What is making my OS 91 FX engine "cackle" even at full throttle when it's in the air? Would it be the high end, low end, replace glow plug or what? I am running a 13-8 prop on it and using 10% wildcat fuel.

I'd like some possible answers please.

thank you

Could be bad bearings

Could use some more info. IS the engine new, just breaking in, or been running a few years????

Bearings? It only has about 50 flights on it. One thing I did do today was opened a new bottle of wildcat 10% 2+4 and dumped the remaining 5 ozs. of cool power 10% from the old bottle into the new bottle of wildcat. I don't think mixing the 2 is causing the problem though. I know, I know.....old fuel could have had moisture in it....Nope, I just opened the cool power 3 days ago and had the 5 ozs. left that I dumped into the NEW wildcat fuel. I should have just tried a new plug while I out there today!

If by cackles you mean it seems to be misfiring then it sounds very rich. Try leaning it and see what happens.

Engine is less than a year old.

I leaned it in and my tack read 13,600 today when I was out flying. I pinched the fuel line and it actually didn't want to rev much higher (just a tad) before it wanted to die out then. So I backed the high end back down a bit so it wasen't running to lean. Nothing was clogged in the needle area...I blew it out and my fuel filter was also clean. Before it would cackle at a lower speed (about 1/3 throttle) but then it started to cackle at lower and higher speeds.

Yes, it sounded more like it was misfiring. Maybe a new plug would help...I don't know.

This sound is pretty common in OS engines I think. I think I've heard that in every one of my 5 OS engines thus far. Some people say there's a possibility that it has something to do with the amount of play in the crank shaft, especially when the engine gets warmed up. Last time I actually paid attention to it I heard the sound when letting off throttle, and it would go away with the slightest bit of throttle advance.

I think if it is something to do with the mixture it is a symptom of being on the lean side. A few older experienced guys said they thought the sound was the engine pre-detonating, in which case a colder plug would help.

What plug are you using?

Most people have not been able to figure out the sound, but judging by the amount of cases I've now seen and heard of, ALL with OS engines, I would say it is 'normal'.

Remember that everything you are reading here are just thoughts and ideas. It's hard to diagnose something without actually being there, and most solutions are stumbled upon by experimenting. Try a small change and see if it helps. If not, change it back and try something else. If you are that concerned about it, I would either try a colder plug or a richer mixture.

Yea I thought that cackle sound was normal too.  I kind of like it!  Sounds like horsepower. 

All of my OS engines cackle a little bit at certain speeds but never at full speed!

My plug I am using is an OS #8 and I just put that in new last week. It has a good glow to it right now also.

Actually after flying it 8 times earlier this morning I was getting so tired and lonely there from no one else there flying, I decided to pack up and worry about it later. It didn't do it as much when I first had my 14-8 apc prop on it today but when I put my 13-8 apc back on it got worse.

If the glowplug, piston dome, piston skirt.....are all relatively smooth looking and not discolored or "burned" looking.....it should be OK.

If you don't feel and roughness in the crank, and it turns over velvet-smooth, and has good compression....it should be OK.

If it runs well at all stages....it should be OK.


I've owned several engines that have had their "signature" sound. Take the baffle out of the muffler, and run it....then tell us what it sounds like and if the cackling goes away. Otherwise, I'd enjoy that great engine.

The top of the piston looks good. It turns over good and has plenty of compression. It's doesn't hardly cackle when I am running it on the bench before flying, only cackles in the air! I took the baffle out about after my first 5 flights when it was new and I got more power out of the engine by doing that.

I just checked my low end needle and the book calls for it to be opened 2 1/2 turns for openers. It was out 2 3/4 of a turn. When I have an os engine I usually have to turn the low end "in" if anything. Maybe it was out to far at 2 3/4 turn out as it was at the field today. I wonder since the low end is so rich at 2 3/4 out if that would be causing it?

I think it may have something to do with the prop your using.  A 13 x 8 is way below any recommended prop.  I use a 16 x 6 on my 91FX.  The only time I get that "cackle" is when I level off after coming out of a dive where my throttle is pulled way back and the prop has been acting as a brake.  When I bring the throttle up 1/2 way or so and the engine is firing, but still not working very hard it cackles.

I'm not sure what the technical term is, but I think your engine just isn't working hard enough to spin that prop, and it's causing some sort of detonation issue.  Try a bigger prop, at least a 15 x 8 and see if the engine likes that better.

