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Good evening, Let me start this off by saying that I am no engine expert, so my terminology and descriptions may be technically incorrect, but I will do my best to explain and appreciate in advance any help and assistance offered. I do have a basic understanding of 2 stroke engines and am quite competent when it comes to trying to fix something - or at least I'm willing to learn. I have a Parajet Zenith with Liquid Cooled Polini Top 80 engine, flash start. I have owned this second hand since October, and prior to me the previous owner had it from new for a little over a year - I am reliably informed that it only had one flight in that time and approx running up a maximum of 3 hours running time. Since October I have had one flight in November, then 2 flights last week. I have tried to run the engine once a month or so. Last Thursday, I started the engine - always starts on first pull. Gently warmed it up by alternating the revs and running for approximately 5 minutes. I had the feeling that I wasn't quite getting full power output, but maybe put this down to my inexperience of not only the Polini but engines in particular. Takeoff fine, easily enough power to sweep me up in a short distance, full throttle up to 1000 feet before easing off and making some gentle turns - from then on when I tried full throttle the engine would slightly 'Bog' - I think that's the technical term. Basically lose power and sound like its going to cut out completely (although never actually cut out thankfully) - It would either rectify itself within a second and then power up fully or I would briefly reduce throttle and increase gently agaoin which seemed to get rid of the issue. The engine never cut out completely. I wasn't happy so landed, checked over the engine visually with my limited know-how, all seemed to be fine. I found a small bubble in the fuel line which I tried to get rid of by flicking it through the line. After a bit of googling and tinkering decided to try it again, Had a great takeoff , again plenty of power on full revs up to a decent height and played around for 15 minutes. This flight the fault only re-occurred once, but sorted itself out again. Although whatever the fault was seemed to have improved as time went on, I would rather understand what the issue is and rectify it if possible. Through these very helpful forums and much Googling I have found a number of suggestions of what the issue could be, but am really looking for advice and suggestions from the ParamotorClub community. Some suggestions are: Old fuel, Carb not adjusted properly, air in fuel lines, wrong mixture, not warmed up correctly, filter needs changing etc etc... The motor has had no service since it was new, so although it has only done about 5 hours from new, should I look to get this done? Thanks in Advance Sash