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I have an old side valve Soviet era motorcycle which runs on leaded fuel, which is no longer available, so I have to use a lead replacement additive. I had to drain the fuel tank, so as not to waste the fuel I filled up the tank on my KZ1300 with it. Now it is entirely possible that I am the victim of my own over-active imagination, but I'm pretty sure it's running a lot better. I was out on it today and it was running as sweet as can be. Do any of the forum boffins have a theory to explain this, or am I indeed a victim of wishful thinking. Or has anyone else tried running their bike with a lead replacement additive.I am aware that these engines are designed to run on lead free petrol.

Lead additive which is Tetraethyl lead was also an octane booster along with other benefits like preserving valve seats and valve faces. You're using the lead additive to offset the valves and valve seat in your Russian bike. The benefit you're seeing is the octane boost of the TEL. You might also see the performance increase by burning Mid grade or premium fuel although to achieve that octane boost today, they use ethanol in various percentages to achieve the octane rating. The only problem is that ethanol has a lower heat BTU and will reduce the power output. Sort of a catch 22. That lead additive is not cheap.
One of the more common ways to deal with the older engines is to have the valve seats and valves updated to materials that can handle the unleaded fuel.Cost wise not too bad. 4 cans of the lead additive would pay for the new valves and seats. Food for thought.

Kawboy wrote: ... Mid grade or premium fuel although to achieve that octane boost today, they use ethanol in various percentages to achieve the octane rating.

In these parts (British Columbia) the higher octane fuels, 91 (Ron+Mon)/2 and up, typically contain zero ethanol.

This is a bit off track, but I love the ethanol in the fuel. It keeps the fuel system cleaner, helps remove water from the tank. In an old vehicle it pays to rejet as it will likely be a tad lean.

We also use E85 in two of our race cars. $2.50 gallon vs $15 a gallon for 110 race fuel. The E85 also runs cooler so we can run smaller radiator opening. Not as cool as Methanol but we street drive these cars too and there are 30-40 local stations that carry E85. As far as power loss, that is only true if you don’t tune a car specifically for it. Without EFI E85 is not manageable as the % of ethanol varies between fill ups. These cars are 550rwhp (3L V6) and 900rwhp (4L V8).

Interesting comments Frank.
I have nothing against ethanol in the fuel. One just has to recognize the short comings and update a few things and manage the fuel if it's allowed to sit for any length of time.
Changing out the Buna N orings in the carb to Viton orings and changing out the old standard fuel line for fuel injected rated fuel line (which is Viton based rubber hose) is all that needs to be done mechanically.
Ethanol based fuel is considered stale if left sat for more than 2 months, so if you want to ride periodically and have fuel sitting in the tank for extended periods, just counterbalance the effect with a fuel stabilizer for ethanol enriched fuels . Sta-bil 360 marine will keep ethanol enriched fuels fresh for over one year. You just can't use regular fuel stabilizer because it's not engineered for ethanol enriched fuels. The Sta-bil 360 is almost 5 times the price of the regular Sta-bil fuel conditioner. Why they even sell the regular stuff is beyond me. 95% of the fuel we buy now has ethanol in it.

Kawboy wrote: ... and manage the fuel if it's allowed to sit for any length of time....

Absolutely. The gunk from 10% ethanol fuel that sits for a few months seems much much worse. I use Sta-bil in my lawn equipment and pay the price in spring if I've forgotten it...