OK-- Let me say I really like OS engines. I am not new to either engines or the sport. I have / have had 40,46,61,108 OS's.

IMHO this 91 is a POS! Perhaps os compromised too much trying to compact the size! Let me be specific--it's the only engine I've heard preignition in--it nocks like hell!! Have blown out two glow plugs--yes I had Clarence Lee helicoil the head-- the head bolts are constantly loosening and I've tried 10% fuel to 20% fuel with negligable diference. The prop is a ProZinger 14/6

If any of you gear heads can help me out I'd appreciate it-This thing has a 51 year old man ready to cry!!!!!!

Do a search on this, there have been many threads about the good and BAD of this motor. My solution to the problems of this motor was, I sold it (CHEAP), and bought a Jett 90. problems solved!

Gary

Good Evening,

I'm running a OS91 FX in my Viper 91 pattern plane with out any pump and tank on the CG and it runs great, I think your have it under prop, I'm swinging a 15X10 on mine with 25% cool power.. The other thing that can be done is add 1-2 head shims, (Tower has them) like a extra .007 will work.. the other thing is run the OS four stroke plug in it.. If you get a lean run this will also cause plug problems.. I have not had any pinging or loose screws with mine.. This engine is a true power house..

scott

Thanks Scott-- I'll try the Four stroke plug out--I have some OS F plugs in my box. Another thing I have done is add a
fourmost one way valve in the vent line to hopefully get a little more fuel to the engine. It is upright mounted in a GP Big Stick 60 with the stock sport muffler. I have added another .007 head gasket. Maybe the 25% nitro is cooling her a bit for you !?? I generally run 20/20 in just about everything. I haven't had a chance to take it out with the Fourmost valve installed yet (rain) but will as soon as I get the chance--Glad to hear you are having good luck with yours. My flying buddy has two new 91's on incompleted planes and he is quite concerned!!

I know the feeling about the rain,, It's raining here in TN today... Below is a pic. of the OS Viper' project, mine red/white/blue... The check valve sounds like a good idea, the other thing you could do is run a small 2 oz header tank behind the fire wall and feed it from the main on the cg.. When we put the OS viper project together that was in the design stage but is not needed.. Good Luck and let me know how you made out..

scott

Hi!
Why do you run so much nitro!??
The pinging sound you hear is from too high compression! (the combustion front hits the piston too early ).
This engine will run as good on just 5-10% nitro! Have you checked your fuel? Water in it perhaps.....
The glowplug supplied (OS 8 )or an Enya 3 is perfect for this engine!
Do use APC props for best performance.

Regards!
Jan K

jaka

I read on a German forum a comment that suggests that the bubbles are due to the fuel line from needle to carb being to close to the engine and of too thin a material resulting in alcohol vaporiztion, and that OS has changed the tube to a heavier material and routed it away from the engine because of this. Have you heard or do you know anything about this from a European perspective. As a note mine run fine bubbles or no bubbles

HI Jan K-- We americans are spoiled with nitro cost/availability here in the states!!! I have noticed that a lot of european motor manufacturers recommend 5-10% nitro. It is my belief that if they recommended 20-25% nitro they wouldn't sell motors because of the prohibitive cost of fuel over there.

Anyway I digress--I run 20/20 because I have various engines to feed --OS 46,61,91,1.08 and three mokis. If my understanding is correct the higher the nitro content,the lower the heat of combustion. If indeed preignition is my problem, my line of thinking follows that more nitro=less heat=less preignition...

My Moki documentation says 10% nitro max and no all- synthetic fuels-- I tried it on Byron's 10% and it ran ok-- I popped in a OS #8 glowplug, 20/20 fuel and those engines KICK ASS!! That kind of demonstrated for me the power of NITRO!!! If indeed Moki's concerns are justified and their bearings go away, I am prepaired to put Boca ceramics in them and fly on !!

More nitro = more heat. You can't get a power increase without a heat increase. When engines start having detonation when you increase the nitro content, you have to lower the compression ratio or delay firing to stop it. A colder plug may help. A four-stroke plug is a hotter plug, and may only make the situation worse.

Nitro is an oxygenator, and releases oxygen during the combustion process, which helps with the fuel burn. When running more nitro in fuels, the engine turns higher RPM, which means that more fuel and air have to go through the engine. More fuel and air means more heat generated. If the engine can't cool fast enough, it will start to detonate. To delay firing, a cooler plug must be used. A O.S. A5 is a cooler plug than the #8, and the #R5 is a colder plug yet.

It's a fallacy that nitro makes engines run cooler. It doesn't. People have seen condensation, even frost, forming on carbs when running alcohol and nitro fuels, but that's not because the fuel's burning cooler. It's because of the cooling effect caused by the vaporization of fuel in the carburetor. When a liquid is quickly vaporized, it pulls heat from its surroundings. If the surrounding air is very humid, you can get icing in a carburetor, though that's not been any kind of a problem in model engines, but condensation on the carb or a 4-stroke engine's intake pipe is very common to see in the summertime.

