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Help - Bog down on Full throttle - Polini Top 80 LC

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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



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Hi Simon,

Thanks - I bought fresh fuel, when I bought the motor, in October. I drained the old fuel and made a fresh mix. The 6 litres or so I put in have been sitting in the tank since then (7 months or so)  - Does fuel go stale in that period of time? Could that have been the issue? I hope so as that's an easy fix.

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For a start you don't own a Polini top80 ;-) what you will have is a Thor 80..  But joking aside 

It would help if you said what carb you had... But as it's a PJ i would guess it's a walbro carb... These do not like sitting around being unused for periods of time and can also get clogged up or get blocked if your not filtering the fuel correctly. there is a little wire gause in the carb that can block or partly block or you could just be running really rich top end causing the bog... 

Or you could have a leak in the fuel pump gasket these are known to be a bit iffy sometimes...   

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You say this is an engine that is essentially brand new?

If its got very very low hours, take off seems fine but is bogging down when you climb to altitude, my first point of contact is mixture settings on the carby.

Just remember that as an engine is run in, carby mixture settings often need tweeking. Time of year and temperature also affect these settings to...so if you are flying in a different area mixtures may need altering. 

Fuel mixture settings on 2 strokes are never set and forget for life, location and altitude. To some extent mixtures are more likely to need adjusting on a new engine than one that has been run in and has a season or 2 of use.

Edited by adamjedgar
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Hello again,

Thank you all for your replies!! I knew the para-community would step up to it. My paramotor is stored elsewhere, so next time I am with it I will take photos of the engine, carb and mixture settings and post them here so hopefully this will answer a few of your queries and help to pinpoint the problem. 

Morgy - Hopefully once I post the photos you'll be able to tell me what Carb I have... seeing as I even got the name of my engine wrong :) 

Adam - I haven't touched any of the mixture settings as I don't really understand it enough to do so and don't want to make it any worse. Maybe this is something I need to learn as it's likely that its something I'm going to need to do  with some degree of regularity.

I think I'll start with the easiest suggestion from Simon and throw out the old (7 month) fuel and getting a fresh batch mixed up for the summer. 

I note that no-one has mentioned the Spark plug?... again, please don't shoot me down as I am an engine amateur however just wanted to double check that the problems I have described aren't anything to do with that? Its the same plug since new, should I stick a new one in?


Thanks again



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