Glow plug problems
 

I have a problem with burning out glow plugs on a O.S. .91 FX and am hoping someone can give me some advice. I have been using Fox's "miracle Plug" and am getting only one flight per plug! I run 15% 2 stroke fuel (Omega) and a 1.5 volt glow driver.

The engine wants a rich mixture and will overheat very quickly if too lean. Even holding the model vertical with no loss of RPM can result in a lean run if not careful. I can see the stream of exhaust in a clear blue sky, and I know it's ok then. Does anyone have a suggestion as to cause, a better type of glow plug for this engine, or other suggestions? I would appreciate any help.

Fred Lomax (W4RCF) [:@]

RE: Glow plug problems
 

Seems to me that there are several issues going on that may be in addition to a slightly aggressive needle setting. If I,am not mistaken that fuel has no caster at all. A small percentage of caster in the oil mix will do wonders. Also you are using a very hot four stroke plug. Remember plug heat is the one easy method we have of controling the ignition timing in our engines. A hot plug will advance the timing slightly and a cold one will retard. Retarded timing = a little easier starting, slightly less power and a lot cooler running. Suggest you try the McCoy MC-9 (not the MC-059 thats a hot one) a genuine cold plug and run don,t walk to the LHS get some fuel with a little caster in it or a bottle of caster and spike some in the stuff you are running. Avoid like the plague all four stroke plugs that are very hot by design.

John

RE: Glow plug problems
 

My friend and I fly identical ancient Cermark Sukhoi's, with .91FX, pitts muffler, and 10% Cool Power. 15% makes 'em die in the air. With no other modification, 10% works for us.

Great engine, on 10%, with a 14X6. A 13X8 will make it a rocket, a 15X6 will give it great braking, really slow it down for landing. I prefer the 14X6 becuase it revs almost as fast as the 13X8, and I don't want it to slow down for landing as fast as the 15X6 causes it to do.

Good luck,
Dave Olson

RE: Glow plug problems
 

Fred, OS recommends their #8 plug. I've had really good luck with them. Have you given that a try? For the adjusting, sounds like you may be trying to squeeze a bit too much out of the engine. When I point the nose of the plane up I expect the engine to actually speed up a bit at first, and then stay there. In the past (long time ago when I was young and foolish) I found that if the engine did not speed up a little when I pointed the nose up, it would lean out in flight.

RE: Glow plug problems
 

Another common problem with the OS .91FX is its tendency to go lean once in the air. I think this has been traced to the needle valve housing getting too hot and constricting the fuel flow. The case of the .91FX is identical to the .61FX but of course the piston is larger. I think some have cured this problem by going to a remote needle valve assembly.

RE: Glow plug problems
 

Assuming the engine is not new of the shelf. Pull the back plate off and look at the bearing, is it rusted? That would explain the overheating and the lost of glow plugs, but only if it is severely rusted.

Good luck

Joe

RE: Glow plug problems
 

W4RCF,

I have nearly the exact set up you describe in my GP Extra. The only differnece is the Glow Plug. I use the OS #8 as recommended w/o problem. One other "issue" with the 91FX is the prop size. I had the overheating problem you describe when using the props initially recommended in the manual. I am currently use a 13x6 APC and all is well.

Kent

P.S. Morgan's Omega does have castor oil.

RE: Glow plug problems
 

Thank you Kent of that fact I was not sure of as noted.

John