I have 2 91fx motors .one in a midwest little cap the other in a hawker sea fury and as of today I have 2 less airplanes to fly Cap totaly destroyed the sea fury may be fixed dead stick**dead9 stick**deadstick** I bought one about a year ago the other about 2 years ago.one had grey O ring the newer one had black ring..I just started flying these two planes last month..( os sent new black O rings and said it would stop running problems but it didnot ) I ran both of these motors on test stand for about 8 tanks each (14%) 14x6 ms prop omega 10 fuel...motors ran very good on test stand with black rings no air bubbles in fuel lines grey ring had small amount of air bubbles but still ran good..motor put into little cap 2 tank runs one with cowl one without (14%)..the motor ran very good on the ground no air bubbles in lines ( 14x6 ms omega 15 fuel slim line pitts muff. dubro vib mounts ) about 10600 rpm pinch test on fuel line was good idle was a little rich mid range trans. was a little rough but still good..first flight 3 min. dead stick .. 3 other flights with richer settings all endedin dead stick before 7 min....New fuel tank.new larger fuel lines..3 line dot fuel setup. new o rings back to the fieled more dead sticks...crashes remove cowl .dead sticks try differant prop 13x6 ms 11200 rpm..dead stick crash destroys slim line pitts try bisson (91 large) worst flight ever stalls evon on idle.. get stock muffler will fit know with out cowl no better then slim line 10600 rpm dead stick crash etc new slimline new needle valve assembly( black O ring )motor check for dirt .new fuel line again..(os claims they have very little problems with 91fx ) run motor in plane 4 tanks inverted upside down-straight up etc..NO PROBLEMS this motor runs great on the ground just gets AIR SICK WHEN IT GOES UP***in 30 flights only 6 was the motor still running on landing..the only way to stop it was to land after 3-4 min.turn needle one turn to super rich then complet the flight and hope it dosnot stop on idle..on last flight started off super rich only lasted 4 min.dead stick again-Why did I park the tomato harvester their--clipped it whent in to spin nothing left**SEA FURY with the newer 91fx only lasted 2 flights first one lasted about 6 min. last one about 10 min...flew very fast with 13x6 ms..had GP motor mount no air bubbles in linse ran very good on ground pinch test straight up etc..ran it four 3 tanks before first flight and it RAN VERY GOOD ON GROUND..over estimated glide ratio landed in corn field wing destroded and another slimline muff.. THIS motor has the same problemes as the other one... BOTH PLANES LOST BECAUSE OF 91FX MOTOR PROBLEMES>>>CAN THESE MOTORS BE FIXED AND MADE TO RUN RELIABLE>>> one at club said he fix his by a remout needle valve change ..8 flights latter he now has a hanger9 mustang to fix dead stick again>>>>HAS ANY ONE GOT THE REAL FIX FOR THESE MOTORS>> I would like to fix these motors ase I like the idea of the same weight as 61fx but a lot more power>>>> I have a WM super CHIPMUNK built ready for a motor if i cant get these to run reliable ill have to use the good old 61fx<<<< ANY ONE OUT THEIR HAVE A FIXED 91FX**********