low revs in 5th gear

I have a Honda Sonic 125 4stroke (Thai motorcycle). As standard the top speed was 120kph. I have fitted a performane exhaust, carb, cdi unit and changed the sprockets (bigger front, smaller rear). These mods produced loads more revs (10,000) and a lot moe acceleration. However, in top gear the max speed remains the same at 120kph and the engine revs at 4000rpm. I have tried various jets sizes in the carb, as well as various sprocket combos. I can get 120kph in 4th gear, but when i cange into 5th there's no extra speed, in fact there's a tendancy for the bike to slow to 110kph. I have consulted a few mechanics here in Thailand, but the best answer i could get was "this is a common problem with this model, 5th gear isn't very good." Can anyone give me a better response than this?

Given your change of sprokets, you should get the acceleration results you got, but you should not be able to reach the same top speed. When you compromise gearing like that, you compromise top speed. It is like a Catch-22 if you will.

hang on.

am i wrong but with a larger front and smaller rear, acceleration would decrease and top speed increase.

any way the power of the bike may have trouble producing torque in 5th gear as it is only small capacity and not designed for top speed.

at that speed the motor is working to hard and is losing power through wind resistance, internal friction but the big one is unsprung weight.

its wasting alot of power just trying to keep the rear wheel/tyre spinning

Correct, the gearing change you made not only hurt accelleration, but top speed will not increase because your bike doesn't make enough power to pull the taller gearing. You hurt it all the way around.

Okay, so I'm dumb and I didn't read the sprocket change carefully. I am hoping that the order was typed in reverse, upon more careful consideration.