Engine sounds rougher than usual at 31 mph and above

[150pcxer]

I realized that when at 31 mph and above, the engine sounds rougher and/or louder than usual.

I had the engine oil changed at roughly 1000 km prior to this phenomenon happening.

What are some of the possible causes?

[Mel46]

First make sure you haven't lost any oil.
Next, check your spark plug. How many miles on it? Gap ok?... etc.
When was its last tune up? (Valves, rollers, belt, etc).
Clean the throttle body or put some additive like Seafoam in your gas.

Any one of those things can affect acceleration

[Old Grinner]

Next, check your spark plug.

Good point Mel.

I've actually seen spark plugs loosen up over time if they weren't installed properly.

[you you]

Check your exhaust isn't coming loose. Expecially the exhaust port end.