RCU Forums - Don't run right

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My OS .91 FX has some kinda leak somewhere, won't richen up. No air bubbles from the needle to carb so I sealed the carb and set screws with some Hi-Temp silicone. Haven't tried it yet (silicone not dry) but will let you know. The jury is still out on this motor, not real happy so far. I'm running 5% and a 3 blade 12/7. Seems to keep the High rpm just right.

Here is an excerpt from a Clarence Lee column;

A few issues back a modeler wrote in about an O.S. 91FX that would not run at part throttle. I have one that had the same problem. I also noticed a large amount of bubbles in the fuel line from the remote needle valve to the carb.
The problem was found by removing the fuel line at the carb and connecting it to my six shooter fuel pump. By drawing fuel from the tank through the needle valve I was able to duplicate the problem. When I removed the needle valve and plugged the hole, the problem went away. I tried a new O-ring and a new needle valve. No help there. I solved the problem by having another identical O-ring groove machined in the original needle valve with a space of about .030" between the grooves. I wish I could say this was an original thought. It is not. I found the two O-ring seal idea on a Tower Hobbies .46.

This is part of an old thread from another board;

There is light at the end of the tunnel! There was a follow-up article in R/C Report magazine that just came out yesterday about the .91 FX problems. A friend of mine in our club scanned the page and emailed it to me. O.S. has released a new part for those engines that fixes the air leak around the needle valve - a 6.5 mm o-ring. It's a little wider and a little bigger than the std. 6.0 mm o-ring that comes with the engine and is causing the problems. The author of the article stated that he installed the new o-ring and the problems disappeared. I ordered two of them yesterday!

None of these may fix your problem, but may give you more stuff to chew on_Bob

Thanks Bob, I'll try a new O-ring ASAP!

Secret
If you don't see air bubbles from the needle to the carb then your problem may be you have the idle screw set to lean. Richen it a half turn and then see if you can richen the high speed needle.

Roger that, I'll try it tomorrow before I order an O-ring, Thanks

I didn't ask before, but if you're using a pitts style muffler this could be the problem. It may not be providing enough pressure. If you are using the pitts muffler you might try partially restrickting one pipe. BUT try the low speed needle first.

Last year I posted the following on RCOnline

djlyon
YS & OS Bubbles Posted 10-25-2001 23:20

--------------------------------------------------------------------------------

My YS53 has bubbles in the fuel line between the regulator and the carburetor when running, from idle to half throttle. At full throttle the line seems to be clear. I don't do anything about it because the engine runs great through out the throttle range and produces great power and besides it is a common condition, which the experts have blessed as normal.

On a RC online thread awhile back it was discussed at length and I presented a far fitched (imo) theory that perhaps the bubbles were vaporized alcohol, the vaporization taking place as the fuel exited the pressure regulator. I didn't really believe this. I thought there was an air leak in the system some where. No body on the thread commented on the theory and I lost interest because my engine and others that I knew of ran just fine.

Now to the OS 91FX, a fine running motor that I've been running for a long time. Good idle, good throttle response and makes really good power. However posts on RC online and magazine reviews have complained about air leaks in the remote needle and discussed what to do to fix them. A few weeks ago I noticed that at low and mid range my engine had bubbles between the needle valve and the carburetor. I ignored it because the engine was running well. Last week I even spotted fine bubbles at high throttle. I suspect the bubbles were always there and I just hadn't noticed. I generally don't try to find out what's wrong with a good running engine.

A couple of days ago I put in a new needle "O" ring anyway because I was sure there was a leak. It still had the bubbles and it still ran really good. After the flying session I started to drain the remaining fuel from the tank and noticed I was getting almost solid foam. I had hooked the fuel pump to the line between the carburetor and the needle instead of the needle and the tank. As a result the pump was sucking the fuel out of the tank through the needle valve. Then I noticed that the fuel coming out of the tank had no bubbles. It was solid fuel. Conclusion, big leak in the needle valve requiring fixing, broke or not. Latter that day I put a clamp on the line from the needle valve to the tank and drew a vacuum on the line to the carb. It held the vacuum, NO LEAK. I filled the tank with fuel and then began sucking it out through the needle valve with the pump. Sure enough, foam again, but still the line to the tank was clear fuel with no bubbles. I left the pump run and clamped off the line to the tank. All flow of fuel, bubbles, foam, air or anything else stopped coming out of the needle valve. Again NO LEAK.

Now I'm really beginning to believe that what I and maybe many others have been seeing in the OS remote needles and YS 4 strokes is alcohol vapor bubbles and not air bubbles caused by leaks. I not sure it makes a difference if it's alcohol and not air. Uneven fuel flow is still uneven fuel flow.

Any comments? Any fluid dynamics persons out there that can give some insight? Fluid properties folks? I'm an aerodynamics major but that's just a small segment of fluid dynamics and doesn't help me here a bit.

IF it won't richen, flush out the carb with fuel. There may be some dirt in there. I've had that problem before with other engines. No fault of the engine, it just happened. I have a 91FX that has not given me any trouble at all.

The muffler on my .91 FX is a pitts "style" muffler, not the traditional pitts but the one that pretty much just diverts the exhaust down. The engine is also inverted? I haven't run it with the new fixes yet so let me do that tomorrow and I'll let you know the outcome. Thanks for the overwhelming response.

Hi all,

About the o ring thing. If OS now has fixed the problem with a new part. Should they not offer this for free? It sounds to me like a manufacturing/design defect.
Any thoughts?

Jimmy

Hey Guys, just wanted to thank everyone who helped me "work through my problem". I ran my .91 FX today after the various mods from your suggestions and she ran great. Problem solved. I wish I could say what did it, whether it was the needle valve o-ring, or the sealant around the base of the carb, or the idle needle setting, but whatever it was it worked. Thanks again.

Just for grins and giggles, I ran 5% sig fuel (my ducted fan fuel), an OS #8 plug and a graupner 12/8 three blade prop. She will idle at 2100 rpm and max lean at 11500 rmp. I have her set at 11100 and she is pouring smoke out, after 24oz of fuel through her the head was decently cool (could keep my finger on it for a second or so). She is mounted inverted with a "pits style" muffler (diverts the exhaust straight down) and she is going into a "slightly modified" Great Planes 40 size Extra 300 ARF. HEE HEE HEE HEE...I want LOTS of vertical power!!