Sbc vibration at 3000 rpm

[Cole1969]

I have a 1969 el Camino with a sbc 350 and a th350 and just rebuilt the engine in it after it had been sitting for a while. Right when I stopped driving it a couple years back it started to develope a vibration around 3000 rpm and now just recently I pulled the engine and trans and rebuilt the top end of the motor replacing heads, cam lifters, intake, Tourqe converter, but I kept the water pump, alternator harmonic balancer, carb, distributor, and pretty much everything from the converter back, so it only seems to vibrate when in gear it's not speed dependent. I kinda think it would be either something with the drive shaft or maybe the balancer. But I'm also using a short water pump that is who knows how old could that be causing the problem? Any ideas are appreciated thanks

[truckster]

You say it's not speed dependent. The driveshaft is definitely speed dependent. The flex plate, however, is dependent on engine speed and is an item that needs to be balanced. Is it possible your flexplate lost a weight? Also check the rubber in the balancer.

[7tee]

You might try removing the fan belt for a short fast idle to narrow down these items run by the fan belt...Ed

[garyd1961]

Make sure the bolt in the balancer is tight. I had a 383 I bought that vibrated at 3000 rpm and the bolt that holds the balancer on was missing. It was just pressed on and had worked loose, chewed the crank up.
Also make sure you have the right balancer and flex plate for the motor.

[GRX]

Drive shaft is speed dependent - rotational speed that is. If that were the problem it should theoretically vibrate at 3,000 RPM speed in each of three gears. Is that the case here? If not x2 on checking dampener & flex plate.

And welcome to the forums Cole1969.

[Cole1969]

thanks for all the help guys I had the drive shaft out today and after looking at it there's a place that looks like it could have had a weight on it althought it's hard to tell but I'm going to take it to a shop and get it balenced and checked out btw it does vibrate in all 3 gears so if the drive shaft doesn't fix it I'm gonna try the harmonic balencer next. Also could a driveshaft vibration be causing the tranny to leak out of the rear seal around the yoke because mine was leaking and after I took it out saw it was ripped thinking that was the reason it was leaking I replaced it but it still leaks any help is appreciated thanks
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[Alex V.]

A driveshaft imbalance could damage the rear bearing/seal in the transmission, but if the vibration occurs at the same approximate engine speed in all gears it's from your torque converter forward.