Vincent::: Myself and a close friend both have Wren 44 engines running in the Savex L39's We have flown them to death this year, between us we have approx 30 hours operating these engines. (Fantastic engine!)
What you describe is not like anything i have experienced with these engines. Some times there is a bit of popping as it ramps up, but once running it is as smooth as silk.
It seems strange that you have this problem on both engines?
Can you describe your installation for me? Where are the tanks? where are the pumps? do you have a UAT? have you any filters, have you modified anything at all, no matter how small? it may be relevant.
Wren are one of the most responsive companies i have ever dealt with, if you have a problem they WILL solve it.