there are no weights that I can see...
I have also removed all the belts and the vibration persists..

Any thoughts as I'm frustrated with this now..
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by slotcarone » Tue Sep 04, 2018 10:50 am

Do you feel anything when you are driving or only standing still? :)
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by Barrie » Tue Sep 04, 2018 10:59 am

AndrewP wrote:<sigh> ...ok..part deux and I'm open to all suggestions as to things to try.

Pulling the plugs, all were dark brown but one was noticeably wet with what I think was fuel.
Any reason why one plug would be wet when it's fed from a common manifold?

The main reason for a plug being wet is no spark.
Having new plugs and leads is no guarantee that they are all working.
Suggest you transfer the wet plug to another cylinder and see if it or the other plug becomes wet.
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by AndrewP » Tue Sep 04, 2018 11:30 am

Thanks for the replies all,

Slotcar
As things have progressed, Ive found its either/or. So driving, I get the vibrations as well as sitting still, in my driveway, in N.
Its 100% related to engine RPM, but not proportional to RPM (i.e the vibrations are cyclic and increase/decrease every 1-2 secs @ ~ 1300 RPM)

Barrie,
Will try and see if it follows the plug or the cylinder/lead/dizzy cap.
This was the old set that was pulled before putting in a brand new set.
#8 was the most noticeably wet

1,3,5 & 7
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2,4,6 & 8
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by Steve Payne » Tue Sep 04, 2018 7:25 pm

Are you sure those are the correct grade of spark plugs?

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by RayR » Wed Sep 05, 2018 12:37 am

Hi Andrew, an out of the box comment. Did you have the problem before you put the Edelbrock performer manifold on? I had a similar effect on my car, but not to the same extent, and exactly the same rpm. On mine I have noticed the effect changes with carburetor tune and almost disappears when the engine is fully hot. My thoughts are that there is a ""bad spot"" in the manifold design (as well as the really bad head port and carb matches!) that causes, I think, a lean/rich stumble at that certain rpm. I have accidentally 95% cured the issue on mine by putting in a larger, and radiused, spacer between the carb and manifold. This has also significantly improved the throttle pickup which was the goal - the improvement at 1500rpm was a side benefit.

Of course this may not be your problem at all!!

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by slotcarone » Wed Sep 05, 2018 2:14 am

So I am trying to understand--is the engine vibrating or is it shaking from a misfire? Big difference. :)
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