RE: Os 120 4 stroke overhaul help needed

RE: Os 120 4 stroke overhaul help needed (Full Version)

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Jaybird -> RE: Os 120 4 stroke overhaul help needed (4/23/2008 7:23:30 PM)
Successful Maiden flight. After installing the new parts supplied by Neol Jensen and several months of refurbishing, I had a successful test run and first flight of my 20 plus year old BUSA Cub and OS 120. The engine started right up and ran very well. I only had to adjust the low idle setting on my transmitter and a slight highspeed needle adjustment to get a good transition and power. I taxied up and down the grass strip a couple of times and then pointed it into the wind. After a short roll at 3/4 throttle it lifted off and headed up! A few clicks of down trim and it was flying very nicely. Loops and stall turns are very nice and the landings are a non-event with a good idle. I look forward to many years of flying this big bird. Thanks to all for the help with rebuild and parts. The engine is a single cylinder but looks like a twin, thanks to the very well built wood dummy engine on the left side of the cowl! I have a short video of the sart up I'll try post as well. Jay

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/6/2008 2:42:44 PM)
Here is a link to an engine start and flight video. http://www.rcgroups.com/forums/showthread.php?t=854104 I'm finding that i'm having tuning and run issues as time goes on that are perplexing. I've listed the problems on the video thread. Does anyone know what o-rings there are in the carb and what size? Jaybird

Ken6PPC -> RE: Os 120 4 stroke overhaul help needed (5/7/2008 1:58:34 AM)
I don't think that your left cylinder is firing at all! <GRIN> Knowing what engine was supposed to be in your plane, I did a double take (pun) when I saw the picture of a twin. That looks VERY nice! Seriously, your engine sounded like it was running very well in your videos. What happened?

Waco Driver -> RE: Os 120 4 stroke overhaul help needed (5/7/2008 1:34:39 PM)
I have found that a Perry oscillating pump improved the running characteristics greatly on my 2 OS 120 FS engines. No more leaniing out as the fuel level drops.

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/7/2008 10:31:49 PM)
It was suggested that i try switching from my cool Power 15/18 fuel to 20/20 fuel by a modeler who had the same engine and same problem. It did run great for that video and then started running poorly the next few flights afterward. I'll keep you posted. Jaybird

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/8/2008 8:39:39 PM)
I did a quick test run with the new 20/20 fuel and it seemed to like it. it started right up and idled very nicely without any external glow driver. I'll have to continue tuning to make sure it continues to cool enough. It did seem to transition well from idle to full power without loading up as before. What I do still have to contend with is something metalic rubbing somewhere and causing bad intermittent elecrtrical glitching. Not sure where to start. I can duplicate it without the motor running by touching a screwdriver the cylinder head, but that seems typical. I'll keep looking. Jaybird

submikester -> RE: Os 120 4 stroke overhaul help needed (5/10/2008 3:31:01 AM)
Are you running muffler pressure on your 120? I run it with mine and it runs well like that on 15%; I've tried 20% in it but it didn't seem to run as well. I prefer 15% in mine. Also, I'm using a Zinger 15x6 prop.

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/12/2008 8:11:42 PM)
Hi Sub, Yes I am running muffler pressure to the tank. The motor had a 15x6 Master Airscrew on it when I bought it. I replaced it with a 16x6 Master Airscrew for my first run. I was thinking of trying different props as well. It's been too windy to do any test running or flying around here this past weekend so I don't have any new information. Jaybird

submikester -> RE: Os 120 4 stroke overhaul help needed (5/12/2008 11:44:25 PM)
I have only ever flown mine with the 15x6 zinger - I got it with that prop and it ran so well with it that I never considered changing it. Originally it was flying on a 13+ lb 4*120 and typically I would use 15% power master fuel. I tried a tank of 20% in it one time and was uncomfortable with the way it sounded, it's been a while so I don't recall exactly what made me uncomfortable but here we are. Now I'm finishing up the assembly of a SIG P-51B and putting this 120 in it with the same prop (15x6 Zinger) and hoping for some majestic flight...

