Vinales had a positive start to the test when he rode both the 2016 and 2017-spec Yamahas on Tuesday and reported good feelings with both, topping the times.

The next day, the Spaniard's task was to further compare the two types of chassis, but Vinales was only sixth in the order, nearly half a second slower than on Tuesday.

Vinales said it is still unclear for Yamaha what is causing the problems: "[The] first day was not so bad so I'm quite happy with the first day, I was feeling really good.

"Today was a totally different day. We lost, per lap, 0.3-0.4s. Same bike.

"Something still we don't understand, and to decide which chassis to take, which way to take, you have to be very clear and sure about the decision."

Yamaha will next test at Sepang next week, and Vinales said he will continue trying out both the '16 and '17 chassis in Malaysia.

"For sure we have to test more. The test here went well the first day, not so good today, so... difficult to take a decision right now," said Vinales.

"We have to continue working, continue testing the things. You know, when you can do two or three full days on a good mood, you can decide. But right now it's very difficult.

"With the '17 I have better traction and better acceleration, and with the '16 I feel better with the front. It's what I was feeling during all the year.

"Finally let's see - [whether] we take the '16 and we work to make the acceleration good, or we get the '17 and try to make especially the brakes and the corner speed better.

"We have to decide which way is the easiest to be fast."

Fellow factory Yamaha rider Valentino Rossi only rode the '16 chassis on Wednesday, as his task was to test a new engine and compare it with the old one.

"Today we had some work to do because we had to decide and test well some different engines," said Rossi. "It was a long day but productive.

"We concentrated on that [engines], we tried also something else but nothing great and in the end was not so bad.

"[With the new engine] trying to have more power without losing the character from the bottom [low-revs] and the smoothness from the bottom.

"We tried some different things and we had some positives and negatives but is not so bad.

"At the end I decided what is my favourite and now we'll confirm in 10 days in Sepang if it is the same or something else."

Additional reporting by German Garcia Casanova