Mystery vibration at 2200ish rpm's

[yellomalibu]

Hmmm. I don't know as to if the crank has been positively identified. What exactly is the difference between the 2?

From Lingenfelter's book:
"The best way to spot a 305 without having to spin it up on a balancing machine is to lay the crank down horizontally and position a straightedge along the machined surfaces of the crank as shown. A 305 crank will allow the straightedge to lay flat across all five points. A 350 crank will not allow the straightedge to sit flat. "

I don't think you can do this test with the crank and rods installed. Perhaps a calibrated eyeball?

[Deaf Bob]

Huh? Firing order is the same...

[STINEY]

Its the counterweights, they are larger to match the larger pistons.

[Deaf Bob]

Its the counterweights, they are larger to match the larger pistons.

Oh.. Never took a 305 apart...

[yellomalibu]

The counterweights are in the same place, same firing order, etc, but for some reason, the square notches that are machined into the counterweights (for balancing, no doubt) line up on the 305. On the 350, they do not.

[Beagle]

I wouldn't have thought about it, but it makes sense. The counterweight would be smaller for the ~3 3/4 v. 4" piston, (very) roughly 12% more weight for the big bore.

[Beagle]

Huh? Firing order is the same...

the cam dictates firing order...

[boosted2.3]

Using a 305 crank in a 350 will do that. Or maybe it has a 305 flexplate with a weight on it.

I've seen flywheels do this , also seen cracked flex plate when they first crack then as it get
s worse it will sound like a rod knocking. jmi

[Scott Liggett]

The 305/350's with one piece rear main seals use the same flexplate. It is weighted because there is no bob weight on the flywheel. In most cases, it's not the flywheel or the balancer. It's a sticky valve, valves out of adjustment, cracked spark plug, bad spark plug wire. The last two are easily found by running the car in the dark. You will see the electric light show.

[STINEY]

I am glad to report that this issue is resolved.

The aftermarket harmonic balancer was replaced with a different brand aftermarket harmonic balancer and the vibration went away permanently.

[Deaf Bob]

Glad it got fixed before catsrophic consquences!

[Beagle]

thanks for closing the loop. Did the balancer look bad?

[STINEY]

Nope. Looked like brand new.

You can't quite see it in this picture, but the bad one is in there. Like Ragu.

[antmnte]

Could you see it being bad or did you just wing it?

[Huskinhano]

I had a vibration similar to that in my Mustang when it had the C4. When I converted it to a 4 speed, I found the bushing in the front pump was shot when I removed the C4.