I have been looking though all the old threads on this problem and thought I would start a new post so that I could let you now what I have found out without digging up an old 2 page thread. The problem with these engines were determined to be that on the old engines, with the solid cast metal rear needle valve was that the engine would run lean at mid range. I discovered this and so for the most economical way to fix it, I cut the needle vavle off the back plate very carefully and attached it with two threaded screws to a plastic piece that was attached with the engine mount bolt to the firewall of my big stick 60. WELL, this did not fix the problem. I have the exact same problem as before cutting the valve off the back plate. The engine continues to mis at mid range. I flew an entire tank through the engine at mostly full throttle (it flew fine, head was cool enought to touch), landed and refueled, took off and did a few touch and goes without raising the throttle above half and all of a sudden when I went to full throttle, the engine would not pick up rpms and died on landing. It was hotter than a two dollar pistol. I hope no damage was done in the short 4 minutes that I was flying it at half thottle. I assume from this that when running at full throttle (and it also idles well) it was fine but it was lean at mid range and finally got too hot to run. My next course of action will be to put a supertiger carb I have laying around on it and see what happens. I am thinking that instead of a heat issue, that this is really a vibration issue and that the engine vibration travels though the engine mount and into the needle valve mounted to it.

I am using 15% magnum fuel with 18% oil blend, a 14x6 MA S-1 prop. Peak rpm about 11,500 richened up to 11,000. The engine has new bearings.

You have to use the later plastic needle valve body that is attached to the back plate with a stamped steel bracket

Perhaps a hotter plug. It would make you run the needles on the high and low speed slightly richer and thus the midrange.

My 91FX did not get many runs on it as it too was not responding to high throttle. For about 4 flights, it would only run for about 5 minutes before I would notice that there was no increase in RPMs when given full throttle. I use 15% Power Master and a 15X6 Prop. It idled great, ran well on the ground, and it responded normally to throttle after takeoff. But, at or about 5 minutes, the baffling issue would return.

Because there was a lot of burnt fuel or varnish around the head, it was easy to see why. The head bolts were loose just enough to lose compression. It was after my plane's last flight that I found the reason. The cylinder and piston had blackened a lot. A 24 hour antifreeze bath in a crock pot got rid of most of the black stuff. I have yet to try it out to see if the issue was solved.

I don't know if this is your problem.

There seems to be kind of problems with bigger props. 14x6 or 15x6 are a bit too heavy. If you stay with 13x8 or 13x7 they look lot more 'acceptaple' by this engine. It is not low RPM engine as many conider it. Also, over 10% nitro is not making any good, just heats too uc the engine. OS8 plug seems optimum. Cheers, Nick.

It was my understanding the orings in the original needle valve assembly played a huge part in the issues the engine has. Some say it's heat while others say its vibration all I know is this the new NV replacement W8ye mentioned fixed mine for good.

My modified FX61 had needle valve problems for a long time. I tried replacing O rings, and even a new needle, I could not get ridof the leak. I could seebubbles in the hose from the needle to the carb. I took the stock needle valve assembly off of the backplate and replaced it with a remote needle valve attached to the plastic motor mount. Problem solved for good.
I agree with the previous poster, this is not a low rpm engine. I run either a 12x6 or 13x5 at about 14,200 on the ground slightly rich. Mine has a Performance Specialties piston and cylinder, ported crank, and larger carb. I run 15%, but I live at 5000 feet.

It is my understanding that the needle valve position in the original lead to vapor lock.  It is located behind the cylinder so reduced airflow. The needle valve is attached directly attached to the backplate so heat transfer is high.

Maybe best solution is to use Perry pump; it is a bit more expensive solution but the cheap solutions take away very precious time...

I can tell you that even with the needle valve detached from the back plate and attatched to the firewall, it still gave the same results. I did not get the new needle but did cut the old one off the backplate, so I think the problem is with the needle valve itself. I put the 61 super tiger carb on and now it is running good. It will sit and idle for 5 minutes at 2400 rpm and then jump right up to full throttle when prompted. Mid range on the ground sounds smooth but for some reason in the air it just sounds funny. Maybe it is just the sound the stock muffler makes.

Bubbles from remote needle valves are not caused by o rings unless perhaps at idle. The fuel is under pressure, the O rings are there to prevent fuel from leaking out, not air in. The bubbles are caused by fuel vaporizing and are vapor bubbles not air bubbles. This is common to all remote needles and is not a probblem if plumbed so that bubbles do not collect and go through the engine at one time. Howver sometimes that is unavoidable and is why many manufactures are going back to needles mounted on the carb. Though the fuel will vaporize, it will vaporize where it needs to, in the carb.

I had all of the above with my OS91FX mounted in a GP Big Stick 60. I read a lot about this problem and talked to Hobbico Services and this is the fix that worked. The problem, as explained by a tech, manifested itself in the 91 even though the needle assy is mounted the same in the 46 and 61. The fix is to remove the needle valve assy from the back plate or mod the engine with these parts. The needle valve assy for the OS65LA is fiber and will not overheat and cause vaporization of fuel and the recommended parts to mod the engine are: needle valve assy - http://www3.towerhobbies.com/cgi-bin...&I=LXMW56&P=OW, needle stay - http://www3.towerhobbies.com/cgi-bin...?&I=LXGBG0&P=M,
cover plate - http://www3.towerhobbies.com/cgi-bin...&I=LXANA7&P=OW and last but not least a gasket set
http://www3.towerhobbies.com/cgi-bin...&I=LXCM87&P=OW

I did the above mod and my OS91FX is running fine!

_____________________________

AMA 16066