Any ideas why my 125 is so underpowered?

I recently brought a tmec enduro 125, (Chinese copy of a Honda xr125) and it is drastically underpowered for its engine size, I previously had a 1984 Honda mtx 50 which was faster than it, I've seen videos of the same bike doing in ods of 70 odd and I can't understand why mine is so slow, it has a top speed of around 46-48 mph, which is amusing seeing my old 50cc moped used to do 45, it revs out okay I just can't understsnd it, any help? (And no not the "buy another bike" "push it down a hill" sorta answers please!)

POS also stands for Piece of Shanghai. Think about it.

Check the chain and sprockets are in good nick, it's amazing how worn ones waste power. Then binding brakes, air filter missing or clogged, wrong plug, under inflated tyres, stale fuel, are all possible causes, but don't expect much better than 55 on the flat. Chinese are cheap as they use prehistoric tech.

When you run a small engine at wide open throttle all day long, they DO tend to get tired or just plain worn out.

The Chinese bike is a poor knock-off of a popular motorcycle brand. If you hadn't been so cheap and bought the real thing, you'd probably be happier right now.

So you know the story-line, get a new bike or only ride downhills.
A 'copy' means very little, get the real one.

If it revs out okay then the transmission is not transferring those revs. You can probably improve it by changing sprockets but you might just create acceleration issues.

ETO
"it woudlnt have to power to have bigger sprockets"
That does not make sense.

Let's call out your FAKE NEWS. A Moped is only allowed to do 28MPH and mopeds are UNDER 50cc otherwise they are classed as a motorcycle and you got to have a MOTORCYCLE LICENSE AND INSURANCE AND A PLATE.

Your 125 might be underpowered because the rear sprocket is too small. A smaller sprocket gives you lousy 'Wheelie abilities & instant take off" but higher top end speed." That is "TO A POINT" then engine HP comes into play when you are going high speed… It is like riding uphill ALL THE WAY.

If you change the rear sprocket to a larger size, then you can almost do wheelies. And go fasted on dirt tracks where you are not going to do 60MPH on a rough course.

The gearing works identical to a 10 speed pedal cycle. Refer to that if you are confused with which sprocket goes where to do what.

Or maybe you are just OVERWEIGHT? So you really need a change is sprockets because the bike was made for a person average weight 150-180lbs

Fat people don't ride bikes. They need 4 wheelers.

Underpowered in what way? Top speed or climbing ability? Highway or dirt track racing? Sure there's a good middle of the road sprocket combination that gives you FAIR in ALL areas, But you got to be more specific when you say underpowered… Looking at YOURSELF first. I'm betting the bike will perform much better if you were not sitting on it.
. The difference between Japanese and Chinese is not going to be much at all.

If the air filter is clean than restrictive air flow is not the problem. Pull the plug when the bike is cool and look at it on the end. It should be TAN IN COLOR. That is the correct heat range.

Ok… I just checked a video out on it and it sounds like a Honda(straight gas); not a 2 stroke engine. Honda's were gutless wonders when they first came out. It is a road bike and gets you around town. Can do 70MPH and that is more than enough speed to give you a severe ROAD RASH. They are not wheelie machines. Just city trippers. Very few places in China that is open highway… That is not congested.
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If you bought it new, take it back to the dealer & have them fix it. But Chinese bikes have a rep for poor quality, so you shouldn't be surprised. If you want Honda 125 performance, then I suggest you buy one.

With a top end of 45MPR. Is it underpowered or under geared? Many Asian motorcycles will top out at 45 MPR. Do to gearing. They are made that way so you can strap a pig on each side, a few crates of chickens behind you. A goat on the handle bars & pull 10% grades to market in the next town. It might be a work bike. Those bikes are great. More power than a waterbuffalo & 3 times as fast. Put it in second hook a plow behind them Have the wife trot behind the plow keeping it up & state. You can quadruple the work you get out of a good woman with one on the farm.

Because it's a Chinese 125.