Valentino Rossi has revealed he has tried a version of the 2019 Yamaha MotoGP engine in testing and was underwhelmed by it.
The Italian rider was speaking after finishing eighth in last weekend's Aragon Grand Prix, a race that marked Yamaha's longest-ever losing streak in its MotoGP history.
Having previously indicated that the character of the M1 bike's engine could be contributing to Yamaha's ongoing issues, he said he was dismayed to discover that an early version of next year's motor was not much different.
"I tried the first edition of the 2019 engine, yes," said Rossi. "But I think and I hope it's not the final one, it's just a small modification, because it's very similar.
"I hope they continue to work, because the engine for me is a problem for us. They have to make it better."
Rossi also clarified that he had not tried much else besides the engine for 2019, bucking a trend of recent years.
"A worrying thing is that usually the Brno [post-race] test was the moment in which we tried the following year's M1, but the last two years this hasn't happened. Where is next year's bike?
"Unfortunately in testing we haven't tried many things for next year."
However, when quizzed on what he wanted from next year's Yamaha, Rossi insisted it is not up to him to instruct the Yamaha engineers - merely to point out existing problems with the M1.
He said: "For me my job is not to say, 'I need the V engine, or modify the chassis in this way'. What I say are the problems.
"We are in big trouble with the marriage between the tyres and the bike, especially the rear. So, this is what I try to explain.
"But the area to work is not only one [thing], for me. They are different - electronics, engine character, etc.
"But I don't know. They have to know. I can just say the advice, every time I say the same."
Team-mate Maverick Vinales admitted at Aragon that he has "zero expectations, zero motivation" for the upcoming races, and Rossi said he was likewise struggling to maintain his enthusiasm.
"It's a difficult situation, because it's one year that we are in this technical situation," he said.
"Now also more, because our opponents, especially Honda and Ducati but also Suzuki, did a big step.
"I am a little bit faster than last year, because last year I was injured, this year I'm in good shape, also because the bike is the same.
"The problem is last year I finished fifth, this year I finished eighth and [Marc] Marquez and [Andrea] Dovizioso are 10 seconds faster.
"In this situation, because we are a top team, we are difficult to find the motivation to fight [only] for the top 10."