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Your People has a metric odometer. Did you remember to do the conversion from kilometers to miles?

Nowhere you mention if it is a 4 stroke or a 2 stroke...you mention you have a light grey plug. Running hot, which means too lean with gas. It seems to confirm your stumbling problems. Can you see if your airbox is stock. If too much air gets in with the stock jets in the carb, it will run lean. Try closing the air intake a bit to see what happens. Usually, a lean engine runs better when ambient air is warmer.

Try reinstalling your original jet in the carb. Or, remove restrictors in the air box. How is the exauxt tip? Is it moist whith fuel? What color is the plug? When you say it slows down, is it the engine that is losing revs or is it just slowing down as if someone was applying the brakes? Have you checked the oil level in the rear transmission? Is the rear wheel spinning freely (normally) when the scooter is on the centerstand? Could be the transmission seizing up. Is the fuel properly reaching the carburetor? is the fuel filter clean if one is installed? Is the fuel petcock functioning properly? Does the engine regain its power after being stopped for a while? Is there gas leaking on the ground when it stalls?

What i'm trying to get at here is the fact that you are not clearly describing what really happens. It is therefore somewhat difficult to try to find some kind of explanatio to your problem!

Funny thought here: Ever noticed if this is an interference engine? What I mean is, will the piston be able to hit the valves if they are not closed when the piston reaches its top position. Important fact to know if your engine easily reaches its rpm maximum, the speed at which the valves open and close might overwhelm the springs, therefore valves might be floating and the noise you might be hearing could be the piston hitting the valves. If so, quickly get a fix or you will soon be faced with a seriously damaged engine. On the other side, what is perplexing in your case, is the somewhat conservative top speed for an 80 cc engine. You dont mention if the 70 kph is speedo indicated or GPS. If speedo, keep in mind that they are usually very optomistic, about 10%. So, if you read 70, you are actually going about 63. My people 50 cc 4 stroke with a dealer unrestricted cdi and nothing else changed, easily and regularly reaches an indicated 68 kph with no funny popping noises, even wide open throttle, going down a steep hill at about 72-73 kph. What you describe as the popping noise could also be the sound of the engine simply ruinning with the valves floating without the piston hitting the valves, in which case, no real harm should occur if not happening for prolonged periods.

Like someone mentioned previously, have your variator checked to see if it is limited there. An 80 cc engine should easily get you to about 80-85 kph with heavier rollers in it. Hope this helps!

Why a high performance plug? What is a high performance plug? someone says a high perf plug doesn foul as easily. A too warm or too cold reading on your plug is the result of carb setting. Adjust the carb, not the plug. A fouling plug, on a two stroke, is generally the result of too rich, in gas, carb setting, or too much oil in the gas. Once broken in, a two stroke, especially small ones, needs less oil. I used to run my old 2 stroke lawn boy with a 50:1 mix even though they recommended à 32:1 mix. Running this way, the little two stroker ran smoothly for 38 years. Never fouled a plug. In the end, a crankshaft oil seal gave out and it was the end of it. No lack of lubrication here. Back in the seventies, yeah i'm that old, my 125cc 2 stroke kawasaki used to foul plugs quite easily. Usually right after starting a cold engine while mixture was still rich. I leaned out the oil pump as much as I could. Plugs lasted much longer without fouling and the engine ran much smoother. The engine on that thing went for a good 20 000 miles before everything else broke on the bike. I think going to a ''high'' perf plug just bypasses the real problem. Fuel, air and oil mixture.

I get about 90 miles per US gallon. It's a S50 4 stroke. I too use the throttle like an on-off switch. With my DL650, I regularly get about 48 miles per US gallon.

 I have a 2009 people s 50 4t, as in 4 stroke.  When new, it topped out at an indicated 57-58 kph. Completed the breaking in period and got my first oil change at the dealer. While it was in, I got in contact with Kymco reps via internet. They got in contact with my dealer which proceeded to derestrict the CDI. Result: 68-70 kph. Still, I believe there is still some restriction but I dont care, that speed is good for city trafic. After that, changed the stock rollers. They were 7.0 g., even though the owners manual cites 8.0 g. So, after trying some new 7.0 g rollers, I got some 6.0g rollers. Better acceleration somewhat better up hills, but still, not enough power going up. I mean, pretty steep stuff. I live in a mountain. Did the GPS thing, got 63 kph for an indicated 68-69 kph on the counter. It translates as 39 mph. There was no restrictor on the variator. I tried removing the restrictor in the air filter, but the engine cant take it with the stock jet. It just starves for fuel as there is to much air coming in. I think  the last remaining thing left to be done is getting a .85 main jet in the thing and opening up the air intake. As I said, I dont need more speed, just more climbing power. Funny thing though, acceleration seems adequate, never seems to slow down traffic, as a matter of fact, usually the contrary. But the minute I encounter a slight incline, it slows down to 50-55 kph, slower on steeper ones. I'm just lazy about the carb and my wife, which is much lighter than I am ( I'm 200 lbs) doesnt seem to complain. My real bike is a Suzuki DL 650. But still, scooters are so much fun around town....

All this might help you, since the mechanicals on the People s50 4t are about the same as your Super 8 4T. I wouldn't change the variator, I believe It will not give more performance, maybe smoother performance, but I find it acceptable on the Kymco stock one. What I havent figured out is if changing the big pring in the rear part of the trans mission will allow the engine to raise its revs when in need of more power going up hills. As it is right now, it just seems to settle at a constant rpm and just slow down to about 35 kph on the steepest of hills and just stay there until the incline levels off, then the needle on the speedo just sweeps up again.

All in all, maybe not the most performing little machine, but up to now, a very reliable one and easy to drive. Also very cheap on gas.

When you say its leaking from the transmission, where is the puddle? under the center stand or under the rear wheel hub? Remember, the only transmission with oil in it is in the rear wheel hub. You did not specify where the leak is.