Rpm related vibration

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pjc360
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Re: Rpm related vibration

Post by pjc360 » Thu Feb 15, 2018 8:33 pm

BB70 wrote:
Wed Feb 14, 2018 9:34 pm
In my post above I advised to hold the rpms at 2500 and remove one plug wire at a time. When the plug wire is removed , note the rpm drop for each cylinder. They should all drop aproxx the same rpm. The one that barely drops is a cylinder that needs looked at. That test is called a power balance test. You said it pops in the exh at that rpm, if it stops popping when a plug wire is removed that is your problem cylinder. Just because you have new parts on doesn't mean anything. Good luck and be careful .

John
I can only hear it missing when the throttle is held at 2509 rpm while it’s in park.
That’s why I’d like to have it monitored by an engine analyzer.
I don’t really want to be pulling plug wires off at 2500 rpm and noting the rpm drop.
It would be a pain in the ass and the hedders would be super hot and it just sounds like an accident waiting to happen with ether getting burned or shocked or both or even damaging my Msd box or my coil.
I’m still not ruling out an imbalance issue with the converter or flex plate.
I’m just leaning towards thinking that it’s not an imbalance issue because I don’t feel anything below 2400-2500 rpm.
But I guess that doesn’t mean much.
I’m going to pull all my plugs again this weekend and check on them since I haven’t looked at them since putting this MSD Ignition box inn.
Maybe I had a fouled plug because of the issues I was having with my crane cams hi-6 box and that’s what’s causing the fuss.
I kind of doubht it, but it is worth a shot checking them out.
My crane box did appear to have some kind of problem though, I would have super bad missfiring at 2500 with the crane box m, bad enough to where the truck would be missing so bad while driving at 2500 you’d feel it jerking and sputtering.
I put this msd box in and separated the trigger wires from the coil wires and it got much better, it doesn’t sputter and jerk like it was with the crane box, but I still have a rough spot under load.
It would be nice if it was a fouled plug prior to installing the msd and it would be nice if putting a new plug inn solved it.
Guess we’ll find out on Saturday.
I still need to call a few shops and find someone with an engine analyzer cause I just want to rule out ignition/ internal engine issue like a valve spring or valve before I spend the money on a new flex plate and converter.

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Re: Rpm related vibration

Post by ozyfordman » Thu Feb 15, 2018 8:57 pm

Can you put your hand on the cap and wires with motor running without getting a boot?

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Re: Rpm related vibration

Post by BB70 » Thu Feb 15, 2018 10:42 pm

They make insulated pliers, it's a quick way to find a weak dead hole. You can also use a test light,ground the clamp lead- lube the probe with dielectric grease and go between the boot and plug wire on the distributor end of the wires. Just trying to help.

John

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Re: Rpm related vibration

Post by pjc360 » Sat Feb 17, 2018 3:52 pm

BB70 wrote:
Thu Feb 15, 2018 10:42 pm
They make insulated pliers, it's a quick way to find a weak dead hole. You can also use a test light,ground the clamp lead- lube the probe with dielectric grease and go between the boot and plug wire on the distributor end of the wires. Just trying to help.

John
I have some of the insulated plug wire pliers but I’ve always ended up ripping my plug wire boots with those things.
The only way I’ve been able to get the plug wires off the spark plugs is to twist and slightly pull at the same time down at the very bottom of the plug wire boot.
If I do it any other way I always end up damaging the wire or the boot some how.
I could dis-connect the wires from the cap much easier then I can dis-connect them from the plugs while it’s running.
I’m going to pull all 8 plugs ether today or tomorrow and I’m going to do another compression test just to see if anytning has changed since the last time I did a compression test on all 8 cylinders.
If everything checks out then I’m going to hook the timing light up to each plug wire and bring the rpm up to 2500 and see if can isolate which cylinder is missing by watching the strobe of the timing light.
I’ll report back here soon with my findings, condition of all 8 plugs, compression numbers, and if I was able to determine which cylinder is missing if any actually are or not.
I just find it more simple to eliminate ignition first before diving into valvetrain issue or imbalance issue due to a bad converter or flex plate.

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Re: Rpm related vibration

Post by MadBill » Sat Feb 17, 2018 3:57 pm

Usually you can pop the all the wires loose from the cap before starting the engine, and then just insert them loosely in the towers. This makes them very easy to lift away once the engine's fired, or better yet, leaves openings for insertion of a grounding wire, which won't risk burning an arc path into the cap.
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Rpm related vibration

Post by Dan Timberlake » Sun Feb 18, 2018 6:03 pm

"insertion of a grounding wire"

A short Snip of paper clip wire slid into each distributor cap tower makes it easy to short out a cylinder with a test lead to ground. Probably nicer for the ignition box too.

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Re: Rpm related vibration

Post by pjc360 » Sun Feb 18, 2018 10:45 pm

I haven’t gotten a chance to mess with the truck this weekend, the weather decided to get super nasty Friday night and it’s stayed all weekend.
It’s negative 9 right now, and it snowed another 12 inches.
And I don’t have my garage organized enough to pull it into the garage cause I just moved into this house.
So it’s been a huddled up inside in the house type of weekend cause of the weather.
Every time I plan to do something outside the weather has a different plan.
I really need to figure this out though cause it’s driving me insane.