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/13/2008 3:10:40 PM)
I've pulled out the homemade onboard glow driver system as it wasn't working well and I think may have been a sourse of radio interference. I'll also go back to the 15x6 prop (I'll buy a new one since I have no idea how old the old prop is) and see if that helps. It's still really windy around here and rain is expected later in the week so it will be a while before I can do any more testing. Jaybird

submikester -> RE: Os 120 4 stroke overhaul help needed (5/14/2008 9:26:35 PM)
For your cub a 15x6 should be very good but in my Mustang I have just recently been testing it with a 14x8 APC and the results were very nice - I was quite pleased.

bogbeagle -> RE: Os 120 4 stroke overhaul help needed (5/21/2008 12:20:20 PM)
Hi, I've just acquired one of these old OS 1.20 singles...with the twin rocker covers. It does run, but I can't get it to idle below 3,700, so I guess that something isn't right! Anyway, I'd like to confirm that the valve timing is correct, but I lack any info regarding timing marks. This engine has been dismantled by a previous owner, so you never can be sure. Hope that there is someone out there who can help with this information. TIA David

Ken6PPC -> RE: Os 120 4 stroke overhaul help needed (5/22/2008 2:14:47 AM)
I check valve timing this way: http://www.rcuniverse.com/forum/fb.asp?m=7442072

submikester -> RE: Os 120 4 stroke overhaul help needed (5/26/2008 9:41:37 PM)
Well, been flying mine a lot more recently and I have to say my original comments are now modified. I am concerned that I may be running it too hard but I guess we'll just have to wait and see. I put this engine in my Sig P-51B, previously I had putted around with it in a 4120. at 13lbs the 4 was in no way fast but I never tried to make it fast either. The Mustang however is only 8.5 lbs so I was hoping for some speed. I also had it installed inverted which I have been successful with in the past - it's just a matter of getting the tank in the right place and the mixture set correctly. I had previously said that I was uncomfortable running anything more than 15% in it but with the 15% I simply couldn't get a good reliable idle inverted like I did with 20%. So, 20% it is and it is running really well. 20% with a 14x8 APC and it's darned quick, not as fast as my buddies H9 mustang with a YS110FZ but quick none the less. I may replace it in this airplane with a 110FZ myself but for now the 120 is running well (until I blow the bottom out of it...). I looked into the heads recently to check the valve clearance - it's tight in there and difficult to work. Good thing it is within tolerance. Also, I noticed on the intake side there isn't much lube getting up there where as the exhaust side was quite wet.

Cub Man -> RE: Os 120 4 stroke overhaul help needed (5/28/2008 3:19:48 AM)
Well I finally took mine out for a test run and flight. A club member helped me to tune the engine in. All is great but the idle is high. I’m running it with only a pipe, it came with no muffler. The question is how can I add back pressure to the tank to help with idle.

submikester -> RE: Os 120 4 stroke overhaul help needed (5/28/2008 6:10:48 AM)
Buy a muffler - I use a muffler off a Magnum 91 RFS (cheaper than buying the OS).

Jaybird -> RE: Os 120 4 stroke overhaul help needed (5/29/2008 9:07:26 PM)
I was able to get in a couple of quick flights last Thursday and it worked well. I had removed the on-board glow system and changed back to the 15 x 6 Master Airscrew. I tried it first with the 20/20 fuel and it sounded funny so I went back to the 15% MV Cool Power for the flights that I did. It took a bit more tuning to get a reliable idle and transition without the glow driver but ran very well once that was set. Towards the end of the flight and taxiing back there was an odd rattle sound that would increase with engine RPM which had me worried. During my cleanup I found that one of the cowl screws had backed out and it allowed a large section to vibrate at different RPMs. If the 15 to 20 MPH winds would ever let up I'd give it another try. I think that was my odd sound with the 20/20 fuel so I may use it later sinxce it's so expensive. Jaybird

submikester -> RE: Os 120 4 stroke overhaul help needed (5/30/2008 4:06:37 AM)
I wasn't very 'happy' with the way mine sounded on 20/20 either but it seems to have mellowed out and sounds fairly normal now. I don't know if that is good or bad though.

