125-200cc CR144 Flywheel weight, Rekluse or both?

I do what I find that works from experiance. You can't tell difference when ridding.
Per the DID Chain Spec site : http://www.didchain.com/chainSpecs.html
520VT2 T ring Enduro Chain 3.28lbs per 100L
520ERV3 X ring racing Chain 3.3 lbs per 100L
520MX Non sealed racing chain 3.43lbs per 100L
520ERT2 Non sealed racing chain 2.99lbs per 100L
520NZ non sealed racing chain 3.54lbs per 100L

Doesn't appear to me that Oring chains are out of the hunt at all on chain weight. I even prefer steel sprockets too although maybe a MX guy won't. If you can find a Sunstar Z steel sproket that fits , I find they last longer , clean better in mud and weight almost same as an aluminum sprocket.

FWIW, I've been using the same O ring chain and steel sprockets on my 125 MX bike for the past 4 years. I couldn't tell the difference switching from non O ring, and aluminum sprockets, and I haven't made a chain adjustment in 2 years

Coming from the bicycle side chains do effect power output (new vs broke in vs worn) so I would expect an o-ring chain to considerable power especially when new. It is also possible to make a chain 15% lighter in the same pitch and width by hollowing pins and links but likely not done on motorcycles. My guess is the power loss is significant and obviously much more noticeable on a 125 vs a 450 both due to the larger percentage of total power and the much greater rotational inertia of the big 4 cycle. So maybe everyone is "right"?!

What about 18" vs 19" wheels? Seems like the OD is about the same but the tire is more substantial on the 18". However the rim and spokes are smaller so weight is saved. Have any of you ever compared same rim/spokes/hub in both sizes with same width tire. Is one wheelsize considerably heavier than the other?

I have thought of going to an 18" rim on the back of my CR mainly because I still have some tires to fit and they seem cheaper/more available. However maybe there is some performance advantage I do not understand on the 19". Interesting that the "off-road" bikes still come with 18" while the track bikes come with 19". I am sure we can start a great discussion on this topic!

I'd love to have an 18" rear! My favorite rear tire is substantially cheaper in an 18" ($20 cheaper)

Your absolutely right that the chain has more noticeable effect on a 125 than a 450. The 19 rear wheel is supposedly lighter but I never weighed them. How bout tubes? A moose hd tube is about 1 pound more than a standard tube and makes a huge difference in the flywheel effect on a 125. I guess the point I should have made is that an off road guy should do the common sense stuff like, oring chain, hd tube, 18 rear wheel, maybe ever steel vs aluminum clutch before putting a flywheel weight on a 125. The smallest things make huge differences on these bikes.