the engine is an OS 61 - I am trying to tune it so I started at 2.5 turns counterclockwise and it's starts ok, but dies when I take the igniter off the glow plug?

too rich? too lean? == any tips tuning this motor?

Start the engine at full throttle and set the high speed needle for a rich scream. This setting should be a noticable drop off from peak rpm. Don't be afraid to look for peak rpm, just do not lolly gag there.....open the needle up as soon as you find peak rpm to a lower note. This is a base line starting point, follow the OS manual for the rest. If the engine is forced to run with too rich of a setting, then it will die after the ignitor is removed.
If the plug is contaminated...or used up, it will light but will not support the catalytic action....so a new plug might in order, too.

You need a new plug.
Your 2.5 turn setting is close enough. The plug has an oxidized build-up on the coil and without the volt boost, there's not enough heat to keep running. Contrary to popular belief, the glow plugs need to be changed more frequently than the cars spark plugs.

Two good answers so far.

If the engine is rich (Or rich on the low end) that could cause the engine to quit or drop RPM when the glow igniter is removed.

So it is either that, or a bad plug

Yeah, my old OS 61 used to behave exactly like that. Of course, I was fairly new in RC and was not really sure when it came to tuning the engine, but I managed to fly it ok (Tiger 60 ARF, by the way). But, it kept getting worse, got to dead-sticks once or twice that I could not explain.

Even though I had a bunch (10 or so) glow plugs of various types (all OS, but various types within the brand), I did't even think about that. My instructor was there one day and told me to swap out the glow plug. Voila.. instant fix. I don't recall what I put in there, if it was a kind-for-kind (F for F, A3 for A3, 8 for 8.. whatever) but I swapped it out and it ran just fine.

But, I must add that he spent a couple of minutes man-handling my high speed and then my low speed needles (or the other way around, I forget.. maybe back and forth, but he played with them). But I do recall that I swapped it out and it ran much better. But, he wanted to show me ( Yeah again, probably for the 124th time.. jeech..) how to properly tune the engine.

CGr.

It's probably not the high end that needs a tweek but the low end setting....is this a new engine? don't even try to tune a new engine until you have it broke in

Do they still make the OS 61? Mine was an older 61 SF that I picked up at an estate sale way back when I first started out in this hobby.

CGr.

It's al older OS 61 SF -
I feel weird leaving the plug in when I put the engine at full speed because it doesn't smoke as it should. But then when I take it out (at open throttle) the smokes starts coming out of the muffler as it should. does leaving the glow stick on the plug for a few extra seconds while you are fiddling with the engine running damages the engine?

Leaving the glow starter on will not hurt the engine. Onboard glow drivers are available that allow programming the shutoff (for example, when you move past 1/4 throttle). There is also at least one post (can't recall the site and am not at my usual computer right now) that uses a simple on-off switch and alkaline c cell for an onboard starter that he leaves on for the full flight. This was to solve an issue with 2 stroke inverted motors that can have a problem with oil pooling on the plug causing flameouts at low revs, but both these examples demonstrate that leaving current (at the right voltage, of course) to the plug will not hurt the motor

I remember once forgetting to remove my glow driver from a plane and didn't notice it till I was landing.....First off I'm surprised the thing stayed on the entire flight, and 2nd' I sure was wondering why my engine was running so well Leaving the glow driver on while you tune is not a problem at all. Usually if your engine dies when you remove your glow driver means that you are rich (plug is getting too cold). As cheap as glow plugs are, you might want to just replace it to be on the safe side.

so what is the best plug for an OS 61 SF

Alex:

OS #8

Happy Landings!

It won't do the engine any harm at all to leave the glow running, but running an engine with a powered glow at high revs will cause the plug to burn out quickly so it's not advised, if only for your wallet's sake. It can also mask mixture problems if you're trying to get things set up - a powered glow plug will burn a richer mixture happily that will flood an unpowered plug.

There's three reasons why removing the glow driver will snuff an engine - first; if the engine's so cold that the catalytic conversion won't happen (it only takes 10-30 seconds to warm up though), second; if the platinum on the coil's worn off to the degree that it won't support the reaction (get a new plug), or third; if the mixture (typically the low speed mixture) is so rich that it's flooding the plug and cooling it off. If it's the third case tune the engine correctly and if it still happens you can get glow plugs with "idle bars" which stop the plug being splashed - HOWEVER make sure you tune it right first. Using an idle bar instead of learning to tune an engine is a band-aid solution and is only masking the problem instead of fixing it.

The other thing to check is your fuel - too little OR too much nitro can cause all sorts of problems, as can fuel that's absorbed water through exposure to humidity. When I started I had a bunch of problems with my Super Tigre engine idling till I a: learned to tune properly, b: got help from people at my club trying out different borrowed plugs and fuels mixes.