Vacuum in 2 stroke reservoir

[CFS]

Hi all,
Whist prepping for an engine removal to replace the crank, I noticed some air bubbles in the 2 stroke oil supply lines to the motor.
I also noticed that the 2 stroke reservoir tank had the sides sucked in as if there was a vacuum in the deposit. Opening the reservoir cap released the vacuum and the tank went back to its original shape.
Can anyone explain what could have caused this to happen?
Have had air in the oil lines but never a vacuum in the tank.

Thanks
Carlos

[karl pa]

I believe the tank has to vent through the cap on the plastic tanks, if the vent system in the cap is blocked it will build a vacuum in the tank.

[Alan H]

Lots of info here - https://www.oldjapanesebikes.com/GT750_ ... e/page-A57 - but I usually drill a small hole through the top of the oil filler cap to stop this happening.
NEVER put anything on top of the filler cap - cloths, waterproofs etc.

Gen13-1.jpg

[CFS]

Thanks Karl and Alan.
I will look into the cap.