Exhaust Harmonics?

[Shawn's 78]

So in my quest to eliminate and determine my vibration around the 3000-3500 rpms, i've replaced all my belts, fan clutch and the harmonic balancer. The harmonic balancer was in the worst shape; I could just about push the two pieces apart with my hands. :eek
So after that is all said and done, the vibration has lessened, but still exists.
QUESTION: Could harmonics from exhaust cause this type of vibration?
It's a stock L48, new clutch and mods made above with 2-1-2 exhaust in which the cat has been modified (pellets removed by previous owner). The exhaust is original from what I can gather (production dates on the actual exhaust itself).
Is it possible that the exhaust is causing a "harmonic" due to the modification of the cat?
Has anyone run into this or noticed a change when they went to true duals?
Would it behoove me to replace the exhaust first, then dig into the tranny / clutch (previous owner replaced the clutch, turned flywheel etc). Our would some of you vets think it may be the clutch?
Any advice would be helpfull
Shawn ;help

 

[69MyWay]

Shawn, you may have a bad motor mount or two, or a bad trans mount. It may be that the exhaust is touching the frame as well.

You could also have a poorly balanced engine. During those year models Chevy was not building the highest quality work product. The days were numbered for the Type I sbc, and they were retooling for many new things.

My Dad worked for Chevy at the time and said the engine plants were a mess. They were re-using old stock, etc.

So, you may have everything dead on, but have a major internal balancing issue in the engine causing it to be off.

No way to know for sure, but I would start with the motor/trans mounts right now and see if they are getting worn out and transmitting too much engine vibration.

Also, is the felt sitting still in neutral, or just cruising down the road. If still in neutral, or park, it could be any of the above. If going down the road, I am back to U joints, make sure you have the proper lube in the diff, and good wheel bearings.

 

[Shawn's 78]

I did a check on the motor mounts - they are solid. The tranny mount seems to be the same.
I can recreate the vibration in neutral. Rev her up and shake she goes. But I noticed it is mainly through the tranny. The motor does not show excessive signs of vibration (visually), thus I tend to think it is either the clutch, flywheel, throw-out bearing or a tranny internal situation (any or all).
I have a friend that owns a tranny shop. I'm going to run it by his place tomorrow. Maybe he can help determine the problem.
thanks again for the advice.
Shawn

 

[Shawn's 78]

Just to check, I pulled the motor and tranny mounts - the tranny was weak, so I changed it - and I had one seperated motor mount (couldn't tell by visual inspection), so I replaced both.

Now I have vibration all throughout the powercurve! UHHHHHH!!!!

I would hate to think that my motor was one of those "piece together with the parts we have lying around" ones. If it is an internal balance issue, I quess I just have to live with it right? (That is unless i want to pull the motor and start over).


To date the following have been replaced:
Clutch
Tranny mount
Motor mounts
All belts
Harmonic Balancer
Fan clutch

Any other advice on what I should be looking for???

Thanks for you help - Shawn :s

 

[JohnZ]

Remove the fan belt(s) and fire it up briefly - if it's a belt-driven accessory issue, the vibration will stop; if the vibration is still there, it's engine/transmission-related.

 

[JHL]

Have you had the wheels and tyres checked yet, it would be the first I would check out, exhasts can give problems particularly stainless ones which tend to resonate more than mild steel but I doubt if this would be noticable as a vibration as you drive.

J.

 

[Shawn's 78]

J.
It is consistant at drive and standing still.
So that rules out the driveline and tires. That I guess is good, but this sure is frustrating. :confused
Shawn

 

[geekinavette]

I know (from experience) that a cracked auto flex-plate will cause the vibration as you describe it. Since yours is a 4 spd...maybe a cracked flywheel or pressure plate...etc?

Bill

 

[Shawn's 78]

Bill,
I think that is my next project after the first of the year. As i'm replacing the exhaust (to true duals), I will pull the tranny to check those two things out. If it isn't there, i'm about out of options - then it sounds like an internal balance issue.
Shawn