McWilliams in action on his new Ilmor bike

The 42-year-old is recovering from breaking his leg in November and he and team-mate Andrew Pitt struggled at this week's test session in Losail.

"We can't underestimate how much work we still have ahead of us," he said.

"I'm not where I want to be speed-wise but we'll have some new parts in Jerez. Hopefully that'll make a difference."

Team managing director Steve Miller said the Losail test had taught them a lot of valuable lessons.

"We needed to understand the exact requirements of the engine before we could really play around with the chassis," he said.

"The engine has such an enormous impact on the bike's rideability, the riders can't position the bike on the track effectively if the engine isn't doing exactly what they want it to do.