av8r4aa

My Feedback: (82)

I wrote what I found out about this engine in the post called" 91 FX leaning out"
It is in this forum.
Anybody else found this to be true I am interested to know.
Greg

Hircflyer

Perry Pump fixed my OS .91Fx !!!! Today ran entire tank without goiing lean and deadstick.

See 91 Fx leaning out thread.

Gone to a Higher Plane

I have finally had time to balance the crankshaft an my 91 FX and removed as much material from the conrod pin area on the crankshaft and also removed material from the conrod in an effort to smooth this motor out at idle and high speed. When removing the crankshaft it was so tight in the bearings that I had to press the crank out with a bearing press which is way to tight so I chucked the crank up in my lahte and used 280 grit wet/dry sand paper with oil to try to get the OD of the crank bearing surfaces down so it would slip in the bearings with very little effort. I was successful in getting some of the vibration out of the motor but it is still way heavy on the con rod side and there is just not enough counter weight on the counter balance to help balance out the weight of the conrod, piston, and wrist pin so I got it as good as I could. I was also able to get the crankshaft to slide into the bearings so there is a few thousands of end play but not enough.
I just finished running the motor on a test stand with a MAS 13/6 prop and Omega 15% fuel...there is no bubbles in the line from the carb to the remote needle but I never had that problem when it was in the plane. I got the engine adjusted good but I think 15% may be to much nitro because even at a very rich setting the motor runs very hot.
I don't think the carb is the problem nor was it over proped to cause the heat problem but there is still alot of high speed vibration. I was not able to really check for idle vibration because it was on a solid test stand.
My biggest complaint about the motor is vibration and heat...I also think maybe a bigger carb might help. I don't see where a pump is going to help me much because I am not seeing a lean out problem but I am not ready to throw in the towell on this motor yet. I have a Super Tiger 90 that is as we all know in the same size case as the ST .61 and somehow ST did not suffer any of the problems we are seeing in the OS.
This is the first glow motor I have ever had that the crank was so tight in the bearings that it had to be pressed out and it is a problem that I have never seen in an os engine until now which leads me to beleive OS is having to scale back on QC to keep the cost down. Any comments about the heat and vibration would be appreciated.

Hircflyer

I agree about the heat issue....I tried to use 15% when I was playing with everything trying to get this engine to run right. It ran a lot hotter with the 15% than with the 10%, so I quit using the 15%.

Since I installed the pump, the engine runs fine on the 10%. I believe the extra fuel flow helps with the heat. Just like a full size recip the leaner the mixture the hotter the engine.

AS far as the vibration, I followed the advice someone mentioned in another .91 thread and aligned the prop vertical with the piston at top dead center and that seemed to help.

bewaretheengineer

My Feedback: (1)

Fantastic discussion! Glad I found it. I am having the exact problem with the 91FX motor I got second hand. I have tried many things from installing a bubbleless clunk to replacing all the parts in the high speed mixture assembly to different props and fuel and glow plugs. Bubbles after the high speed needle at lower than full power remain. I pulled the backplate and immersed the high speed valve in water and pulled a vacuum and applied pressure, no leaks whatsoever. What I have discovered is that this engine runs very hot. I have measured 330 F at the head stabilized at full power with an infrared device. That was even with an extra head seal and lower nitro fuel and a cooler plug. My conclusion is that the high speed mixture being attached to the engine overheats the fuel going through valve and causes vapor formation. At full throttle there is enough fuel flow to cool the housing enough to clear the vapor, but there is not enough fuel flow at lower flow to prevent vapor. The only fix I found is an externally mounted high speed needle assembly.

tony0707

HI
YOUR problem is a common oneOS knows of this problemif you contact OS repair at TOWER hobbythey will send you FREEa new back plate-needle valve assembly and mount , for the asking heat and vibration from the current back plate location ( one piece ) is the cause of the problema poor design of some of the OS fx series of engines is having-some 46's also -,been there -done thishope it helps get your engine running as it should-OS has been replacing many internal parts on the 91 FX if you have done damage to the engine as a result of getting air into it and therefore running it leanwhich is the end result ENJOY BEST RESULTS TONY

Taurus Flyer

bewaretheengineer, I am an instrumentation engineer!

