microdon2

My Feedback: (47)

I have a pre-owned OS 120 4S engine in my GP 70" Revolver. Generally a very good engine, though I do have to adjust the lifter gap once in a while - no biggy - takes 15 mins. The engine does seem to run a bit rough at idle, but once warmed up it's good. But lately I've lost three spinners while in flight - they just pop off in flight. The last time - two weeks ago - the engine pulled the firewall right off of the engine block. I was lucky to be able to land it. A senior guy at the field said it's probably pre-igniting and recommended a colder glow-plug and higher nitro content. Is that's what's happening? Is it some kind of backfire that causes everything to stop for a second? Whatever it was the last time if was powerful enough to rip the firewall off.

I'm using 15% Cool power now - would hate to go up to 20%, just for this one plane. I already ordered a "colder" glo-plug from TH, the OS #10 A5 plug.

Does anyone have any experience with this situation? Does it sound like pre-ignition, and will this colder plug solve the problem?

Thanks.

Rodney

More likely running to lean. I get by nicely with 10% nitro on several different brands of 4 strokes. Try with a little richer run and see if that does not help.

microdon2

My Feedback: (47)

Rodney - Thanks for the feedback, but how would running too lean cause spinners to fly off and the engine to rip the firewall off? Wouldn't the engine just cut out? (as it has on other planes..) The symptoms seem to indicate a sudden stopping and almost reversal of torque. (to be fair that firewall may not have been completely secure - it's been a few years...)

1QwkSport2.5r

A lean 4-stroke will backfire resulting in the prop nut coming loose. I had a prop fly off my little Enya 60-4C while bench running it; It got a little lean and at mid throttle kicked the prop. I double nut the prop on my 4-strokes now to help prevent that.

I use a tach to set my needle and usually set it 300-400rpm rich of peak.

microdon2

My Feedback: (47)

1QwkSport - thanks. Did not know about the backfire. I'll try running a bit richer.

kenworthbob

If memory serves my OS 120 surpass 3 came with a double tapered lock-nut assembly, I would use it if its coming loose. Use only a type f glow plug. 15 percent nitro is fine. OS recommends at least a 18 percent content of lube in the fuel. Do not use four stroke fuel, not enough lube in it. If you don't hear any weird crackle noise under full load its not pre-ignition. Happy flying :-)

blw

My Feedback: (3)

Have you richened the lowspeed needle to see if that stops the problem?

w8ye

My Feedback: (16)

As mentioned by BLW, if the low speed needle is too lean, you can not get the high speed needle rich enough to run properly at full speed.

This is due to the way the carb is made.

microdon2

My Feedback: (47)

If 4 stroke Nitro doesn't have enough oil in it for 4 stroke engines, why sell \ buy it? Is it just a marketing gimmick?

Btw - I also have an older OS 91 4s that has some rust in the lifter housing. Anything I can do for that?

Thanks.

Gizmo-RCU

My Feedback: (27)

I helped a friend with an older (pre-surpass) 120 that did what you discribed when too lean. I also had an older OS 60 fs of about the same vintage and it also behaved badly when lean, 10% castor/syn worked just fine in them. Both would appear
to run fine until you moved the throttles when warm, that set off some bad things........going richer took care of that.
Don the rust may not be doing any harm, if the bearings are not smooth which is difficult to determine on cam bearings you may have to replace all bearings in the engine after tearing it down. Unless you have some experience/help I suggest you start
researching the project to learn how to do the job. Over the years there have been many good sources here on RCU. Removing Cam bearings can be a little difficult..........4 strokes are not the best to start with unless you fully understand how they operate.

microdon2

My Feedback: (47)

Gizmo - actually, the last time I started the older OS 91 4s it sounded very metallic - and the engine was not running well, so I've already decided to change the bearings (maybe they have rust in them, too). I bought this engine used, so... I have changed front and rear bearings before on other engines, but have had trouble with the cam - ended up leaving in the old one (different 4s). Tried heating it a few times, but no luck getting that bearing out. Do you have any advice for getting those out? Would prefer to change them all while I'm in there.

