AeroFinn

Dear Bax,

Referring your kind instructions elsewhere of this forum,

I normally adjust the needle valve of my O.S. FS-91 w/pump to the peak and leave it there since the engine is a pumped version. I have tried also such a procedure that I open the needle valve a bit from the peak, say 100-150 rpm drop just to stay on the safe side in case the engine would run hotter in the air during a 14 minutes flight. Is this unnecessary?

I get 9600 as peak RPM (15% nitro) with 14*6 APC, is this normal or am I putting too much load on the engine? The O.S. official site says "practical RPM range 2000-11000

The full throttle rpm fluctiates a bit, though. Not much, say between 9400 and 9650 or so. Is this normal? I have tried a new plug (O.S F)with the same sympthomes. Well, I assume there may be many reasons causing the full throttle rpms fluctuate but what might be the most PROBABLE reasons to start work with? Regulator ? Adjusting the valves? Pump?

Thanks in advance

artto

Bax

My Feedback: (11)

9,600 RPM is good for that engine and prop. The "useful" RPM range is the RPM range O.S. has run the engine at when they work with it. The lowest RPM is an idle with one prop/fuel combination, the high end would be with a different prop. You can't get the full range with one propeller. If you go for maximum top-end, you lose low-end and vice-versa.

AeroFinn

Dear Bax


Thank you for your input so far;


Yesterday I was flying and the weather was a bit colder than last time, so my FS-91 w/pump peaked in 9900 rpm with APC 14*6. I continued leaning the high speed needle to the point where the engine started to hesitate or detonate. At this point I richened the high speed needle to the point just a little bit.

I was under the impression that such a procedure to adjust the high speed needle was per your instructions concerning pumped O.S. four strokes..

However, in the air the engine started to loose peak rpm so I landed the model. Right after landing I realized that I had to richen it to reach again the peak rpm 9900 and to avoid a lean run..


Why did this happen? What did I do wrong with my first adjustment of the high speed needle?


Thank you again for the great support !

Bax

My Feedback: (11)

As an engine warms up with running, the mixture can go lean a bit. Richening is normal after a bit of running, especially with the larger engines. That's why it's a good idea to let them "warm up" a bit before flying.

AeroFinn

This came clear, thank you Bax!