Granted that is not much rise. I had the same problem with height.
Do you have an anti siphon valve? Many with C-30's and E-35MKII's have eliminated them as getting enough "height" to make them function well is problematic. I eliminated mine and ran a bleed to a cockpit drain and Neil eliminated his too I believe but I'm not sure where he connected his. I'm sure Neil will chime in with that info.
If the engine is running well once running I would first check the integrity of the manifold as an internal pin hole leak would take a long time to get much water into the manifold like when sitting for a long time with the engine off. If the A-S valve is sticking it would not take long to get water into the cylinders.
If the manifold checks good a compression check may yield some answers.
Dave Neptune
