1972 GMC Shakes at idle - where to start

[dman535]

I went through and did a full tune on our 350 GMC C10. Plugs, wires, caps, rotor, carb rebuild etc. The truck runs great, except for a shake at idle. As soon as you are off idle the shake is gone. I have popped the hood and the vibration is pretty substantial. I thought it might be a balancer, flexplate, toque convertor issue but I don't detect any driveline vibrations going down the road.

I played with timing an mixture and have it manageable but want it licked before I destroy something. I did a few open hood brake torques and the motor doesn't seem to be moving around a whole lot. I thought it might be a bad cam, but it runs to good at higher RPM to be losing a lobe.

I thought maybe my motor mounts are shot. Before I throw any parts at it - looking to get some input from the group.

Thanks

Derek.-

[geezer#99]

What did you set your timing at?
Are you hooked to manifold vac for the vac advance?

[hamjet]

Sounds valve train related. most likely a burnt valve, or worn guides. just two cents... P.S... it did do it before tune up, right? One more thing, put a vacuum guage on it to see what it does...

[dman535]

the truck wasn't in the greatest state of tune when we bought it - but yes I think it did it before. I did plugs and they all looked good coming out of the engine, no major carbon or oil fouling. I did put a vac gauge on it and was getting 20' at idle - so its pulling good vacuum. I set timing to 8 degrees and it didn't run the greatest. I have had the timing all over the place and right now - its set to give the best performance out of the motor.

I figured my next step would be a compression test to see if everything is good. The truck had a later Target motor in it and I don't think those are known for any major problems.