Yetti, remember this one thing, and you'll do OK.

HS needle is adjusted only when the engine is at wide open, adjust it just the rich side of lean.

LS needle is adjusted only to tune the idle running, and improve transition.

Tuning an engine one does an iterative process, in other words, do the steps once, then repeat them until things run the way you want.

Assuming the engine is close enough to start and run, if roughly, then:

Step 1: Start engine, go to full throttle, tune HS needle. First richen it a bit, to ensure you're on the rich side of things. The engine slows a bit. Now lean, and the engine speeds. Continue leaning until the engine stops speeding up. If it begins to slow down, rich quickly a couple clicks to catch it before it quits. Now rich it until the rpm's just begin to drop from peak, a distinct but small drop.

Step 2: Idle the engine. Idle at least 30 seconds (if it will). If it slowly slows down and quits, sounding like it's drowning, lean the LS needle a touch, and restart. If it idles a bit, slowly picks up speed (sounding rather smooth, not drowning) and then quits, rich it a bit and restart. Each time you restart the engine while tuning idle, it's important to go to full throttle for a couple seconds to clear the engine out, or fuel pooled in the crankcase can lead you to bad assumptions. Once the engine will idle 30 seconds or better, it's time to start fine tuning the LS needle. Start, and clear out the engine (WOT), then go to idle. Idle it at least 30 seconds. Go to WOT. One of 3 things will happen: The engine will stumble, blubber, spit, and either quit or finally clear out and go to full power. This means the LS is rich. Lean it a touch. Or, the engine will immediately flame out, without burbling, spitting and with no smoke. This means LS is too lean. Rich it a touch. Third possibility is the engine cleanly accelerates and goes to full power. You're good then.

Once you've done step 1 and step 2 once, do step one again (retune the HS needle, at WOT). After you've done that, do the acceleration from idle test again. Adjust the LS needle as needed to correct the performance, just as in step 2. If you didn't end up needing to change the HS needle more than a few clicks, it'll likely still be great at idle and acceleration.

When you make changes to the LS needle, make them small ones. If it's way off, do 1/8 turn at a time. If it's close, just a screwdriver blade width is enough. It should take maybe half an hour to complete this exercise.

Now, once you've done this, in the future you'll never need to adjust the LS needle. I like to verify the tune of the HS needle before the first flight of each day. The LS setting should be good until the engine is worn out, or you change fuels. The HS needle is affected alot by weather, prop loading, temperature, etc.

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One more tip, on Dead Sticks and avoiding them:
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If you're flying and the engine suddenly begins to sag, to try to quit, wether it be because you've leaned it too much, or the muffler came apart or whatever, the first thing to do is to reduce the throttle to fast idle. At idle, the engine makes the best fuel draw and is least affected by tank pressure (which is near zero at idle anyway). This will often keep the engine running, and if you gently advance the throttle, you may be able to squeeze a little power out of it, which may make the difference in making the runway or ending in the rhubarb. I learned this from my LA25, which typically tries to quit on the first inversion of each flight, when the air slug comes out of the fuel filter, temporarily leaning the mix and sagging the engine.. If I throttle back when it sags, it always recovers. If I don't, it'll often quit.

Good luck! Read all you can about tuning engines, and realize that they're all about the same thing, with different methods to achieve the result. HS, then LS, then rinse, repeat. Make 1 adjustment at a time, test the result, and continue according with what you observe.

J

Actually his engine was just fine.

His muffler fell off loosing pressure.

I commented to him that I had the same engine which ran fine w/o ANY muffler pressure, and that maybe he had the HS needle a bit too lean...

He tried adjusting it a bit to try to do the same...

That set it a bit off, but he knows where to put it back to.

Thank you for your input. One more question. When changing the prop on my O.S. 40 LA, I inadvertantly took off the backplate. Well, I put it back on, but it is not on evenly, or it is bent. When I am at idle, it is clanking against the engine. When I move beyond idle to higher rpms, it no longer clanks. How can I get the backplate off? It's being difficult.

Did you forget the washer under the prop drive?

If you lost or forgot it, the crankshaft will slide too far back as you tighten the prop nut and cause the rear of the shaft to hit the backplate ( why doesn't O.S. fix this? )...

As for the backplate...

Remove the four screws and lightly wedge a screwdriver in to gently pry it off.

yep, that's it. where can I get another? Do I really need it? heh.

Yes, you really need it.

You can get another from Tower or maybe your local hobby shop if they stock OS parts.

It''s a really thin washer, so I don''t know if there would be problems with just finding or making some other washer that fit. Too much space would make it easy for dirt to get in there, and might cause problems with the prop hub''s fit on the crankshaft.