I tached my OS Surpass 52 running on GX-5 (5% nitro) fuel swinging a 12.25" by 3.75" prop at 11600rpm top end and 4400rpm idle. Do those numbers seem reasonable? I have tried to get a lower idle but the engine cuts out after about 10 seconds on a slower idle than that, which is no good for slow passes etc.
I'm sure it used to idle lower, but with the old props I used which were 11x6 if I remember correctly, but surely a bigger prop will increase the flywheel effect and allow a slower idle? [&:]

Cheers,

Maybe the low speed is a bit rich, and thats why it can't idle for more than 10 seconds under 4400.

when you snap to full throttle from idle after a good 30 second pause, does it hesitate?



As far as your RPM range is concerned, I was just chiming in because a friend uses the same engine and we were also curious.

The low speed is as lean as it will go while still being responsive. I spent quite a bit of time carefully tuning the engine so it is as responsive as possible, I know what you mean though, intially leaning out the low end a little resulted in a pick-up in idle rpm and a slightly smoother idle.
The plug is new and the engine is pretty well broken in now, so I am at a loss as to why it is determined to idle so high.

G'day Mate,
If this was a new engine, & now jsut run in, have you checked your tappet clearance, they do bed in, after a while & may need to be adjusted, to get back to consistant running, both full throttle & idle.
When they bed in, the valves don't open as much, ie, the gap gets bigger, & you won't get a good fuel charge, or a good exhaust. This causes the engine to load up at idle.

thanks Alan, I'll remember that for my friends os52

what should the clearance be, do you know?

Don't run that engine over 10,000 rpm static as you may float the valves when it unloads in the air and it will be out of its power band. 9,000-10,000 is about right. prop 12/6,12/7, 13/4 etc. I would go with a non wood prop for flywheel weight. Idle will come after proper breakin and tuning. Their is a learning curve with 4 strokes but once set up they run and run. Good luck.

Thanks for the reply. I may give a larger prop a go at some point then and see what happens.

No one asked this, so I will. And this is a very basic functionality. Are you getting adequate tank pressure? It may be leaning out after full throttle with inadequate pressure. Just a thought. I don't fly four strokes, although I did have an OS 52 Surpass for a short time and it ran just fine all the time I had it, so I am by no means an expert. But sometimes a basic question can help someone get through a problem. But what I do know about them is running them over about 10,000 RPM is not supposed to be very good for the engine, and for the reasons already mentioned by earlier posters.

Good luck with that. Let us know what you find when you do get it running right.

DS.

2500 to 10000 RPM is a good workable range. 4400 at idle is fast. It is possible you have the idle mixture too lean. Does it shut off like you threw a switch or does it burble, decreasing in rpm then shut off. I suspect your idle mixture is too lean, especially if it just shuts off.

I've had three .52's and ran them all on APC 12X6. Very good all around prop for this engine.

You can download a copy of the manual from:
http://www.osengines.com/manuals/index.html
See page 18.
The clearance, cold, should be between 0.0015 and 0.004 inch.

How new is the engine?

O.S. told me in an e-mail that you cannot expect reliable idle until you've run 1-2 galons of fuel through the engine.

Initially I was happy to get 4400 reliably. After 2 galons of fuel I'm at 2100 RPM and BELOW.

Hi everyone, well I reset the clearances and it seems to be running a bit more smoothly now, but the idle isn't much lower (I haven't tached it yet tbh, British weather is preventing me doing anything outside!). I'm going to buy a 12x7 prop today and throw that on it and see what it gives. The valves weren't out by much, but I reset the clearances anyway.
It will idle smoothly at the current setting for about 30 seconds then slowly slow down, eventually seeming "misfiring" a few times then eventually stop.

G'day Mate,
Sounds like your idle mixture is a bit rich, lean it out slightly & try again, it should idle for 30 seconds to a minute, then pick up cleanly if idle mixture is set properly.

If you have not already bought your prop get a 12x6 instead of the 12x7.
Almost everyone runs the 12x6 on this engine.