Thanks for the feedback on the "too lean" situation - 3 opinions saying the same thing sounds pretty convincing (and I'm glad to hear it's an easy fix!). I'm now bolting that 120 4s back onto the 70" Revolver and will fly it tomorrow here in LI, NY (forecast 30 F, 8 WNW). The cowl is open underneath so I do have access to the mixture control valve, if needed.

Mike

Gizmo-RCU

My Feedback: (27)

Sounds like the engine has potentially a serious rust problem. Do not run it until you check it out completely. A friend ran an OS 160 2 stroke that was full of rust and it took out the piston, ring and cylinder
in very short order, re-built it for him and it still is going strong.
I suggest you disassemble the engine if you find rust on the bearings when you open the case it's time to re-build. Boil out the case in antifreeze after removing the bearings so everything is clean
Mike there many, many sources on RCU on how to do this whole project and it would all be just a re-hash for me to go over it. You can however obtain bearings at most bearing supply houses at much less cost
than the mfg. I just take the old bearings with me and they will measure them to find the right ones. Stay away from the Chinese bearings (OS and others are using them and they are sub-standard).
Do not use a lot of force to disassemble the engine, you will break something!
The older 120 I was talking about required about 4 (in that range) turns on the high speed needle to be right as I recall?
Good luck!

SrTelemaster150

Put the breather nipple in the cam housing & plug the rear breather. You can put an out check in the line from the cam housing for a more steady out flow.

If you want positive cam lubrication put an inlet check on the back plate & an outlet check on the cam housing.


BTW, "4-stroke" glow fuel IS a gimic!

SrTelemaster150

I also think that your LSN might be the problem W/the kickback. It is a pre-ignition problrm. That causes the backfire that momentarily stops the rotation of the prop. You are lucky it doesn't spit the prop & all of the nuts/washers off.

Richen up the LSN until you get smoke & hesitating on transition followed by the engine reving to max RPM. Lean out the LSN just enough to get rid of the hesitation. Fuel mixtures usually lean out in the air, especially as the level of fuel in the tank decreases.

bogbeagle

Sounds like a lean run, causing pre-ignition. That could definitely tear the engine from the firewall and destroy spinners.

Buuut, since it's a second-hand engine, it might be worth checking the valve timing. It's not unknown for engines to be wrongly assembled by tinkerers.

I'd be quite concerned if I was having to "adjust the lifter gap once in a while". The need for this should be vanishingly rare. Is the head properly secured?

Lots of possibilities to explore.

Rodney

Do check the valve clearance settings. Even on a new engine, you need to check the settings after the first 1/2 of hours running as they wear in. Once set after a break-in, you usually only need to check them once a year or so. When new, or after a rebuild, there is often quite a change after the first run or two, evidently minor flaws wear in.

blw

My Feedback: (3)

You gotta check them from time to time.

jaka

Hi!
First of all change the plug to an OS F plug! All OS four strokes are run on that plug! Nothing else!!!!! The OS 5 will not work!

But...If the engine back fires and throws props I would definitly have a look inside the engine. Perhaps the ball bearings need replacement. They shurely do if the engine is several years old.
Also the valve steams might have lots of carbon build up , if the engine has been run on Castor oil, that needs to be cleaned away.

Prop size also matters. You should run a 15x8 or 16x6 on that engine if you fly at sea level. Best props are Graupner G Sonic, RAM or APC.
And the most important....always set the engine a little rich.

SrTelemaster150

How would bad bearings cause a backfire?

The only sentence that seems pertinent is your last one.