I have had this discussion with many a pilot and race car driver. I used to think along the same lines as you, Mr. Bax. I thought lower octane meant lower heat, thus better for my car. I was out buying a new lawn mower from a local lawn and garden equipment shop, the ower of the shop informed me, If I used higher octane gas my engine would work better, run cooler and last longer. I said, "Bull hocky!" Well, I tried his suggestion, it really works. Higher octane in higher compression, no detonation and better idling etc, which also goes along with what all my stock car racing buddies claim. Nitro is the octane in glow fuels, Decompress your engine, which should retard your ingnition slightly. Then the higher nitro should give you an effective means for better idle and transition. YS 20/20 or a good equivalent fuel, all the difference in the world. Oh, I am not an OS basher, I think you should try one last time with your engine. Then find a suitable replacement, if your not satisfied with the outcome.
wra

Check for bubbles in the fuel line between the needle and carb, that is the standard problem, plenty of posts on here about it. If you are getting them, it will be leaning out the engine, potentially causing preignition.

I do not have any such problem running mine on an APC 15x7. To cure the bubble problem I cut the needle valve assembly of the back plate and attached it to the firewall.

Also your idea of using a check valve is not a good one without a regulator to control fuel flow. As you run the engine, preasure will change in the tank, causing changes to the amount of fuel being forced into the carb, it will get richer over time. If you then lean it out, it will be too rich when the preasure drops. You need a regulator such as the Cline or Iron Bay.

There is no comparison between nitromethane and octane in automotive fuels.

Nitromethane is an oxidizing agent that I often call a liquid turbo or liquid supercharger. By containing oxygen, nitromethane allows more fuel to be burned over a given time period. Use of high nitro fuel usually requires decrease in compression ratio (more head shims) and/or retardation of engine timining (cooler plug or change in engine timing)

Octane is a measurement of a fuel's ability to resist detonation/knocking in an engine. More octane in an automobile designed for 87 octane doesn't relate to more power. High octane fuel's ability to resist knocking allows engine manufacturers and race tuners to increase compression ratio, increase turbo-boost to increase horsepower without the engine self destruction. High octane fuel contains less BTU per gallon and may result in slightly lower gas mileage per tank of fuel and possibly create carbon buildup within your engine if run in a normally aspirated/normally compressed engine.

With respect to the OS .91 FX, use 5-10% nitro, a slightly cooler plug and a 13 inch prop to unload the engine. This has helped many people with their engine problems. The .91FX doesn't care for 14 inch props much.

Strange since, nitromethane has such a high point of vaporization. What chemical reaction would you suggest is causing the oxygen to be released as a free radical so it may be reduced during its combustion?

OK Gentlemen--I really did not want to start controversy.

My comments on nitro lowering combustion temperatures came from reading Clarence Lee's column in RCM! Maybe I read to damn much!! Anyway he related how pattern fliers run high nitro in their abc motors and practice ALOT. At some point when the pattern flier decides his engine is 'shot' he will usually find sport fliers willing to take the motor. They in turn will run 10-15% nitro in the motor and it will run great for their purposes because the higher combustion temperatures of the lower nitro fuel (10-15%) will expand the piston to provide a servicable life. If I have misconstrued or perverted the direction of that article I certainly apologize to all!!!

Now I got to fly my 91 again today...as you recall it is in a GP Bigstick 60, mounted upright with the original sport muffler. Guess what folksI found the source of my heat problem. This is a fairly new motor and has the extended OS silencer muffler. I took the baffle out of the muffler and eliminated 85% of my problem. I realized that all my other OS's arerunnig bisson pitts -type mufflers and remembered we used to pull the baffles out of the .30-46 engines all the time. It ran great--the best it has ever run.Perhaps the fourmost one-way valve in the exhaust was a good addition considering the lost backpressure with the baffle removal.

Anyway here is the configuration that is running best for me--20/20 fuel, 14X8 prop (Zinger PRO) OS #8 glow plug, Stock sport muffler with baffle removed, Foremost one way valve in pressure line. I had gone to a A-5 plug before removing the baffle and it did help, but not near as much as removing that baffle. Motor sounds much better and will only knock when obviously tuning to the lean side

The lesson I take away and offer the forum is if your motor is RUNNING HOT---ELIMINATE EXHAUST RESTRICTINS..Tks all !!!

Hi!
The more nitro used -the more heat produced inside the engine....but using nitro(small quantities 5-10%) could make your engine run cooler...the reason is .......you can set the highspeed needle more exact when using nitro!
20% is way too much nitro for all sport engines and will only make needle setting more difficult and wore out the engine sooner.
15% Nitro is max in most sportengines....as they have fairly low compression ratios.
The pinging sound is from too high compression ( too much heat is generated in the engine)!
By using a less restricted silencer you help the engine get rid if the excessive heat but it will not like it.......better to use 10% nitro.
Regards!
Jan k

I have the 91FX 14x7 graupner prop on my Corsair and its runs great! I use Power Master 15% nitro 18 castor and syn mix blend. No problems at all. One thing to mention is air flow across the head are these engines that are detonating being cooled properly and are we sure (I'm refering to the engine tuner) that we are tuning the engine properly? Check that fuel too, Synthetic oils have up to 100 degree less cooling ability then castor oil.

I have the 91fx mounted to a H9 Mustang with a 13x8 prop its runs very well any problems very fast plane.

rflash, I think you misread the article. When a high nitro ABC engine gets worn you start running lower nitro in it because the heat is less and therefore the cylinder does not expand as much keeping the compression in the useable range. Remember, the cylinder is the part that is tapered and expands out with heat. Less nitro = less heat.

wldcwbypilot, putting high octane fuel in a lawn mover and it runs better, not unless the lower octane gas was bad to start with. I would suggest changing service stations.

Ed M.