Jaybird -> RE: Os 120 4 stroke overhaul help needed (6/9/2008 3:28:24 PM)
Well, the glitching problem does seem to be solved, but I'm still having engine idle run problems. For the first flight of the day the engine runs great.. it starts right up and transitions nicely and will idle without problem. After that flight however, it becomes uncooperative at idle. If I increase the idle it will stay running but it's too high for landing on a short field, which I could do the first flight. If I leave it at the original idle RPM (I haven't tached it so I don't know the number) it will idle for 10 to 15 seconds and just stop. It doesn't rev up or bog down it just stops. I'm suspicious of the crank bearings as this unit is 20 years old and I have no history of it's maintenance. The crank vent line has a brownish/blackish tint to the fluid coming out of it and collecting on the left gear. I'm wondering if as the engine heats up the bearings turn less freely and so the lower idle becomes a problem. I'm using 20 % nitro/ 20% oil fuel at the moment but have also used Cool Power 15% MV fuel Any thoughts out there? Would O.S. take a look at it and replace bearings??? Jaybird

Jaybird -> RE: Os 120 4 stroke overhaul help needed (6/20/2008 9:58:17 PM)
An engine guy in the club took a look at it and after adjusting the exhuast valve and checking the bearings set it up on his test stand and it ran great. It would easliy tick over at idle and transition well up to full power. I'll mount it back on the plane this weekend and make sure the fuel system is working properly and see how it does. It was pretty impressive on the test stand! Thanks Art!!! Jaybird

Jaybird -> RE: Os 120 4 stroke overhaul help needed (7/25/2008 9:44:19 PM)
Well, it looks like a trip to the repair shop is needed. I did mount it back on the airframe in it's original horizontal position but soon had the same problem of not beeing able to maintain the idle level needed to land at a proper speed. I took the fuel filler out of the circuit to see if that made any difference and also tried it with and without the exhaust pressure line but nothing made any difference. I took it off the airframe again and opened up the cam gear cover. The cam follower for the exhaust pushrod has a definite scooped shape to the bottom that rides against the cam. The cam adjuster screw is also extended quite far out of the rocker to get the proper valve clearance. The intake adjuster is in a more "mid-travel" position when set. I don't know what the cam follower seat should look like so I don't know if it worn out or not. The problem right now is that the hobby shop hasn't sent it in because they aren't sure the service center will work on it due to parts availablity. I hope they find out soon and get it sent in. I also gave them Noel Jensen's contact information as he has many out of production parts available. Jaybird

blw -> RE: Os 120 4 stroke overhaul help needed (7/25/2008 10:14:01 PM)
You could put the cam follower in a drill and grind it down to the proper shape on emery paper. I've done that with good results.

Jaybird -> RE: Os 120 4 stroke overhaul help needed (8/15/2008 9:22:36 PM)
I took the engine to my LHS and they have sent it in to a repair service that was willing to take a look and repair it if they can. I'll keep my fingers crossed!! Jaybird

w8ye -> RE: Os 120 4 stroke overhaul help needed (8/16/2008 12:45:54 AM)
Cam followers are flat and smooth on the bottom with a small radius on the edges.

Jonathan2121 -> RE: Os 120 4 stroke overhaul help needed (8/22/2008 6:05:50 PM)
Hi everyone, I just got the OS 120 III w/ pump and was wondering what fuel should I use. Is 15% nitro ok or should I use 10% nitro. Does it make a difference?

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