ORIGINAL: KenChoo
Hi Everyone,
Wanted some opinions on a mid-range issue I'm having. Engine is a 1.20 2 stroke which runs great at idle and full throttle. Needles are set fine at both ends (pinch test for idle, and backed off 1/8 turn/couple hundred RPM at full throttle).
At about 40% to 60% throttle though, on an OS #8 plug, engine runs fine for about 2 1/2 seconds, then becomes a little erratic - dropping RPM then up then down (surging). If I roll the throttle smoothly and quite slowly back to full after no more than 2 1/2 seconds, it's fine (otherwise it quits). For a while I thought the issue was with my needle settings (mixture) but yesterday tried a number of plugs with interesting results. First, OS F - again, fine outside of the midrange, but after 2 seconds at half throttle the engine suddenly dies. No surging. If I try to advance the throttle back to full after 2 seconds the engine still dies, no matter how gently I roll the throttle. I tried varying the needle settings at both ends with a number of combinations (richer, leaner) to no avail at all. Then I put on a high quality cold plug and can't remember the exact results, but I think similar to the OS F.
So the engine runs best with the OS #8 (at least I can still change the throttle no matter how rough it is at idle or how short a time I can spend at half-throttle). Wondering if I should try a different plug? Maybe hotter? Like ENYA #3?
Thanks,
Ken
P.S. Forgot to mention, engine is mounted inverted.
Ken,
Let us start with the idle mixture adjustment... It is not 'set fine'. The 'pinch test' gives you a good starting point from which to continue; and by no means is it necessarily the correct final setting for the low-speed needle.
So, you should start from this preliminary setting and adjust from there, to achieve a perfect transition from low to high throttle.
In a 3D plane, it should be carried out, with the model at hovering attitude and at hover power (as the 'low' point), to 'launch' (full power).
For a sport/scale plane or a trainer, high and low are from off-idle to full, with the model in level attitude.
The OS #8 is 'medium heat'... According to [link=http://www.os-engines.co.jp/english/line_up/plug/pluindex.htm]this OS Japan page[/link], engines larger than .60 need a cold plug, such as their own A5/#10 glow-plug.
The OS F is actually hotter, but 'behaves' slightly colder in larger two-stroke engines.
The Enya #3 is the hottest plug they have...
As I wrote, you need a COLDER one.
I don't think the inverted mounting of your engine, causes fuel to pool in the plug's cavity, but you should change to the more appropriate side-mounting position.