Frustration with 91FX
01-06-2003, 03:01 AM
#1
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Frustration with 91FX
I have a 91FX and it's failing. The engine keeps quitting on me and has "dead sticked" my plane several times. It also is overheating.
The "quitting" happens at high throttle. When going to full throttle, after about 7 seconds, the motor just quits...as though a "light switch" was turned off. There is no warning. It just shuts off.
I have also noticed that even with the needle valve wide open, I can't get the motor to run rich...to the point of quitting. (I have adjusted both the idle screw and needle valve in testing this.)
Further, it appears the engine has overheated...to the point of damage! I pulled the head and found the sleeve had seized to the head! Since then, I have replaced the head and sleeve with a used one and have the very same problems (deadstick and overheating). Also, I have replaced the carb....same problems.
Just to let you know, I have the tank near the motor (approx 1.5 inches) so no pump should be needed. Further, there is no foam, bubbles, or kinks in any of the fuels lines. Several new (OS 8) glow plugs have been used....and I am using omega 15% fuel with a 14/6 prop. All fuel lines are new and the fuel line to the "clunk" has been replaced...just in case.
The plane I have this motor installed in uses a cowl. However, I am not at the point of having the cowl installed. So the engine has been run without a cowl (eliminating air flow issues as a cause of overheating ).
To eliminate the possibility of a "lean condition", I have pinched the fuel line and the engine will increase in rpm significantly.
I am at "wits end" with this motor. I have 4 other OS motors and have never had such a problem with them. I'm afraid I am going to "toast" this motor again. I am about to give up on this OS90 and look for a comparable motor.
What could be wrong?
The "quitting" happens at high throttle. When going to full throttle, after about 7 seconds, the motor just quits...as though a "light switch" was turned off. There is no warning. It just shuts off.
I have also noticed that even with the needle valve wide open, I can't get the motor to run rich...to the point of quitting. (I have adjusted both the idle screw and needle valve in testing this.)
Further, it appears the engine has overheated...to the point of damage! I pulled the head and found the sleeve had seized to the head! Since then, I have replaced the head and sleeve with a used one and have the very same problems (deadstick and overheating). Also, I have replaced the carb....same problems.
Just to let you know, I have the tank near the motor (approx 1.5 inches) so no pump should be needed. Further, there is no foam, bubbles, or kinks in any of the fuels lines. Several new (OS 8) glow plugs have been used....and I am using omega 15% fuel with a 14/6 prop. All fuel lines are new and the fuel line to the "clunk" has been replaced...just in case.
The plane I have this motor installed in uses a cowl. However, I am not at the point of having the cowl installed. So the engine has been run without a cowl (eliminating air flow issues as a cause of overheating ).
To eliminate the possibility of a "lean condition", I have pinched the fuel line and the engine will increase in rpm significantly.
I am at "wits end" with this motor. I have 4 other OS motors and have never had such a problem with them. I'm afraid I am going to "toast" this motor again. I am about to give up on this OS90 and look for a comparable motor.
What could be wrong?
01-06-2003, 06:06 AM
#2
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Location: seoul, KOREA, REPUBLIC OF (KOR)
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Frustration with 91FX
Hi,
I had exactly the same problem with my webra91 abc. I could never run it rich. Fortunately, the engine was strong enough to bear all those overheating during test run.
I did not even think about expensive pump since fuel tank was already close enough. I thought it was carb. The carb probably had incomplete structure causing not enough fuel delivered.
As a work-around, I used YS check valve in the middle of back pressure line so that the pressure in the tank could make fuel flow actively. I also enlarged the nipple hole with a 2mm drill. The nipple attached on the muffler seemed to have too small hole. But you need to be careful not to break the nipple itself. Bingo! A lot better. No overheating any more and I could get the pick as I did on other engines.
However, still there is one problem. When you stop the engine after flight, the fuel still flows if left due to remaining pressure in the tank. you need to open back pressure immediately to release the pressure.
Hope it works in your case, too.
SeungWan
Seoul, Korea
I had exactly the same problem with my webra91 abc. I could never run it rich. Fortunately, the engine was strong enough to bear all those overheating during test run.
I did not even think about expensive pump since fuel tank was already close enough. I thought it was carb. The carb probably had incomplete structure causing not enough fuel delivered.
As a work-around, I used YS check valve in the middle of back pressure line so that the pressure in the tank could make fuel flow actively. I also enlarged the nipple hole with a 2mm drill. The nipple attached on the muffler seemed to have too small hole. But you need to be careful not to break the nipple itself. Bingo! A lot better. No overheating any more and I could get the pick as I did on other engines.
However, still there is one problem. When you stop the engine after flight, the fuel still flows if left due to remaining pressure in the tank. you need to open back pressure immediately to release the pressure.
Hope it works in your case, too.
SeungWan
Seoul, Korea
01-06-2003, 05:36 PM
#3
My Feedback: (11)
Frustration with 91FX
rcnewbie,
If an engine quits after a few seconds at full throttle, then it's a fuel feed problem. Since you aren't seeing any air leaks, the problem has to be elsewhere.
You might want to try larger fuel tubing from the tank to the engine, including inside the tank. Maybe drill out the clunk.
You might also try richening the idle a lot, start the engine at part throttle, advance throttle to full, and lean the high-speed needle. Then throttle down to adjust the low-speed mixture. Sometimes the low-speed mixture can be leaned enough that it interfered with high-speed fuel flow. There should be about a 4-5mm gap between the needle and the nozzle when you look into the venturi when it's opened to full. Start there and lean the idle mixture only after the high end has been set.
If an engine quits after a few seconds at full throttle, then it's a fuel feed problem. Since you aren't seeing any air leaks, the problem has to be elsewhere.
You might want to try larger fuel tubing from the tank to the engine, including inside the tank. Maybe drill out the clunk.
You might also try richening the idle a lot, start the engine at part throttle, advance throttle to full, and lean the high-speed needle. Then throttle down to adjust the low-speed mixture. Sometimes the low-speed mixture can be leaned enough that it interfered with high-speed fuel flow. There should be about a 4-5mm gap between the needle and the nozzle when you look into the venturi when it's opened to full. Start there and lean the idle mixture only after the high end has been set.
