Spring engine throttle problem

[Gianluca]

Hello. I am trying to make a throttleable spring engine. It's a 4 cylinder, 40mm x 40mm engine that I added a throttle that rotates (like those old they used before), and everytime the vehicle it's mounted in hits an irregularity on the ground, the engine's pistons literally blow out from the cylinders

well, it really happens when it's not full throttle, I've tried more mass on the pistons and cylinders (originally 10kg, with 10kg cylinders), less spring constant, but it loses A LOT of power even with slight changes

https://www.youtube.com/watch?v=-dXakaLQaOs here's a vid I recorded that demonstrates the fail I'm talking about

http://www.algodoo.com/algobox/details.php?id=249278 here's the scene too