91fx weird throttle while flying

[katman]

We got an Aeorworks Edge 540 60inch with a 91fx on it. The combo has some wheres between 10 and 15 flying hours on it. Well the airframe was changed out once but thats a different story. Recently it has developed a weird thing with the throttle. About every minute or 1.5 minutes while flying it goes to a high idle rpm for a second or two, from full throttle or at least above half. Then it comes back and winds out again. We thought it might be a fuel delivery problem due to aerobatics but the last time we flew it the other day it did it while flying straight and level at full throttle. It is just like you pull the throttle back to idle for a little then hammer it again. We can't find anything wrong with the fuel system, no leaks, pinched lines etc. And like I said its been flying this way for many hours so its kinda puzzleing.

We had been flying it with Cool Power fuel which is really junk stuff and it had the main bearing completely full of oil or some junk to the point where it was seriuosly robbing power so I took it apart and cleaned out the accumulated oil and put it back together. That brought the power back up to what it was when it was new but soon after is when this problem started. The compression seems low although when I had it apart the bore and piston look like they are still brand new. There was not a mark to be seen on the cylinder.

Any advice would be appreciated.

[2walla]

My 91fx was a POS... It was an early one and the remote needle would start to vibrate at certain RPM's and would leak air and go lean and run very erratic.. Are you sure you got the case put together good? The oil in the fuel shouldn't build up on the bearing to the point of causing problems? I did have some cool power years ago that developed a white waxy substance in it and it plugged up the carb, never figured out what caused it but shoot that was 20 years ago.. I will tell you all fuel manufacturers can screw up.. I had one very good supplier send me a batch of fuel with 0 oil in it.. After smoking a motor they they assured me that it was computer mixed and couldn't be bad.. Come to find out the new guy was changing out the oil barrel on the mixing machine and forgot to take the plastic bag off of the pickup tube before sticking it in the new oil barrel..OOPS. I would look it over good, check it for leaks on the case, clean the fuel system and try some different fuel..

[Kiva Flyer]

Not a radio fail-safe problem, is it?

[katman]

Next chance I get I will check all the screws since I did have it all apart. The head has previously came loose on this motor but we checked that and they are all tight. We are pulling out of the same jug of fuel for several different motors and they seem to run fine. We have pulled the tank twice and replaced the clunk and suction line but there is still some fuel line on there that has been there for probably a year so definitely will replace it.

It could be a radio problem as the radio being used to fly it has an issue with the rudder. It is a Futaba 6EX 2.4GHZ and it has been sent back once already because the rudder screwed up and now it is doing the same thing again. If you hold full rudder it won't go all the way every time and when it doesn't then it chatters around. Have had several of those radios around here do the same thing. My JR 72 MHZ is rock solid. I hate 2.4 but thats another subject I won't get into as my disgust for them pertains to our flying situation here not world wide.

Thanks

[ta154]

I have a good friend who had recently experienced all sorts of uneven running and variable RPM at fixed throttle problems with an OS 91FX.

We checked all the usual things as you did, with no success. Finally, we found that although there was no other indication, both cranshaft bearings were worn out, allowing the prop to have considerable run out. You could wiggle the prop blades back and forth quite a bit.

On tearing the engine down, it turned out that a repair was well beyond any economic value (rusted, pitted crank, bad con rod, crappy bearings). So he replaced the engine with another OS 91FX in the same plane with no other changes and all the problems were immediately gone.

Amazing what trouble you can get with just bad bearings.

[katman]

One other thing I totally forgot to mention. It seems like the compression is pretty low on that 91. If you turn the prop over and hold it against compression there is no resistance it just falls through. Maybe that is how it is when its new I can't remember. Just to save someone from saying it I know that high rpm 2-strokes don't always have a lot of compression if you turn them over slow but this one seems a little excessive. If you turn it about one round every 5 seconds you can hardly feel where the compression starts or stops.

I will check the crankshaft slop next time I get a chance but the way I remember is felt like brand new still. I am dubious of the bearings because of using that Cool Power crud. I wouldn't use that stuff to light my wood stove.

