OS 95AX Mid-Range Problem
 

Does anyone have any tips or tricks on how to set the low end and high end mixtures on an OS 95 AX to get the engine to run good in the mid-throttle range? I am able to adjust the low end mixture to get a reliable idle and then go to full throttle rapidly without any problems. The problem is that I can't get the engine to run reliably/smoothly in the mid throttle range (~5000 rpm). When I run at mid/half throttle, the engine starts surging and eventually dies. It's as if someone is closing and opening the throttle every 2 seconds. It does this no matter if I turn the low end mixture leaner or richer. I discovered that if I turn the high speed needle valve 1/2 turn in (lean) from where it reaches max rpm at full throttle, that the engine runs great at mid throttle. But this is too lean on the high end and I don't dare fly at this setting. You would think this would indicate that the low end is too rich, but leaning the low end does not help the mid-range problems.

My set up is as follows: I am running a 15x6 APC, new #8 plug, new 12 oz tank mounted centerline of carb, fresh fuel (10% nitro, 18% oil), with no cowl. It is not a fuel flow problem, the clunker is installed correctly and the fuel lies are new with no visible bubbles. I also recently sent the engine in to Hobbico for diagnosis and they found nothing wrong after running the engine. My first flight after I got the engine back was a dead stick landing!

I have tried an F plug and that helps. But, the mid range problem is still there just not as bad. Besides, it should run properly on a #8, right?

RE: OS 95AX Mid-Range Problem
 

GR - I recently had a very similar situation with my (relatively new) OS 91 FX. Low speed & idle - fine. Upper to High speed - fine. Mid range - well, it would wander up and down - pretty unsettling.

Inspection showed that the fuel tank stopper was damp - not wet - but some fuel was leaking - but just barely. I found that the screw in the stopper was no longer tight. I went ahead and replaced all of the fuel lines (inner pickup, fuel to carb and pressure from muffler). All is good.

Hope your problem is as straight forward to resolve.

Oh, how easily does your engine start? Iask because mine required closing off the muffler tip (not completely - just partially) to generate enough pressure for it to start with a starter.

RE: OS 95AX Mid-Range Problem
 

I have ruled out any tank problems. I have had the same problem with different tanks and the current tank is new and has been tested for leaks, proper clunker clearance, kinked tubing, etc.

The engine starts easily. Idles good, doesn't hesitate when going to full throttle quickly from idle. But when you put the throttle approx. half open, within 5 to 30 seconds the rpms start going crazy. The rpms will drop out as if the throttle was suddenly closed and then the rpms bounce back up to where they were. It surges back and forth like this every couple seconds until it just dies.

RE: OS 95AX Mid-Range Problem
 

I have a Thunder Tiger 91 that has behaved in a similar fashion. I had many dead stick landings while flying mid- throttle. In sorting out this engine I have found that it takes about 2 gallons of fuel to break in the engine and the factory idle setting is way too rich. It has taken considerable patience to adjust the needles. Once the engine gets close to being broken in, the engine responds appropriately as the low end needle and high end needles are adjusted. I am using SW 15% nitro OS#8 plug and a 14x3w APC prop. It turns at 9000 rpm and the vector flight Sukohi has unlimited vertical performance. No more dead stick landings. Some have recommended using an OS F plug but I have not had to resort to that. The engine starts very easily. It takes time and patience to get it right, leaning the low end and getting the HS needle just right. A lot more work than most engines I have had! Do be sure there is no problem with your fuel tank plumbing.

RE: OS 95AX Mid-Range Problem
 

What muffler?

RE: OS 95AX Mid-Range Problem
 

I would check the muffler pressure lien. Pinhole leak, carbon or tar partially blocking the muffler vent, etc. Also is the engine broken in? Sometimes engines will not draw fuel well at mid range, not as much muffer pressure, and not as much carb suction as at idle, so it is in the poorest fuel draw at midrange. So you have it set slightly rich at full and midrange, it leans out at midrange, speeds up, gets more muffler pressure, then gets a slug of fuel from the muffler pressure, runs rich, slows down, muffler pressure drops, leans out, speeds up, then keeps repeating this cycle. Sometimes just more run time improves fuel draw and helps greatly.

RE: OS 95AX Mid-Range Problem
 

Midrange mixture is mostly controlled by a balance between the idle needle and the high-speed needle. To start out, set the idle mixture to stupid rich. The engine won't idle, so just start it at 1/4-throttle or so. Have the high-speed needle set to a very-rich setting. Start the engine.

Once it's running, advance the throttle to wide open and adjust the high-speed needle to just rich of peak RPM if the engine's had enough running time to accept it. Once it will run "all day" at this setting with little or no change in needle setting, then you can retard the throttle and look for the idle RPM and mixture. Any idle lower than 2,500 RPM or so is good. If you are trying for a lower idle and your engine starts having handling problems, then you have the wrong idle RPM. Trying for a too-low idle RPM is the source of many midrange problems.

One thing we hear a lot about is that the engine won't idle low enough to keep the plane from moving. That's not an engine problem per se, but a problem with the fact that you're on a very smooth surface, the wheels roll freely, and you likely have the wrong propeller on the model. If the airplane will fly the way you want it to fly, and will idle down enough to allow you a nice, slow landing, and the engine handles well, then you have the correct propeller. You will just have to accept that your model will just not hold still on the ground.

