I can confirm that a 13x8 works well on a 91 fx.I use a standard muffler and 10% nitro.I see ,however that Larry in Denver has a problem with his fx-I have exactly the same.It runs very nicely at first from a very low idle to full power.After a couple of minutes in the air it starts to run a bit rough,you can hear the difference in engine note.the lovely clean pick up becomes a lottery as to whether the engine will pick up at all(at best very badly) but, quite often it cuts.the motor is up at 45 degrees and well out in the airflow.Tank top is one half inch below the spray bar.I've stripped the carb,put several different plugs in-all to no avail.The engine is now starting to get on my wick-does anybody have any ideas? regards Marc Sommer

Marc,
It sounds to me like the engine is getting overheated. What do you mean it is "up at 45 degrees and well in the airflow"?

dear AnnMarie,thankyou for your reply.By up at 45 degrees I mean its half way between fully upright and a true sidewinder.Th e cylinder is well clear of the cowling and sits in the full face of the airflow.How about this? If you run a motorcycle engine on fully synthetic oil before it is run in you can destroy the motor by virtue of the fact that the piston rings are not allowed into enough contact with the cylinder.I run 100% synthetic,could this have had a detrimental effect on this OS 91fx???

Dear AnnMarie,I take on board what you said about exhaust systems causing overheating problems ,as I said in my original thread, I use the standard exhaust on my OS 91 fx. I know now of two people with the same problem with this engine and,obviously a few more judging by the web site.Please be honest,is there a problem with this particular engine?,nothing is worse than irritating engine performance but, if one is on a hiding to nothing then, that makes it all the more annoying.Does OS have a problem with this engine?????

Hi Marc! As you spend time here and read things I write, I hope you'll find your own answer, that I don't "hide things"....

the 91FX has done very well for us and we have had minimal problems. The carb on it is the same as on many engines for many years; that said, however, some modelers are finding that something is causing the o-ring at the base of hte needle to fail. For those that have this problem, a replacement needle seat is available.

In most cases, however, the problem is overpropping, based upon the Japanese recommendations in the manuals. They fly these engines VERY differently than we do, and their pattern plane prop range is just not fitting for the average US sport flier. The 13x8 is a GREAT choice and runs terrific for most users.