I agree with Amram. He is an experienced YS user and I don't think his problem has anything to do with his priming or starting sequence.
I think one of the main causes may be premature wear of the poppet valve seats. As I stated in my earlier post, this was not a problem with YS160's. However the pump assembly in a YS170 would appear to be different in several fundamantal respects, including the length and stiffness of the poppet valve springs and the detailed design of the seating arrangement. I am not exactly certain what the logic was for changing the design. No doubt Yamada will have had their reasons. It may have been the need for slightly more volume (YS170 glow versus YS160 glow) coupled with the need for a slightly higher pump delivery pressure. Whatever the reason, YS170 pumps seem to fail much more quickly than YS160 pumps. Around 80 to 100 flights is not uncommon for many reported failures, though I have also know some to fail at the lower mileages mentioned in this thread.
I don't think that ambient temperature is the main cause either.
It would be worth re-reading Arnstein's thread -
http://www.rcuniverse.com/forum/m_90...tm.htm#9001464
Baking hot summers are not something that we normally suffer from in Norway, or in Great Britain for that matter!