Or mount another carburetor for a better midrange and do not use the remote needle valve as I did on my 61 FX.
This is a Moki carburetor, you only need a lathe.
BTW no air in the fuel line, but vapor as result of high temperature above 65 degrees C of needle valve assy.
I did measure temperatures of 400 degrees F is 205 degrees C on plug basis. See picture.


Edit: This post is placed on same moment as post of Tony, you can order an other backplate that's right!

Cees

tony0707

HI
JUST REMEMBERED,AN OS 7D CARB an early one NEEDLE VALVE ON THE CARB will bolt right in ,a very excellent carb( from an OS 91 or 1.08 FSR series ) you can also use a 61 to 108 ASP CARB,THE EARLIER carb that is an OS FSR clone engine ,the super tiger 90 carb will also bolt right in,all above are good options and will help the air line problem-,the OS being the best as is usual-TONY

w8ye

My Feedback: (16)

They seem to run fine with the new needle valve assemnly mentioned above

bewaretheengineer

My Feedback: (1)

Thanks so much for the information. I was unaware that Tower would send out a fix and I will be contacting them. I got anxious last night and cut the high speed assembly off the rear plate and bolted it to the motor mount. It moved maybe 1/8 inch back from where it was, but with that physical separation it should no longer be a heat sink into the incoming fuel. It got towards midnight when I had everything back together and I am flying a cross country tonight after work, training for my private pilot license, so I will get a run with the model strapped to the bench in the morning. I will be calling Tower anyways to see about the free parts regardless (I'm all about free stuff). Thanks for the heads up guys!

Broken Wings

My Feedback: (20)

From:
McDDD

"Quote"

Checking the remote needle valve for air leaks.

The easiest way is to start by filling the tank.
Make sure the inlet and outlet fuel lines are connected to the remote needle valve.
Open the needle valve about 3 turns.
Disconnect the fuel line from the carb nipple.
Plug your fuel pump line fitting into this fuel line.
Turn on or crank the fuel pump in reverse to suck the fuel out of the tank and back into the fuel can.
Look closely at the fuel lines.

If you see bubbles in the fuel line going into the remote needle valve, then the line from the tank to the remote needle valve is leaking air into the fuel system. Find and fix the air leak, then move on to checking the remote needle valve.

If you see clear fuel in the fuel line going from the tank into the remote needle valve, and see bubbles in the line that goes from the remote needle valve to the carb nipple, then air is leaking into the remote needle valve assembly from somewhere. Find the air leak and fix the problem before doing anything else.

If you see clear fuel in the fuel line going from the tank into the remote needle valve, and clear fuel in the line that goes from the remote needle valve to the carb nipple, then there are no air leaks.....and the remote needle valve isn't the problem.

"End Quote"

rcdude7

My Feedback: (264)

The problem with the metal NV assy has always been heating from the engine. The only solution is to seperate it (cut it off) from the back plate or update it to the plastic one like the .65LA. You can hunt for airleaks on the old NV assy till the cows come home, but it will not solve the problem, this was a design flaw from the start.

tony0707

HI
i love to see a HAPPY CAMPER PROBLEM SOLVED- ENJOY BEST REGARDS TONY

bewaretheengineer

My Feedback: (1)

Happy to report back that removing the needle valve assembly solved my problem. I did go through a great deal checking the integrity of the assembly and determining that it could only be fuel vaporization causing my bubbles. I mounted the cut off needle valve assembly to the firewall and ran through a tank a few nights ago on the bench. Bubble free down to the last drop. Thanks to everyone for their generosity of experience!

oldtyme

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!