I've never understood this..

[chuck peterson]

During all the Florence coverage there were lots of Generac commercials...90 degree V motors

engine gif....click on the image to see it move

[chuck peterson]

Beemer flat twin...click on the image to see it move

[oilhed]

A rotary engine was supposed to overcome all these woogity problems but has pretty much faded into obscurity apart from the odd boy racer types.

https://youtu.be/vYo21dkR-cc

[Huzo]

During all the Florence coverage there were lots of Generac commercials...90 degree V motors

engine gif....click on the image to see it move

Hmmm..
Looks like they're both hitting TDC at the same time...
That can't be good..
Or has that one got two crankpins ?

[Testarossa]

That Generac clip makes no sense at all. Looks like two separate cranks. Here's a Generac video showing they use a single crankpin just like a Guzzi:
https://www.youtube.com/watch?v=6OdixA8Ssdc

[oilhed]

Here's a Generac video showing they use a single crankpin just like a Guzzi:
https://www.youtube.com/watch?v=6OdixA8Ssdc

Great, now I want a motorcycle powered by a generac OHV. 

[wirespokes]

Thanks Chuck and Dusty. Here's what seems to make sense to me - let's see if I'm right.

When piston #1 is at TDC or BDC, the crank counterweight moves the opposite direction to cancel forces. Since piston #2 is mid stroke, there are no forces to cancel so is out of the picture. It's like a single cylinder motor with the piston and crank counterweight balancing each other.

As soon as #2 hits TDC or BDC, the situation is reversed and the counterweight again cancels the forces, this time of #2 piston.

I hadn't thought of the crank counterweight before - I was only thinking of how the pistons could cancel each other which made no sense at all.

I think I finally got it. Pretty clever how that all works out! Thanks again guys!

[Huzo]

Yeah, that's helped me out a lot also.
I initially posted the question to find out what "secondary vibration" is, then found out a few things I didn't know that I didn't know.
Bonus...!