Slipping chaing when on on high gear

Apfik

Hi, I am new to the the forum as well as to fixing my bike, so please apologize me if the answer to my question is obvious.
I am fixing my old MTB, which was almost impossible to run as it was standing outside for some time. I had to change a chain as it was very rusty as well as complete chain set as it was somehow slightly bended. Everything works fine, but when I set the chain to the high gear (big chain ring, two smallest gears on back) the chain stars to slip even when I paddle with moderate effort.
Do you have any advices what that could be?
I did check the cassette and it seems pretty good, but I am aware that there can be mismatch between new chain and old cassette, but the problem is only on the two highest gears, so I am not sure about that.
I was thinking about two things: 1. too long chain or 2. there is something wrong with back gear shifter.
Thank you in advance for all comments and suggestions.

The Rookie

Have you checked for debris between the gears lifting the chain off the teeth?

If no debris, worn cassette/freewheel I'm afraid, the smallest gears wear most as less teeth are engaged with the chain.

owenlars

New chain on an old cassette is a recipe for slipping. This link will tell you why http://sheldonbrown.com/chains.html

Apfik

Thank you for answer. I looked closer if there is debris, but it is completely clean. What I discover is that the chain get lifted by higher gear when it is on two smallest back gears. Now, I am thinking about two possibilities.
1. I placed the the two smallest gears opposite way on the cassette.
2. The chain is thicker or is lower quality and some of the connection pins are sticking out and touch a bigger ring.

The Rookie

3. Cassette is worn and the chain is riding up the worn teeth

You can't fit them the wrong way round due to the unever spline pattern

njee20

Worn cassette.