[TONY0707]

hi do keep in mind when OS first released the 91FX it did so with a green ' o ' ring on the needle valve ---OS then issued a statement that the green ' o' ring was leaking air and was replaced-- by OS--- with a black o ring that solved the leaking air problem--i know as i was building a cowl around a new 91 fx---and fortuninitly did not run my engine until the needle valve o ring had been replaced for free by OS--- (as i read about the new o ring issue in a modeling magazine )---i just wonder how many 91 fx engines were broken in with the needle valve leaking--which i know after handling glo engines for 26 years ----MIGHT BE THE SINGLE WORST THING YOU CAN DO TO ANY GLO ENGINE----as the air leak causes the engine to run LEAN --and will burn up --pistons-rings and bearings- from the excessive heat build up-- if the air leak is not identified by an individual who knows to look for this condition constantly when handling a glo engine ----the bottom line is that any 91 fx exposed to the above conditions could be subject to early termination--which just might be what you may be experiencing-- (an early bearing problem )when a glo engine starts to run UP RPMS in an unusual manor---other than the way it has performed in the past--usually accompanied by not taking the same needle valve setting as usual and the engine feeling very hot to the touch when the head is felt ( just tap the engine with your fingers-or you will get burned ) --do start to look for an air leak--from the fuel line all the way into the tank--a loose head --or a loose carb--a lean run will take out a good glo plug at the same time--whewwww-now that was a mouth full ENJOY REGARDS TONY " the omega man " think pink ! !

[god-of-engines]

My OS 91FX's (3 in planes) were tuned with Perry needle valves, not the OS valve. I use larger diameter fuel lines, hotter plug, and have had much better results than with "stock" parts. They are stronger than my Super Tigre 90 engines, and have been flown for 5 years now without showing fatigue. Now you can ante up for the AX series, or try another brand, learning it's quirks.

[TONY0707]

hi the fact the 91 has low compression is a sign --- a lot of engine wear has taken place--either from a lot of run time --mishandling ( running to lean ) or air getting into the engine which causes lean runs-- RPM surges when flying is a sign the engine is set and running lean- in the air---and is caused by the fuel being used in the tank near the end of a flight--that is the reason we set the engine a bit rich at take off--do land as soon as is possible and richen the needle valve a 1/4 turn--then proceed to fly-- if you should go dead stick -as soon as you get to the plane feel the head-it s/b warm--if very hot -you were to lean--LET US KNOW WHAT THE FIX IS FOR YOUR PROBLEM-SO WE SHARE THE KNOWLEDGE--ENJOY REGARDS TONY

[2walla]

If you are using a futaba 2.4 it could be going into failsafe. You can disable the low throttle function failsafe. If it isn't the failsafe the fix is to pitch it and buy a magnum.. I think OS quality has gone by the wayside...

[katman]

The rpm issue started after I had disassembled the motor to clean it out. I highly suspect I created an air leak somewhere. Just don't have the time to pull it back off the plane and check it out. It did not have this problem at all before I pulled it apart. We had been running it on Cool Power fuel for several gallons and the main bearing was clear full of oil sludge. It had very much turning resistance. After I cleaned it out is spun freely and makes obvious more power but now it has the problem mention.

Thanks for all the help and suggestions though. I may not be back on here for quit a while (due to life's many unexpected happenings) so I probably won't post again unless the problem gets solved. If I do solve it I will try to post it on here to clear things up. I don't like loose ends.

Thanks a bunch.

Katman

[TONY0707]

HI KATMAN--do yourself a big favor and get rid of the OMEGA COOL POWER-the stuff will kill your bearings and cylinders--flying for 20 years -used cool power the first two--switched to MORGAN OMEGA-pink--cannot remember the last time i had to change bearings or a ring--the castor oil is a must in two cycle glo engines to protect from the evils of heat--the engine destroyer-in any engine-and many times worse in an aluminum engine--soft metal will warp easily from heat build up --a lean run ENJOY REGARDS TONY

[TONY0707]

hi do read the OS 91 fx DESIGN FLAW --thread here--should help you understand the cause of your engines demise