please help, I have a old 120 gemini that Iam having trouble with,first this is in a 1/4 cub I plan on using a 16/6 prop,I currently have a 16/8 prop on it just because Iam waiting on my 16/6 to get here,I can get it to run but I just cant get it set right to stay running I have taken the carb off and cleaned throughly,and still cant get it right,if someone could give me a starting point on the needle adjustments that would be great,so far the best I cant get to idle and run decent is 3 turns out on the low end and 4 turns out on the high end,and its pretty rich at that point and after about 20 to 30 seconds at full throttle it will just die, the tank center line seems to be right on for the carb,so I been messing with this for 3 days and love any help someone could offer, Thanks Patrick

Which plugs are you using? Try some fresh four stroke plugs. Is the valve lash on both cylinders in spec? While it's running at idle, spray some WD-40 using the little red hose around the carb and intake manifold. If the idle changes, you have an intake leak.

it has new four stroke type f plugs,the valve lash I have no idea on,but I will try spraying some wd40 and see if there is a intake leak,thanks for the help unballanced


ps you wouldnt know the specs for the valve lash do ya

I ran a 15-8 on them Was always a easy going engine.

Is your fuel new?

If both cylinders fire up to the point of stoppage, you have a fuel problem

Here's the manual for the 120 and 160

http://www.osengines.com/manuals/ft-gemini-manual.pdf

Enjoy,

Jim

thanks for the manual link w8ye, I just baught a new gallon of cool power 15% I can get it to fire up and idle nice and even wide open but after like twenty seconds it will just die

It may be getting hot?

Did you try running it a little richer?

You can leave the glow starters on it for a while and see how it does?

it does act like its running lean,I got the high end 4 turns out,and the low end 3 turns out,and it just dumps fuel out the exaust,and I have been keeping the glow starters on, now I have been reading some other forums and posts and I saw someone mention adjusting the airbleed, now are they refering to the low end or is that something else? thanks again for your help

The very first Gemini's had a air-bleed carb. But the MK II came out and most had a low speed mixture screw in the center of the throttle arm. If yours has no screw in the center of the throttle arm, it is most likely the early air bleed carb. The pictures of the engine in the manual are of the air bleed engine.

If you have raw fuel comming out a exhaust pipe, that cylinder is rither very rich or is not running.

You need to be comparing both pipes.

If a cylinder goes out, you can relight it by putting the glow starter back on that cylinder.

How much running time is on the engine? If it's an "old" one that's had a lot of time, it could have the piston/ring/liner fit worn to the point that compression's no good when the engine warms up. If it's had very little running time, then it needs to be broken in.

Initial running of that engine should be done at full-throttle set as rich as possible and still running. You may need to leave the battery connected to the glow plug to keep it going. Run it like that for 15-20 minutes before you attempt to lean the high speed mixture. Gradually lean the engine a bit at a time, and then richen it a bit again. Go rich-lean-rich-lean, gradually working the mixture leaner. You'll need a tachometer to accurately track your progress.

With a 16 x 8 propeller, you'll likely see RPM levels in the high 7,000's when the engine's broken in and run just rich of peak RPM.

Cold fuel coming out of the exhaust pipe means the cylinder is not firing. If it's burning hot, then you have that cylinder firing with a rich setting.

The engine should have no problems running with the fuel you're using and with good O.S. Type F glow plugs.

Set the rocker arms so that the gap is between 0.04 mm and 0.10 mm when that valve is off the cam.

thanks for all the help guys,Ive been messing with it a little bit this afternoon, both cylinders are firing I can get it set to were it fires up nice and runs nice just after about 20 seconds wide open the rpms start dropping till it dies, the compresion isnt the greatest,is there still parts available for this engine to do a rebuild? also its not an airbleed it has the lowend screw through the carb,thanks Patrick

You can get new rings from this guy. Many of the other parts are common with the current 160.

