When i pedle hard the bike changes gears by it's self.

Chain adjusted correctly? (i know it sounds silly, but it did effect a Bike i had).
Was it the correct weight oil? Thicker oil means harder to change.
Personally, i prefer to listen>test ride to determine an issue.
Good Luck with it, i'd love to know how you get on.
In the meantime...YouTube/CrashTestDummyTV.
Cheers n 'Happy Trails'...soon 4u i hope.
>:)

POSIBLE TIMING CHINE

check your compresion make sure it is within specs. if okey check your timing make sure timing mark have not change. this is after you ensure that your not loosing fuel or your carb is working normally.

Terrence >>>> 1. Do not go through the gears while idling on a stand!!!
The rear tire is not moving at the speed that is normal for 4th or 5th gear..AND if you do get the tire moving that fast.. it is BEYOND DANGEROUS!!
This sounds like the 4th and 5th gears may need replacing.. or the shift drum detent for 5th gear has somehow become ineffective.. If you continue to ride this bike , you may cause more trouble and damage to the transmission.

Take this to a qualified professional!!

my 4k cant go to 2nd gear even you do adjustments.what is the problem?

sounds like you need to clean the carb may have a clogged jet

If you just had the bike serviced, take it back to the tech that did the work. If you did the work your self, check the gear oil level on level ground with the bike in the vertical position (riding position). It sounds like the oil is low and this can cause the gears to not shift properly. I hope this helps.

You have the bike in a high gear. Assuming it is a deraleur type, look to see which sprocket the chain is riding on. If it is one of the smaller sprockets (on the rear wheel, not the pedals), put it onto a bigger sprocket by changing gears to one of the lower gears. As a side note, professional riders change gears many, many times during one outing. They keep the pedal speed at a comfortable speed no matter how fast or slow the bike is going. Anyhow, to review, put the bike into a low gear, particularly when starting out from a stop. If the bike makes a wierd clicking sound, or the gears don't change as they should, it is possible that your indexing in your gears needs some adjusting, or there is some other problem. Any bike shop will take care of you, as these are very easy to fix.

Not familiar with the exact model of bike but normally you start riding in a low gear. As you wish to speed up or find it easy to push the pedals just change up in gear and keep pedaling and the gear chain will move to the next gear cog. The secret is to keep pedaling when you select a gear either up or down and the gear will change. Hope this is of benifit.

There is pretty much a false neutral between every gear on an Enfield. You need to ease the gear up or down and do it by feel more than a hard up or down movement