I had this question today.
I usually run my engines with 20-23% lube content which is half castor and half Klotz-200
With all my engines , i tune the HSN so that the RPM is around 1000 less than the max.
However with this Saito 125, mounted on Javellin 90, I seem to have a little less power. So i am running the engine at peak RPM . Since i have enough lube in my oil, is this going to cause any harm ? I cannot use nitro due to availability/costs, and want my engine to have a long life. No compromises here.
Please advise if there could be any harm in running an engine with high lube content lean.

-Gaurav

Running close to the max is always risky, right ?  And the bigger question is, do you really need that kind of rpm ? I have always heard that running engines "a bit rich" helps with engine longevity.

Getting a hole in the piston is a pain, skirting around the peaks.

Not a good idea to run the engine lean. Mind that when the engine is up on air, the head wind will make the engine lean further. Having a higher lube content might not be a solution although it helps. I would suggest to play around with prop choices before leaning out the engine.

To see if the engine has run lean, just look at the piston and see if there is any light blue discolouration. If so, then the engine has been run lean. - Well, this is just to see if damage has happened after the damage (if any) has really happened.

-Ismail

Agree, That running close to max is risky. But why ? Because the engine is going to run hot ? But then wouldnt the 2% extra lube help to cool off, and protect from damage ? Castor is supposed to take away heat much more efficiently than synthetics, and siunce i have around 10% of that in my fuel ( the other 10% is castor ) wouldnt that help ?

Regarding why i need that RPM, is because i have this Saito 125 mounted on a 90 size airplane with a 63" wingspan. I thought this engine was going to be more than enough power for this, but even with the HSN leaned out fully, there is power but not unlimited power i expected.

Intersting thing i obersve is that when the airplane is inverted, there is an increase in power, and the engine runs more smoothly
However since the needle was maxed out on ground, how could it run leaner and run better ?

The fuel tank centerline is nearly in line with the carb centerline.

I will try the prop change today.

The question is, do you have enough power for your style of flying ?  Whether the plane shoots up vertically like a rocket is not really a matter of concern, right ? I mean as compared to risking the engine.

Experimenting with props to suit your style of flying sounds like the right thing to do, and avoid putting your engine in harms way.

I was NOT having enough power for my style of flying, that is why i had to lean the engine. I do not fly 3D and do not need my plane to shoot up vertically like a rocket. but i do need power to help sustain KE and do short take off rolls.

I used a 15x6 prop and it was better, i just hope i havent damaged my engine by running it lean.
I guess time will tell !
-Gaurav

 A 15 x 6 prop is WAY too small for an FA 125. The minimum would be 15 x 8. Also 4C engines need much less oil. Excess oil - particularly castor - will cause rough running as well as carbon formation, which will ultimately foul up the valve seats.

I use 10% Klotz + 2% castor without problems. Also, tune for peak RPM and reduce by 300-400 RPM. That should be adequate to prevent lean burning as the tank empties.

I do not know the AUW of your plane but I have a TEXAN that is 70" WS, weighs 8 lbs. and flies well with an RCV 90 4C engine.

It appears more than likely that your problem is the prop. The excess oils is not helping either.

Thats what i thought too ! I used 16x6 for the FA-125 but then i was getting 8000 RPM on 15% lube fuel, no nitro at HSN fully peaked out !
For a 66" wingspan Javellin-90 this was not enough power to do short take-offs and hold KE. Plus the engine running was not very smooth too !
Then i switched to 15x6 yesterday and i got 9200 peak, and tuned it at 9000 and flew. Performed much better !
Engine was smooth, power was better and overall more satisfied.

This was my first experience with Saito and so far i was not very happy. My OS .90 Surpass gives me 9000 RPM with a 14x6 . and the same RPM i get from the FA-125 with the same fuel on a prop just an inch bigger !
From what i read, these Saitos need Nitro to run properly.

3D and the rocket reference were just my guesses/examples for non-simple flying habits 

Also, it is a well known/respected rule in nitro tuning that you always lean the HSN till you feel the screaming rpm, then dial a few clicks back.  This dialing of a few clicks back is the part that protects the engine. 

BTW, I always use the 3 to 5 second rule to tune engines (unless they are pumped engines etc, where touching the backplate is not possible). 

And whats the 3 to 5 second rule ?

The 3-5 second rule works like this.

First you tune the engine using sound of the engine as a guide line.  Then fly the engine for like a minute or so, at high throttle. Land quickly and touch the backplate of the engine. You should be able to put your finger on the backplate for 3 to 5 seconds. It should not be too hot, that you cannot place your finger there for 3 to 5 seconds (too lean), or too cool that you can place your finger there forever (too rich).   The tricky part is touching the backplate itself,  as many engines are inside cowlings etc, so best to do this before putting the cowling on.  And you have to do it fairly fast after landing with the engine still idling.  If you wait for while, the temperature measured on the backplate rises, and messes up the calculation. 

This procedure is particularly easy for helis, where landing and temperature verification are easier.

With 4C engines the peak is not well defined as there is no "screaming". The technique used by me for years is to -  very slowly - lean out the HSN till the RPM begins to DROP. Then open out by 3-4 clicks. Best to use a tach though.

The "backplate" monitoring technique would - again for 4C engines -  not be reliable as these engines run much hotter.

And - to repeat - reduce the oil content!! Trust me. Have used nothing but 4c engines for over 20 years. With pure castor I used to mix only 9% oil. With Klotz 12-13% total (with max 3-4% castor) should be fine.