New stuff
 

New goody has arrived. :D

Daniel

Here's hoping it solves your issues. :thumbsup:

Still hunting...
 

I got the new trans installed the other weekend and went for a drive and...the vibrations are still there. :omg:

I guess we've eliminated one more possibility= the trans. So, it seems the issue is somewhere between the front of the engine and forward of the trans.

I've been reading various vibration threads and I'm wondering if the vibration is a result of a physical imbalance (i.e.- internal rotating assembly out of balance, flywheel counterweight incorrect) or an imbalance related to tuning (i.e.- misfire, incorrect timing, etc)?

Wouldn't a physical imbalance begin to vibrate at a certain RPM and continue to increase in severity through the RPM range?

In an effort to be concise, I'm going throw out some facts from what I know at this point:
-Vibration is not speed related.
-Vibration occurs around 1900-2100 RPM's, then goes away and returns around 4000 plus.
-Vibration decreases when I push the clutch in and engine speed begins to drop.
-Vibration seems load dependent.
-Vibration isn't noticeable/present when engine rev'd in neutral at a standstill.
-Engine seems to have normal water temp, oil PSI, power.
-Engine DOES have run-on/dieseling when I shut off the ignition. Usually around 3 seconds of continued rough idle before finally dying.

Regarding physical imbalance- I ordered a few different flywheel counterweights and was planning on trying to find a counterweight "sweet spot" via trial and error. Someone mentioned that the rotating assembly could have originally been balanced with the flexplate that came with the engine and now that I've installed a flywheel, it could be out of balance.

On a positive note, I did get around to making some valance opening inserts to match my grill...

great looking front end

What fuel/octane are you running?
Any other vitals for the motor (deck height, head gasket used, piston dome/dish volume)?

Seems you might have answered your own question. I also added to part of it about vibration getting worse.

Also since you've eliminated the drive line from the problem lets take a look at other rotating parts that I've seen cause vibration. The balancer, easy to check by setting cylinder 1 at TDC and seeing if the timing mark lines up with zero on the timing tab. Balancers have been known to slip and will cause the type of vibration you're having. Ironically your timing might be "correct" with the timing light, but if your balancer slipped its off in the engine causing your dieseling.

The accessory drive is another one. If you're using a cheap drive that things can flex around that can also cause a vibration. Free reving an engine doesn't always work, sometimes it needs a load on it like when driving. Misfires are the most common that need a load to show up. At this point I would get your timing corrected and go from there. If you have a GoPro you should mount it under the hood so you can watch the engine and see what its doing when the vibration happens.

Thanks slimjim.

ScotI- I'm running 93 octane. Unfortunately, I don't have all those specs. The engine compression is around 9.5-1 from what I understand.

CamaroAJ- Thanks for the input.

I replaced the original external balanced balancer with a brand new unit the end of last year. I verified TDC and the crank snout and key way looked fine before I installed the new balancer.

I am running a lower end front accessory kit, but I ran/revved the engine with the serpentine belt off and there was no change. BUT, I haven't driven the car/applied load while the belt was off. I might try to get my hands on a Go Pro and see what I see.

Daniel

Drop the original motor back in & see what happens.

The wrong combo on the 383 could be causing both the vibrations & run-on if things weren't done properly. Swapping a different power-plant definitely narrows the scope of things.

Update
 

ScotI- unfortunately, I don't have the original motor.

Well, I've tried a couple different flywheel weights recently with no luck. The original 156 gram counterweight seems to be the best and the other weights (heavier & lighter) caused the vibration to increase.

So, at this point, I'll just cruise around until winter. Doesn't look like I'll miss many/any autocrosses this summer with COVID hanging around. Then, I plan to pull the engine and have an internally balanced rotating assembly installed. As long as I keep the rpm's under 4000, the vibration is not that bad. I put about 40 miles on it yesterday cruising back roads.

I still have a few small details to finish (AC charge, alignment tweak, etc) but overall, the car is "complete."

I'll post a few gratuitous photos for grins.

Daniel

Couple more
 

Couple more photos