Valentino Rossi was hampered by vibration issues with his Movistar Yamaha on the second day of the Sepang test, leaving the Italian rider frustrated after he was unable to build on a positive opening day in Malaysia.

Rossi set the second fastest time on Wednesday, only a tenth of a second behind Repsol Honda's Marc Marquez, but he slipped down the time sheets to sixth place today with a best lap of 2m 00.676s, finishing 0.713s behind team-mate Jorge Lorenzo.

"The chattering this morning was a big problem and right from the beginning I wasn't able to ride at 100 per cent, in fact on the lap with the new tyre I was slower than yesterday," Rossi said.

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"We expected to go a little bit faster, like Jorge [Lorenzo] did. In the afternoon we fixed the vibration, but we had to change the balance a lot and at the end I didn't like it very much - I'm not very satisfied.

"My pace was not so bad, especially because I was able to go a bit faster than yesterday, but I think that we can do better so tomorrow it will be important in the morning to try and make a good lap with the new tyre. After we will do a simulation, I don't know how many laps, to try to understand the pace."

Rossi was at a loss to explain the chatter problems and is hoping for a much better outcome on the final day of the test on Friday.

"We modified something in the bike, we tried something else but in the end we came back like yesterday. Anyway I had vibration so why, I don't know," he said.

"We tried to change the tyre and with a new set of tyres it was a little bit better, but anyway it is three tenths slower than yesterday [on new tyres].

"We will look at the data and try to understand if everything is ok, like the clutch and the engine braking and tomorrow we will try another time."

The former Ducati rider also conceded that Andrea Dovizioso and Andrea Iannone would have the potential to battle the factory Honda and Yamaha riders this season if the new GP15 lives up to expectations.

"They are fast and also the pace, also they have the advantage of the extra soft [tyre] and maybe that was good when the Ducati was slow, but now I don't understand exactly why [it is faster]!" he said.

"But anyway I think they are quite good and it depends with the new bike if they are able to make a step; if the new bike is good then they can fight with us."