Vibration at 2500 rpm

[Ethan0]

Looking for some help at figuring out a vibration/shake at around 2500 rpm. There is a small shake at idle but it smooths once I start giving it gas. Then at about 2500 rpm it feels like the whole truck starts shaking/vibrating pretty badly. It does it in neutral with the clutch in or out, while driving, and with the truck in gear and on jack stands. Always at about 2500 rpm. It seems like something with the engine is the problem. Could the balancer be the problem?
It’s a 1976 k10, 350sbc, sm465, and np205, all stock.
Any advice or things to check helps, thanks.

 

[fast 99]

clutch cover gone off center, fan clutch if equipped. 350's are internally balanced so doubt balancer is the issue. If it vibrates and stationary without being in gear that eliminates driveline.

 

[Scott91370]

Motor mount? I know they are usually felt more at idle but worth look at.

 

[Ethan0]

I know it’s been awhile since posting this thread but I’m getting back to finding this vibration/shake. After checking and replacing a few things nothing fixed it so I left it alone. Now thinking that the problem with the clutch/flywheel not being centered or loose I pulled the trans out and found there was no pilot bushing. Would this be causing the vibration/shake? Probably not helping anything but not sure if I need to keep looking at other things

 

[SquareRoot]

Yes. That might likely be the cause. Unfortunately, you will need to replace the input shaft and bearing in the trans.

 

[Ricko1966]

Yes. That might likely be the cause. Unfortunately, you will need to replace the input shaft and bearing in the trans.

You know I thought about this real hard,since yesterday,wasn't going to put in my .02 cents because I didn't know for 100% positive. But he said he gets the vibration with the clutch in as well. With the clutch in the disc and mainshaft are not spinning,even if the disc were cocked and still contacting the flywheel with the clutch in,the truck would try to creep in gear. IDk I'm thinking although he needs a pilot bearing for sure,but not certain that's his vibration. Food for thought.

 

[Old Guy Bill]

The rubber ring on the vibration damper deteriorates then it starts moving on the inner core.
How about the motor mounts? What shape are they in?

 

[Ethan0]

Motor mounts were shot but I replaced them a few months ago. Didn’t make much of a difference in the vibration. The dampener doesn’t look like it’s moving around while revving the engine but I’m not certain it’s not a problem.