Gizmo-RCU

My Feedback: (27)

Too lean.........some of these old engines run best about two turns more open then the Surpass series (from my experiences), could be exceptions but a good base to start with. I had an old 60 FS that was fresh inside
throw props violently when the carb was advanced rapidly. It seemed to be adjusted properly and this trait was well concealed until I figured it out.

microdon2

My Feedback: (47)

Thanks for all of the feedback so far. On the OS 120 4s, I noticed it has basically no compression (on swinging the prop). So I re-gapped the lifters - no change. (btw - am using an F plug.) Don't remember this engine having no compression. So I ran \ flew the engine yesterday (after opening the mixture control valve a bit) and it ran very rough on the low end, but good on the high end (with a hesitant transition). After each flight I closed the MCV down a bit, and that helped smooth the low end a bit, but still pretty rough, and the engine would still die if I didn't rev it up. Looking at the engine last night it's covered with oil (inside the cowl, too). Seems like I have a leak somewhere, and that may be why I have no compression too (?). I checked the five head-bolts last night - all solid. Maybe a different bolt. Could be I need a new piston ring, too? Taking the muffler pipe off I noticed black, hard soot inside the cylinder. So I'm going to take the engine completely apart, order a new piston ring, gaskets, and bearings. And will check cam timing while I'm in there. Will also try that "boiling in anti-freeze" trick mentioned above.

Basically going to do the same things with the OS 91 4s. So what fuel SHOULD I use to prevent rust, if 4 Stroke fuel is a scam?

Thanks.

Mike D

1QwkSport2.5r

If I were buying commercially made fuel, my first choice would be SIG champion. Second choice would be Omega with a little extra castor added. I run 5-10% nitro 20% castor homemade fuel in my Enya 4-strokes on Enya 3 plugs and have zero problems. Last time I pulled my .60 apart, I had oil come pouring out of the crankcase and cam box and found zero rust on any of the steel parts. Thats how I prevent rust. Long term storage I fill the engine with mineral oil based dexron III and put in a ziploc bag.

For the best piston rings, get rings from Frank Bowman. [email protected]

This regimen may or may not work for you. This is my gospel according to me.

Airplanes400

My Feedback: (349)

First of all, and like others have written, do not use any other glow plug than a 4-cycle plug. The OS "F" plug is perfect, and should always be used.
Secondly, Cool Power 4-cycle fuel does contain 20% oil. I use it on all my 4-cycle engines and never had a problem. I would not use the basic 15% Cool Power for a 4 stroke engine.

Lean mixtures do cause backfires. A backfire on an OS 120 can cause the motor mount or firewall to break off due to the torque and power of these engines. A backfire would be like hitting the side of the engine with a hammer. If only a spinner or a prop came off, consider yourslef lucky.

Let your engine idle for a minute, and tune it to run smooth. If you are able to "gun" the engine after that without any hesitation and get white smoke upon acceleration, the idle is pretty much set correctly.
For the high end; like everyone else wrote and is written in the instructions, richen the mixture for a 300 RPM drop.

Other causes of backfires could be improper valve clearance, stuck valves, carbon buildup, weak valve springs, worn intake valve, worn valve sleeves (which could cause a valve to remain open), or timing (which would be constant backfires).

Airplanes400

My Feedback: (349)

Microdon,

If you have little compression, the most likely cause would be the valves. They could be burnt, pitted, or open due to carbon deposits on the seats. Be very careful cleaning valves or the seats. Use a valve compound to clean them.

If that doesn't work, then just buy the complete head w/valves. http://www3.towerhobbies.com/cgi-bin...?&I=LXAESF&P=Z
It is well worth it, and better than just buying the head without the valves.

I would not recommend the boiling in anti-freeze routine. It discolors the engine and makes it look extremely old. Turns it a shade of gray.

Go to the dishwashing isle in Walmart. Look for Dawn Power Dissolver. Spray it on, wait 20 minutes, then use a sponge or nylon brush (even a tooth brush) to wipe it off. Don't use a wire brush ... you don't need that, plus it will ruin the finish of the engine.

SrTelemaster150

I have used standard 15% (17% oil) Cool Power exclusively in my Saito 4-strokes after Byron's 4-stoke fuel caused sticking valves & cam lobe wear.

After burning @ least 100 gallons of that fuel, I have not had a single issue, even after 14 years of storage in a climate controled enviornment.