SBC Harmonic Balancer Question

[Mac]

I've been living with an annoying slight engine vibration in a roller cam 305 that comes on at 1K rpms and goes away by 2K, most pronounced at 1200.

I thought I'd spring for a new balancer so I measured the one that's on now and came with the engine when I bought it used. It measures 7.25 inches and I find that size referenced to the 400 SB while the 305's are listed at 6.75 size. 400's are external balance and 305's are internal.

If this were a 400 spec balancer that somehow found it's way onto this 305 wouldn't I see more than slight vibration?

[wayne petty]

small block 400 balancers have a NOTCH cut in the outer surface to offset the weight..    ----------------__________---------------____________  as the outer surface goes around..

all other balancers have a smooth outer surface.

=======================================  an even surface on the outside of the damper ring..

[Mac]

Well, mine's smooth. Rules that out. Thanks Wayne.

[chimp koose]

Isn't 1200 rpm about when the torque converter 'stall' speed is ? Could it be related ?

[Mac]

Good suggestion Chimp but this is a manual 5 speed. I had the trans and clutch out and ran the engine to see if it still had the vib and it did. I also took accessory drive belts off and tested and made sure to separate 5 & 7 plug wires.
Some remaining sources are: old mounts under the iron bellhousing, budget FWheel from summit, balancer still, etc..