Your low end needle won't have anything to do with anything past 1/4 or 1/3 throttle.

I think a 16x6 is too big for a .91. That's more of a prop for a 1.08 or a 1.20.

13x8 is a good size for a 90 two stroke.

it could be the prop tips going supersonic this happens when an engine use a smaller prop.especially in a dive does the sound go away in a climb when the prop is under a load?

I was pretty sceptical when Iread the manual myself, their reccomandations went from 15x8 to 17x6 for sport flying and 13x11-13 to 14x10-11 for aerobatic flying. Iwent right down the middle of the sport flying recommendation and did a 16x6. Can't remember what the tach reading were, but they were fine and Ilike the way it flies, engine response is great, so I haven't bothered to change it.

Anyway, it's an easy enough fix to try. Put on a larger prop, see what happens.

OS calls for a 13-6 to a 16-6. I flew it before with a 13-8 and it was fine. The 14-8 seemed to bog my plane down for my style of flying with my tiger 60 today. On the slower side on all manuvers. The crackling sound was in almost all positions I had the plane in. A 14-8 is something you'd put on an astro hog bipe with the 90 2 stroke, not a tiger 60.

When I was going unlimited vertical it was crackling so bad I thought....I better bring it back down before she blows up on me!

Actually it sounded like some old car back firing! Maybe I should leave it the way it is and fly it on the 4th of july!

"cackling" properly described usually indicates detonation or pre-ignition. One or more of the following usually causes this. Too hot a plug (try switching from OS #8 to a A5 or #10); overcompression (add a head gasket); too much nitro (10% should not be the problem in your case); or running too lean (overheating-same effect as too hot a plug). Suggest you try or or more of these. Worst case effect would be a pounded out rod end.

is it all the time or does it do it in a dive more them in level flight ?
could be over reving the motor  due to a small prop . 
i us 14x6 or 15x6 apc  prop and see if it  runs better

Maybe they changed their recommended props, but the manual Ihave, and this one online manuals.hobbico.com/osm/50sx-40-46-61-91fx-manual.pdf
shows the props Isaid before.

Good luck with your engine.

On my last flight it seemed to want to crackle about all the time! I'll try all the above tips and if I can't get it running right, I'll ship it back since it's still under warranty.

Thanks for all the help.

hulio516, try some fresh 5% wildcat in it, you may have a higher compression OS that may run better on 5% fuel, The Fox engines in the 80s used to crackel if higher nitro was used in them as they were designed for low or no nitro fuel. Too high nitro will boost your advance timing and as a result the piston could actually fire multiple times btdc (before top dead center) during a single combustion stroke (that is why you hear crackling) because it is running overheated regardless of needle setting , I had a rossi 60 that hated 10% fuel and it sure let me know it very rudly, It would overheat even if I flew it rich and it would blow a glowplug after every flight, some do and some don't , this one did so I went back to 5% and it cured my problem.. It could also be your fuel-glowplug combination also..You could need to go back to running your OS with the stuff that worked in the first place and it could be that 10% of the same brand needs to be used as before, It could be that you have a finiky or picky engine, let me know what transpires , Best wishes greengoose....

I have two of these engines and really like them.  There is a lot of information here - RCU - on guys that have had similar problems as you are having.  I was one of them early on.  Bill Baxter from Hobby Services/Tower/OS knows a lot about them.  Look up his responses on here and you will learn a lot.   Anyway... here's what worked for me.

First the 13-8 is perfect for this engine.  The only downside on my Skybolt is that it doesn't want to slow down for landing in calm air.  I need a breeze to help.  The props quoted in the manual are too big for good performance.  In Japan noise is an issue, and the larger props help with that.  The 91 likes to rev, but 13,000+ is too high.  Detonation is probably occuring at that lean setting.  The noise you hear is the engine going lean in the air when it gets hot (too hot).  The fuel feed is part of the issue as well.  My solution was a $30 Perry pump.  It provided incredible power immediately, and didn't require anything except reading the instructions the first time.  The noises went away, the plane turned into a vertical rocket (with the 13-8), is fuel efficient,  and with a slimline pitts on it makes a very cool Harley kind of sound at idle.  Jeez it's great.  I use an OS#8 and 20% heli fuel.  Smoke trail is awesome and it is there through the whole flight.  If I get off a click lean, the smoke thins out and that's my clue.  Anyway, 13-8, Perry, 15-20% fuel.... and hang on.    -Tony Iannucelli