Frank Bowman
1211 N. Allen
Farmington, NM 87401
(505) 327-0696
[email protected]

I still suspicion you have the mixture wrong? From my experience with these OS twin engines, they are very mild manered and easy to tune.

Enjoy,

Jim

If the engine doesn't say "FT-120 II" on the case just behind the drive washer, it's the earlier FT-120. There are zero parts available for it any more. the FT-120 II had some parts in common with the FT-160. The older FT-120 has virtually no parts in common with the FT-120 II or FT-160. Most parts for the FT-120 II are still available.

yeah its the ft-120 not the ft-120II,what would be the best starting point for the lowend and highend starting point like if I turn both needles all the way closed how many turns out on each should I start with,thanks

Actually parts are available in Japan. My son is stationed in Yokosuka and I have a good friend that owns a hobby shop in Sasebo. I get parts for old OS engines from them when I need them. I his engine slows down and quits it sounds too lean and is slowly siezing.

Open the main needle about four turns and don't worry about the idle until you get the top end set. If it is too lean open it another turn. Once you get the top end set you can mess with the idle. You have to get it to run before you can do much else with it. Even if that valves aren't quite right, as long as they aren't held open it will run if it gets fuel, has a good plug(s) and and isn't broken.

The early model FT 120 had a metering carb. If the low speed adjustment is screwed in too far, it will reduce fuel flow towards full throttle. Try about 2 turns on the high speed and 4 turns on the low.

Yes the early ones do have a metering carb. I bought one of the first ones when I was in Japan in 1979. Again, don't worry about the idle adjust until you get the engine to run at full throttle. At full throttle the idle metering needle has no effect anyway. You have to get the thing to run before you have to worry about the idle.

thanks Waco Driver, hopefully that works, thanks everyone for all the great help,Patrick

Henson,
I've got a stupid question for ya. Will it re-start after it quits? Or do you re-fuel and start agian? The reason I ask is that you may have the fuel lines reversed. Could be once the fuel is sucked down past the (what is supposed to be) the vent line it quits. Hate to admit it, but it's happened to me. Fought a Fox 19 and finally got fed up and was going to replace the engine when I found my goof.

no,its not that I did figure out the problem and know I feel bad that I wasted your guys time,when I get real frustraded I tend to over look some things, so I will go ahead tell what it was. Well I had 2 lines coming out of the tank one going to the carb and the other for the exhaust but on the twin there is no exhaust port so just out of habbit I plugged the line,unplug the line runs great,so if you guys can forgive me for being a idiot that would be great,thanks again Patrick

I thought that's what these forums were for???[sm=spinnyeyes.gif]

Henson,

I'm pleased to read you worked out the problem

My battle cry in the engine tech support world.... check the fuel system.

About 85% of issues that arrise come from the fuel system, not from the engine.
Something folks should keep in mind as a general thought.

Image here is of my 1/4 scale H-9 cub, Gemini FT-120 power (old one). Utilizes an Expert glow driver on board which kicks on around 1/8 throttle. Using standard 15% sport fuel (power master). Turns an 18x5 zinger prop at 6800-7000 rpm, and provides very realistic performance.

Bob

thanks,beautiful plane bob27s, I have the plane all loaded up, the winds are supposed to be down tomorrow so I schould hopefully have her in the air tomorrow

Is there a source for ANY parts for the old FT-120.....rings or valves....New old stock somewhere.......

I have one on a BUSA 1/4 Cub and want to get it freshened up and tuned.


I am turning a APC 16x6 at 7400 , ....if this thing could turn a nice BME or Menz prop....I would be very happy.

Bob27,

Wow, after looking at your Cub Pix I had to run to the garage to see if my Cub was still there! Sure enough all is well. Mine is A 1/4 scale Sig with the original OS120 twin for power. I'm using a classic 16-6 which seem to be just right for this engine/plane. The older Gemini seems to be very friendly to me. I've been using 15% nitro with half and half castor and synthetic and as always, a little on the rich side. The only complaint has been a hassel from my flying buds who swear that howling winds always follow when the big sucker shows up at the field.