FWIW, bkr started a similar thread on another forum

http://www.benzworld.org/forums/forums/thread-view.asp?tid=1174370&posts=2&fid=10

Not much action yet, but it might be worth watching.

Thanks Paul!

Ok,,, but there are " adjusting " procedures for the mounts... something about loosening and shaking the engine and then retightening....
Are your mounts those with the correct color dots on them ?
Did you resolve the issue with the trans crossmenber mount ?
Does the rear axle snubber on your car require adjustment ?
Have you checked the alldata list of tsb's concerning driveshafts and vibration troubleshooting ?

Yes, yes, yes, can't tell.

There was no BUZZ before installing the manual transmission so logically, it would indicate that the vibration is associated with the changes made during the conversion, not the rear axle since nothing has changed back there.

The alldata list has some interesting things for a 1979 wagon... did you check for your year ?

But that logic would have to include the possibility of too long or too short a driveshaft.. since that unit was fabricated.....which could put pressure on the rear member...
How about the angle of the driveshaft issue... ? I know on farm implements driven by a pto which have two piece driveshafts they give elaborate instructions on angles and lengths...
Your driveshaft ujoints are not CV joints... so this may be critical..

if mirrors are blurring you SHOULD be able to find the vibration running it up to speed.

I'll am willing to give odds its a speed and not a load related issue. You are certain there is no runout on the new driveshaft?

but the time you get to 3rd or 4th itsgoing to be turning a good rpm.

Remember the driveshaft speed is keyed to vehicle speed not engine and gear. If that driveshaft is not true you will have a good sized balance issue. You need to see it running.

Have you tried to get it to happen at lower speeds... but in 3rd or 4th..... ?
You would not normally be at those speeds and not in the correct gear.... but this might point to trans instead of driveline if you can get any indication of roughness at way lower speeds...
How about getting on a hill and turning the engine off and depressing the clutch... and trying several gears...this would still load the transmission....

"Everthing on the car moves when it buzzes. How will you tell what the abberrant component is if the whole damned car is shaking???"--RLeo
Absent some serious harmonic resonance... whatever is causing this is going to have to be doing some serious moving Itself...usually it will be the thing moving the most... so should be easy to either see or feel....
Maybe your rearview mirrors are loose .... LOLOLOL

:) Dumb question: At what speed (mph) is this vibration coming?

You mentioned:

How about if you leave the car on second, rev it up and try to reach the mph that the vibration starts on in 3rd? Does it also cause the vibration in second then? If so, then it is probably not the transmission right?

Thanks,
Adiel

Re: Vibration assistance needed - Manual Transmission Related?
 

1400 to 1900rpm is something like 20 to 30mph in 3rd and 30 to 40mph in 4th. I cannot give you the exact speeds because the speedo calibration is waaaay off after the tranny swap.

It does not vibrate in 1st or 2nd.

It is DEFINITELY NOT speed related.

:) Another Dumb question: How old are your tires?

Thanks,
Adiel

No, that's a valid question. However, probably not an issue in this case since they're brand new...less than 500 miles. And, the vibration is still there with them.

R Leo,

I hate to ask a stupid question, but after rapidly scanning your previous posts, I saw no mention of using an early 300 flywheel.
I seem to recall from Ridge's posts that it was necessary to change over to a different flywheel. I don't recall if it was for balance or for some other reason.

The only stupid question is the one left unasked.

I haven't ruled out the flywheel but, I tend to think that's not the problem for this one reason: I can slowly rev the engine to 4000-4500rpm in neutral or with the clutch depressed and am unable to recreate the vibration I feel when driving in those two gears at the specified rpms.

300D manual flywheels are as scarce as chicken lips. Anyone know where to get one of these?

And, I do believe I now know why the 300's flywheel is heavier: they do it to maintain engine rpm between shifts. In the 1-2 and 2-3 shifts, the engine looses rpm so quickly it is difficult to make a smooth transition. OTOH, it also picks up rpm quite nicely= more fun to drive.

:) Have you checked the oil in the manual trans? Is it just as new as when you put it or has it gotten dark or filled with metal particles? Just brainstorming some ideas...

Adiel