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V-8 Engine Tech

Rhythmic vibrations from 383

Mopar Big Block Talk

by johnw » Wed Sep 05, 2018 5:04 am

6, 7, and 8 look wet. With one dodgy plug or lead the issues can be almost unnoticeable on a V8.

Also, with standard leads, no electronic ignition, stock engine mounts, unbalanced driveshafts with new quality joints installed at home, the cars work perfectly for some, for years on end.

My money is on more than one silly thing causing this. A dodgy lead, and perhaps a couple of misbehaving plugs. I don't think it is just one thing. So if you fix something and it gets slightly better, don't assume that wasn't part of the cause. I would get another set of plugs of the correct grade, from a local shop and replace those 3 with different ones. I also like the engine temp and the IR reader idea. Also some of the parts off ebay could be fake, like NGK and Denso spark plugs for example. Google fake NGK sparkplugs.
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by AndrewP » Wed Sep 05, 2018 8:30 am

Thanks for all the replies.

To answer a few but not in any order.

Vibration due to misfire or engine vibration?...that's the million dollar question. New engine and gearbox mounts all standard rubber units. New leads and new plugs (the ones pictured are the old ones that have been in there for 5000ks or so).

The engine itself, sitting in the engine bay doesn't rock or lurch.
Only when revved to ~1300 rpm does it vibrate through the car (see the YouTube video in my first post).

I'm cautious at assuming the plugs pictured are an accurate reflection of the engine as these are after many hours of sitting idling and short journeys trying to chase this issue down.

I fitted a set of genuine (from a Repco, a large auto supplier) NGK plugs and will take it for a run and get everything warmed up and then pull the plugs again.
New plugs should be easier to spot any oil and potential misfires.
Finally..AFAIK NGK say they are correct for the 383.
BP5S.

Thanks all for the options..I'll work through them over the next few nights.
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by slotcarone » Wed Sep 05, 2018 10:33 am

The way you determine misfire or vibration is a misfire you will see the engine moving/shaking and a vibration you can feel but not see any movement of the engine. :) So this is a problem when the car is parked?
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by AndrewP » Wed Sep 05, 2018 11:21 am

Yep..and moving

The common factor here is the rpm. Always vibrates @ 1300 rpm.
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by slotcarone » Thu Sep 06, 2018 1:46 am

Sorry to be repetitive but when you have the car parked and you open the hood and bring the engine up the where the problem occurs can you see it shaking or is it smooth? Big difference and two totally different groups of possible causes. I am beginning to think your problem is exhaust system related. Making educated suggestions as to the cause can only come from information about exactly what it is doing. Of course if I was there it would be easy! :)
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by AndrewP » Thu Sep 06, 2018 12:27 pm

Hi Slotcar,

No problem at all...im thankful of any help!

So, I've attached a link to a video Ive just been out and taken in my garage, RPM @ ~ 1300-1400 RPM and the engine sits smooth....BUT you can see the indicator stalk vibrating (at about the frequency you can feel through the car).

Im interested to hear about the exhaust theory?...

Video here (updated link)

https://youtu.be/FoN6HaNC-A4

RayR

Sorry I missed your response first time...everything is possible at this moment in time. I dont remember the change of manifold causing this BUT, given that when i changed the mounts, the passenger side (RHD car here is AUS) mount fell out in 2 bits.
Its quite possible any vibrations were not being transmitted to the body due to the mount acting as a disconnect.
Now I have some nice new ones in there...perhaps it more noticeable?
I did remove the insulator (a bakelite type of material) when I fitted the new manifold mind you and just fitted the pad/gasket between the manifold and carb.
Perhaps I need to re-fit that bakelite block?, maybe an airleak here? I was worried that it might make the setup too tall and it foul on the bonnet underside...
As I said..anything is possible.

Thanks again all.

Andrew
Last edited by AndrewP on Fri Sep 07, 2018 12:17 am, edited 1 time in total.
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by Grant » Thu Sep 06, 2018 12:36 pm

If you spray some WD40 around the intake where you suspect it may be leaking Andrew and then anywhere on the joints of the manifold it should (if there is a leak) make some sort of difference in the running and also burn a bit of white smoke, if so you know its leaking Andrew.. but I guess you may already know that trick?Image
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by slotcarone » Thu Sep 06, 2018 9:43 pm

I have tried it twice and the video does not play. When you installed the new engine mounts now it is in a different position. It is now possible that one of the exhaust pipes is touching the bottom of the car. Check it out. :)
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by AndrewP » Fri Sep 07, 2018 12:21 am

Thanks guys..I'll get out into the garage over the weekend and work through it.
I've re-uploaded and updated the video link..YouTube had a Friday moment!

Thanks again..turning into a bit of a saga! I
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by RayR » Fri Sep 07, 2018 3:15 am

Hi Andrew,

I got the new video to play and it sounds very much like mine did, what happens when you rev it slowly? mine would stumble a bit until about 1700/1800rpm then clean up and smooth out. Also interested if engine temp makes a difference?

Good luck on the weekend.
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by slotcarone » Fri Sep 07, 2018 5:06 am

From watching the video it looks like the engine is running ok. My guess is still exhaust system related. :)
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by AndrewP » Tue Sep 11, 2018 1:42 pm

A minor update..

The easy one first that Grant suggested first. Armed with a can of carb cleaner and a generous spray around the inlet manifold to head area and a noticeable change of engine note around cylinders 3 & 5.

So...first cab off the rank is to remove the inlet manifold, clean, check and re-seal it.

Question for those in the know.

The Edelbrock manifold was fitted with a brand new Felpro valley pan and I fitted the 4 supplied gaskets either side of the pan.
It was a tight fit in places, so this may have contributed to the leaks I've discovered.

Whats the correct way to do this?
A previous post I found details the gasket on the engine side and instant gasket on the manifold side (so |gasket|-|valley pan|-|goop|-|Inlet manifold)?

Any advise?

Thanks

Andrew
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by Steve Payne » Tue Sep 11, 2018 4:41 pm

When I fit a valley pan I don't bother with the paper gaskets. I put a bead of high temp silicone around all the ports and bolt holes on both sides. I then allow this to tack off for 20 mins before I assemble it. You will need to be careful as it is easy to knock the soft silicone. Then place the inlet manifold on and tighten the bolts down equally.

Seems to work well for me.

High temp silicone is used to assemble gas chimneys and is available in black and clear, any excess can be removed with a sharp blade once dry.

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by RayR » Fri Sep 14, 2018 12:49 am

The felpro gaskets have a significant compressed thickness that is needed to position the edelbrock manifold correctly in relation to the heads for port alignment, if it is too thin the ports will be mismatched and more chance of a leak. It also depends on if the head is standard, skimmed or ported. I am sure there are some others who can guide you more. Another issue with silicone is that it is possible for the manifold to slip during assembly, best to mark alignment dry and then check it doesnt move during assembly.
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by Steve Payne » Fri Sep 14, 2018 6:30 am

The problem with the paper gaskets they are designed for engines that have not had there heads skimmed. Each time you skim the heads there is less room in the ''V''.

In theory each time you have the heads skimmed you should have some metal removed from the inlet manifold.

High temp silicone has worked well for me.

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