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Re: Rpm related vibration

Post by enginenut2 » Sun Feb 18, 2018 11:36 pm

Some years ago, I swapped a 318 in a van for a used 360 and picked up a vibration like yours. On checking I found that the 360 was externally balanced and I had used the 318 flex plate. The correct 360 flex plate had a small weight added so the vibration went away when I changed them out.

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Re: Rpm related vibration

Post by MadBill » Mon Feb 19, 2018 12:00 am

Upon re-reading the post, nowhere does it say the vibration is a recent development; it may well have been there since the new engine was installed, making the flywheel a likely culprit. There is mention of a 'magnum balanced converter'. Does a 360 need both unbalanced? If the converter has deliberate imbalance, is it phased correctly?
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Rpm related vibration

Post by pdq67 » Mon Feb 19, 2018 12:31 am

Chevy's have crankshaft dowel holes and sometimes dowels to line the F/W and F/P correctly to the engines crank, BUT I don't know if converters are so clocked to the F/P or not.

Can you beg, barrow or steal another smooth running engines converter to switch out to see if it is an out of balance converter?

And while you are in there, dial indicate the bell-housing run-out to make sure it is in spec?? We changed our old 258", 6-banger engine in our old '77 4-dr AMC Matador to a newer one and it ate its 904 front tranny seal because NOW I find that they can be out. Back then, I didn't know better..

pdq67

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Re: Rpm related vibration

Post by pjc360 » Tue Feb 20, 2018 3:59 pm

MadBill wrote:
Mon Feb 19, 2018 12:00 am
Upon re-reading the post, nowhere does it say the vibration is a recent development; it may well have been there since the new engine was installed, making the flywheel a likely culprit. There is mention of a 'magnum balanced converter'. Does a 360 need both unbalanced? If the converter has deliberate imbalance, is it phased correctly?
The 360 magnum is externally balanced nustblike previous la 360’s but they take a different amount of weight.
Because the pistons are lighter.
Part of the confusion stems from the way Chrysler handled the balancing.
In the early magnum years they used a tradional nuetral La flex plate with a converter that was balanced specifically for the 360 magnum.
Then a few years later around 96 or 97 they started using a nuetral converter and a magnum specific balanced flex plate.
So lots of people will say check your converter and make sure the balance is right.
And I have verified I have the earlier nuetral la style flex plate and a magnum balanced converter.
I’m not sure if this is a balance issue or a missfire issue.
Like I said my next step is to have it hooked up to an engine analyzer that can monitor ignition at the troubled rpm and if the ignition checks out and everything is firing right then I’ll be more inclined to thing it is a balance issue and I will order a new flex plate and converter and put them on and see what happens.
I just don’t want to buy a new converter and flex plate and still have the issue because it turned out to be something else like a bad valve spring or a bad valve or whatever.

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Re: Rpm related vibration

Post by MadBill » Tue Feb 20, 2018 4:05 pm

I'm not familiar with Mopar drivelines, but could you unbolt but not remove the converter and briefly run up the engine to see if the vibration is affected?
Felix, qui potuit rerum cognscere causas.

Happy is he who can discover the cause of things.

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Re: Rpm related vibration

Post by pjc360 » Tue Feb 20, 2018 8:15 pm

MadBill wrote:
Tue Feb 20, 2018 4:05 pm
I'm not familiar with Mopar drivelines, but could you unbolt but not remove the converter and briefly run up the engine to see if the vibration is affected?
Unfortuently no, you can’t un-bolt the converter and slide it back like you can on a small block Chevy or else I would have done it already.
It really does seem like a balance issue, the engine feels crisp and sounds crisp when you hit the throttle.
I’ll know a little more about how the ignition is doing once I can pull all the plugs out this weekend and see how they look since wiring the new Msd box inn.
If all 8 plugs look good and I get good compression readings on all 8 cylinders then I might just roll the dice and get a new converter and flex plate and see what happens.

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Re: Rpm related vibration

Post by smeg » Tue Feb 20, 2018 8:26 pm

Disconnect the exhaust from where it is attached to the chassis and support it on stands or whatever and check if it fixes it.
I have seen first hand that this can cause a vibration at certain revs.

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Re: Rpm related vibration

Post by pjc360 » Wed Feb 21, 2018 2:39 pm

smeg wrote:
Tue Feb 20, 2018 8:26 pm
Disconnect the exhaust from where it is attached to the chassis and support it on stands or whatever and check if it fixes it.
I have seen first hand that this can cause a vibration at certain revs.
I’ve dis-connected the exhaust before, and the alternator and ps pump belts, I’ve took the radiator fan off before, I’ve done pretty much everything you can to eliminate accessory’s and exhaust.
I’m left to ether flex plate and converter or a miss firing cylinder.
I just don’t think it’s missing cause it sounds crisp when you hit the throttle and it’s smooth below 2500 rpm.
I’m going to pull the plugs this weekend and check on them, then I’m going to do another compression test to verify all cylinders are putting out good compression even though I just did it not that long ago.
If everything checks out then I’m gonna be ordering a new flex plate and converter.

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