I have heard a few good arguments that most small two stroke engines aren't designed to very high specifications and physically can't take advantage of higher octane fuel. I have used different octanes in multiple engines and consistently not noticed any difference.
What I have noticed is fuel from different companies perform differently (I acquired a good/bad station list when I first started driving.)
Speedway consistantly sucks (but their diesel seems okay)
Amoco/BP is good
Shell has always treated the car really well.
Sunoco is always good too (plus they've got lot-o-options on the octane levels)
Marathon has teetered on the fence.
I do practice using, exclusively, either high end or low end though, in hopes of getting fresher fuel. The middle isn't used as often I think (given the stations location) and I've heard it is common practice to mix the 87 with the middle tank (generally 89) if the station mis-estimated how much fuel they would need and ran out of tank space.
That being said I feel better sometimes getting the fancy fuel and even though I know it may not matter at all, I like to be happy, and I like my engine to be happy. Then we ride off into fields of blueberries and stuff outselves silly till we fall asleep in eachothers arms.
The 70cc/80cc engine has a compression ratio of 6.6:1. You can run as low as 70 octane in it with no problem.
Use as low octane as you can: Reason
2. More energy content (BTU) per gallon. Higher octane fuel has less energy content
3. Easier starting due to higher volatility. Higher octane fuel is harder to start in cool temps or first start of day. I had run100LL octane fuel in a engine and it was impossible to start when cold. It was a very high compression ratio engine (modified) and needed this fuel. I used starter fluid a lot in this engine.
The only time you need higher octane fuel in any engine (to include cars) is when you experience detonation or preignition and/or you engine is higher in compression (or runs hotter) and is specified in owners manual.
Older engines typically have carbon build on on piston tops and this caused increased compression and hot spots that can cause fuel to ignite prematurely. This is why older cars may run better with higher octane fuel.
If you use high octane fuel for any other reason, you are wasting your money and are being foolish.
Some of my qualifications: Engineer (lots of thermodynamics), maintenance test pilot, 20 years of wrenching on almost any kind of machinery.