RE: OS 95AX Mid-Range Problem
 

Thanks for everyones help. Thumbskull, I am running the stock power box muffler. I have disassembled the muffler several days ago and checked for plugage in the pressure tap - it was as clean as a whistle. As far as how new the engine is, it is relatively new (purchased at beginning of season). It was broke in on the ground per procedure using over half a gallon of fuel. The engine has had about 30 flights on it.

Sport_Pilot - I think what you said makes the most sense. The engine is surging up and down just like you explained. This would indicate poor fuel draw at mid range. The engine should be getting good draw though as the tank is installed dead on center with the carb and just behind the firewall.

Bax, I did that procedure. Started stupid rich on the idle mixture at approximately 90 degrees richer than the original factory setting. I then set the high end needle just on the rich side of where it ran at max rpms at full throttle. Then I backed off to low rpms, then increased rpms to mid throttle and listened to see if it would surge/run poorly - which it did. Then, I leaned out the idle mixture approx. 15 degrees and repeated the previous steps checking the engine at mid throttle. I did this all the way through sloppy rich to mean lean on the idle mixture. No matter where the idle mixture was set at, the engine would always surge at mid throttle. It did seem to be somewhat better at approx. 90 degrees leaner than the original factory setting, but not anywhere safe enough for me to trust it in flight!

So, I am thinking a Perry pump might help. Do you think this is worth trying?

RE: OS 95AX Mid-Range Problem
 

How long is the fuel line to the carb assembly? Are there any "connectors" in the fuel line for refueling? As I had your mid-throttle symptoms I was using a lengthy piece of fuel line to the tank that had a white plastic connector inserted in the line and exposed outside the cowl that was disconnected for refueling. The connector was very restrictive (inside diameter maybe 1/2 of the fuel line). After re-plumbing the fuel lines I replaced the plastic connector with a short piece of brass tubing whose ID was much nearer that of the fuel line. With the tank cork tightened and the plastic connector replaced my mid-throttle problem has all but disappeared. Seriously considering a 3 line approach with a shorter carb line with no inserts.

RE: OS 95AX Mid-Range Problem
 

The vent tubing is only about 6 inches with no restrictions.

I ordered a Perry pump today and a spare backplate in case I mess up tapping the pressure fitting. I hope this solves the problem.

RE: OS 95AX Mid-Range Problem
 

I think that will help, but you may simply have a lemon carb. Still the Perry pump could help. Just be sure that the pressure is adjusted so that the high end has to be opened up, yet more than enough to get a good rich mixture. You can adjust it to lean the mid range as well, but that usually means a lean mixture at high speed. Most carbs will run richer at midrange if you have sufficient muffler pressure. The instructions have more detailed info on how to set it up. Just be sure you get it right.

RE: OS 95AX Mid-Range Problem
 

What did you ever come up with?

RE: OS 95AX Mid-Range Problem
 

My guess is that he's left with buyers remorse, wishing he had bought the 120AX for nearly the same weight.

RE: OS 95AX Mid-Range Problem
 

Ihave had good luck using a colder plug to help with rich midrange it allows you to run the high speed needle leaner and thus the midrange is cleaner.

RE: OS 95AX Mid-Range Problem
 

I installed the Perry pump but after an hour of trying to adjust it I gave up. I might not have given up on the pump had I remembered to bring a allen key wrench to the field to adjust the pump pressure.

But alas! I pulled from the depths of my flight box a brand new Fox long glow plug with idle bar circa 1990. I installed it and rotated the engine slowly to be sure the piston did not hit the idle bar. I fired her up, adjusted the high end, tweaked the low end, and up she went. I FLEW UNTIL THE TANK WAS NEARLY EMPTY WITH NO DEAD STICK!!! I made this flight at full throttle the entire flight since I thought it would die if I went into the mid range (as it had in previous flights with the #8 plugs). After this flight I made a half dozen more flights (varying the throttle during flight) with no problems! The Fox idle bar plug seems to have solved the problem.

Thumbskull, The engine i had in this plane before the 95ax WAS a 120ax! I bought the 95ax because the 120ax was too powerful for the plane. At least thats what everyone told me. :)

RE: OS 95AX Mid-Range Problem
 

Funny but the Saito 125 is lighter than both.

RE: OS 95AX Mid-Range Problem
 

Hi!
First of all...it's probably not the engine.;)

As with alla modern engines no lenghty running time in is necessary. Just two or three tanks is enough and all engines can be set up and run fine after that!

Have you pressure tested the tank? Have you tried opening the low speed slightly and see what happens when you throttle up from idle ?

The plug to use is a either an OS 8 or Enya 3. Not an OS F!
15x6 APC is fine.
12oz tank is also a good choice to that large propeller. But how is it mounted? According to the: "tank mounting rule" or what?
For best results use Uni-flow (two clunks, one for pressure and the other fto carb). The "Uni-flow" system gives a more even pressure to the carb.
15% nitro fuel is overkill as 5% nitro is just as good.

RE: OS 95AX Mid-Range Problem
 

Do me a favor. Try putting 2 two! Yes 2 Washers under the glow plug which should be a OS 8.

I fixed my buddies by doing that.

Dave

RE: OS 95AX Mid-Range Problem
 

Interesting. Do you think it lowered the compression / caused the plug to run cooler yet, so the needle could be further leaned which might help the midrange??

Thanks,

Ernie

RE: OS 95AX Mid-Range Problem
 

DLK, i tried the double washer trick but it didn't help. Since the problem has gone
Away by installing the Fox idle bar plug (and not the Fplug)
I would dare to say the problem was caused by fuel droplets hitting the element.