408 Question

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321Scamp

321Scamp

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I have got the car up and running all leaks fixed ready for the road... kinda.
I was taking it out for a spin to see how it did on the road too. Here is the question. I have a wicked vibration just a few RPMs off idle. Like rattle your teeth bad. But it you go up in RPMs about another 100 or so, it goes away. Timing is good. Probable not the best gas in it but all i had at time.

Any thoughts? Thinking it might be a vac. leak but not for sure

It a 408 with comp cams xe274h, stock 360 heads that are port matched, eddy air gap intake, tti headers, eddy 750 carb.
 
It does not matter... park, neutral or in gear... just in the one rpm range just off idle
 
In such a tight range and with or without a load. I wouldn't rule out a vacuum leak, but I would suspect it would be more widespread and not as violent. Wow i'll have too think on this awhile.
 
Did you just rebuild/refresh it?

If you just did a little bit to it, what specifically did you change/fix?

Does it rev freely, to like 5K rpm? Like it doesn't reach certain rpm, 4K or something, and stay at that rpm.
 
Yes it is balanced. Got a stock style damper on it for a internally balanced 360 as well as the stock flex plate and torque converter off my 318.
 
so the dampener is new?? what brand is it?? and there are no weights on the torque converter either correct??
 
Yes it is new... dormain i think is the brand. Factory replacement that you can get at napa (bro works there)

No weights on converter.
 
I could be wrong here but if it is a factory replacement then it wouldn't be neutral balanced cause I am under the impression that all 360's from the factory were externally balanced, although you did state that it is neutral balanced so I don't know can you get a part number and double check to make sure??
 
i have a 408 and had a similar noise.. this may sound stupid. but it was the tti header hitting on the oil pan on the driver side.. it was hard to see and notice.. .. a couple of well placed dings on the header and some engineering fixed it.. now its all good. no power loss, no rattle. it actually sounded like you described it ...
 
It factory replacement brand but yes for an internally balanced 360. I not have a noise... it a full car jarring vibration.
 
i did get it from summit... that probably the problem there. It probable a externally balanced damper that they said was an internal.
 
Summit has done that a lot lately, someone else had the same thing happen to them, you should be able to find out what one they sold you if it is externally balanced and your engine was internally balanced then you have your vibration, and I would not run it anymore till you figure this out. good luck
 
321Scamp said:
i did get it from summit... that probably the problem there. It probable a externally balanced damper that they said was an internal.
Click to expand...

Pictures of the dampner should allow someone to properly ID what exactly you have.

Put you camera to a macro setting and get up close in front and behind the dampner at an angle. Probably will have to reach in an area and hold the camera and point it to the area. Just delate the pictures that aren't aimed or focused right.
 
It's that narrow rpm band that has me stumped. I can think of a lot of things, but when I go back to that nothing seems feasible.

I'll take one shot in the dark.......Are you running ported vacuum advance? Could it be that when the throttle blades open just enough to send full vac to the can, that it's pulling the advance plate in the dizzy and causing a malfunction there or throwing off the reluctor gap off.

Like I said, when you apply the thinking of the narrow band, even this doesn't quite make sense....idk.
 
no i am not running ported. have tried both ways and no change. Maybe the vibration is still there at higher rpm but it just smoother because it faster. idk

we are thinking it more like vac leak... but i think it could be the damper
 
321Scamp said:
It factory replacement brand but yes for an internally balanced 360. I not have a noise... it a full car jarring vibration.
Click to expand...

There is no such thing as a factory internal balanced 360, so unless they sent you the balancer for a 318, I would start looking there. I put a 318 flywheel on a 360 one time and got a vibration as you describe. I would check your flexplate also for any weights, or a large half circle shape cut out of it?
 
come to think about it... i think the back side of the damper has a large mount of material off of it instead of small drill hole in it.
 
321Scamp said:
come to think about it... i think the back side of the damper has a large mount of material off of it instead of small drill hole in it.
Click to expand...

Like a large amount of material of evenly all the way around it?

OR

Large amount of material off only one side of it?
 
Well I guess I might be the only one that keeps all my receipts from parts?? cause if you can find the receipt you can quit guessing and find the part number, then you will know what you have or call summit maybe they can look up your order. my guess is you have the wrong dampner.
 
412 Stroker said:
Well I guess I might be the only one that keeps all my receipts from parts?? cause if you can find the receipt you can quit guessing and find the part number, then you will know what you have or call summit maybe they can look up your order. my guess is you have the wrong dampner.
Click to expand...

True.

And I think you can go online to summit and look at your past orders.

But I would still get a positive ID of exactly what you have. Parts can be mislabeled and mis-packaged.
 
just got done talking to summit. the picture matches the part i have. It has three small holes in the motor side of the damper. that matches there picture and it is for a internally balance motor. However, the pulley side of the damper has a machined groove about 2/3 around it that is machined out to about 1/4" deep. that tells me it is for external... am i wrong? the only problem is that my part was shipped straight from the manufacturer, so they could have sent me the wrong one.
 
Summit has 30 different dampners for mopars and almost all of them are for External balanced 360's there are some internals if you go with ati or major brands, didn't you say it was a dorman?? cause those are all external except for the 318-340 ones, and those shouldn't fit due to different cranks?? give me a part number, now I am really curious....
 
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