The much-needed new engine had enjoyed a series of positive outings ever since a secret debut in the hands of Fabio Quartararo and Franco Morbidelli at the Barcelona test in June.
But Yamaha were left scratching their heads when the latest - and what should have been greatest - version installed for the Valencia test proved no faster than the meagre 2022 power plant.
“This engine was supposed to be a little bit faster than the one we tested in Misano and Barcelona but it was the same as [the current engine],” said 2021 world champion and 2022 title runner-up Quartararo.
“It's not the way I wanted to end the year, but something went wrong.”
Monster Yamaha team director Massimo Meregalli agreed, telling the official MotoGP website: “The same specification was tested in Jerez by Cal [Crutchlow] and he said the engine was [more powerful] but maybe a bit more aggressive.
“So something went wrong and unfortunately only one day was not long enough to [find out what] because we had to spend time on other areas like the aero package and chassis.”
“I think first of all [Yamaha] will check the engine on the dyno and then we still have one test in Japan with our Japanese test rider before Christmas,” he added.
“Knowing the performance we already saw from this engine we are not that worried [because] for sure something was missing and we expect they will understand the reason.”
Meregalli confirmed there is no question of Yamaha switching to a Plan B for its 2023 engine.
“No, something went wrong because this engine was already tested in Misano and Barcelona. We never said in Barcelona, but we started testing it in Barcelona with good results [and then in] Misano, Motegi, Jerez.
“[Yamaha] have to understand why it didn’t perform here as we expected, also because compared to Misano this engine had a little update… for sure there is a reason.”
As such, the engine used at the Valencia test is “most likely 85% of what will be the engine that we will start the season with next year.”
The next official MotoGP test, for race riders, will be at Sepang in February.