Cub Man

My Feedback: (50)

I have a OS Pre surpass 120 that I cannot get to idle. During a flight it went south and dies on me. I know a little on tuning but having a hard time of getting a good low idle. Has great top end. I have pulled the carb apart and cleaned everything out, and also put a new OS F plug in it. Looking for suggestions. Have thought of putting it up on the market for sale due to its rarity. Other then the idle it is in great shape.

earlwb

What fuel are you using?  The engines tended to run best with 15% to 20% nitromethane in the fuel.
Also have you checked and adjusted the valve gaps too? If the gaps close up, which does happen over time), it doesn't let the valves close all the way and the engine won't idle well.

Cub Man

My Feedback: (50)

Running wildcat 15 nitro 18 oil. I did check the gaps when it first started acting up.

earlwb

Does it idle Ok with the glow plug ignitor on it versus off of it?

estradajae

Hi,

Ionce had this engine and had the same issues, and also I was having the same problem on a surpass 91.

To make it transition good, the idle is so richit tends to quit sooner than later....if you lean the low end, it wont transition... no matter what you try on the needles and fuel and so on.
To me, the carbs of that era tend to get worn somehow. On the 91 you ended closing the hsn at about 1/4 turn from the closed position, dont remember on the 120, but was similar. I could not set the 120 right, and I gave it back to the owner ( who lent me the engine while I lived in germany), but the 91 was cured by changing the carb with a magnum carb....idle was fine and so was the transition.

Can you try another carb on it?

Cub Man

My Feedback: (50)

I'm about to the point of putting it up for sale

mmike65

Lot of times we don't run the right size fuel line and pickup,when you get up to 1.20 and bigger you need to go to a larger size fuel line & pickup.
It will make all the diffence in the world.

Cub Man

My Feedback: (50)

All was doing great, until I did a snap on take off and it quit. after that I have never been able to get it back to where it was?

estradajae

Now I got exactly the same problem with a 91 surpass....

jaka

Hi!
The dimension of the fuel line has nothing to do with how the engine behaves! Ordinary 2,5-3mm inside diameter is just fine.

Some advise:
1.On the 1.20 use a 16x6 prop!Preferable APC, RAM or Graupner "Sonic"! In some cases a 15x8.

2.Fuel containing 5-15% nitro and 18-20% oil is fine, mix of castor and syntetic is ok.

3.Tank size: 400-500cc! Mounted according to the: "Tank mounting rule" and preferably with "Uni-flow" system (that's two clunks)...or better yet, using a Tettra "Bubbleless tank!

4. Glow plug: OS F!

5. Check all bolts on the engine!
6. Check the cam setting!
7. Check the valves! On all four strokes : Between 0,04-0,1mm when the engine is cold
8. Check for rusty ball bearings!
9. Check for carbon build up on top of piston, valves and valve steem!

Same for the .90 engine but use a smaller fuel tank a 270-350cc tank.
Prop size: 14x6, 15x4 or 15x6.

mmike65

It does make a difference,but not so much on the 1.20s but it does not hurt.You can check it out yourself.I had a 1.50 ran great on the ground but once up and more throttle it would sputter and lack for power
but I change the fuel line and pickup never had the problem again.

http://saito-engines.info/fuel_tank.html

RandyB

My Feedback: (23)

Cub Man is having a problem with an OS FS 120(pre surpass),not a Saito 150.I have a Sig 1/4 scale cub with the FS 120 that runs perfect with medium fuel tubing.I also have two 150 saitos and a 180 that also run just fine with medium fuel tubing.

estradajae

The problem is not at high power settings as well...and it is indeed the engine running rich....not lean. A smaller diameter fuel tubing would lead to a leaner run!

RevGQ

How much do you want for her?

Cobra1

My Feedback: (42)

Check for air leaks around where the carb slides onto the intake pipe. I believe there is an o-ring in there and they will leak. Some silicone will seal it up. Just a thought.

mmike65

Mine also was not a Satio. Was just trying to help! This was my first time and will be my last.

Gizmo-RCU

My Feedback: (27)

I go with an air leak around the carb, the suddenness of the problem points to a mechanical problem in the fuel system (air leak?). A friend has an old 120 OSFS and as I recall the needles required some innovative tuning on his.
Once that was accomplished it ran great. As an idea you might remove the needle and see if the system has something blocking the flow, it takes almost nothing to cause serious problems.

Cobra1

My Feedback: (42)

Welcome to RCU where no good suggestion goes unpunished. Don't let it get to you. Some of us appreciate and welcome all comments as we realize we don't know everything.

estradajae

Every input is valuable, despite our discrepancies!, that's what is so great about the forums!!

I don't really think it is an air leak problem nor something blocking the needle, since both would make the engine run lean, and the OP, as well as the problems I have been having with the 91 surpass, are about an engine that is running VERY RICH on the low end.

With the 91 I have been working on, I changed the carb and the first time I ran it, I noticed a better low end performance...and then we tried to fly the plane, and that day, it was so rich at the low end we couldn't make it fly again... With the original carb, there was no way to lean the low end to make it run steady, it smoked like crazy and so on...

Then I took the engine back home and checked the new carb, and noticed the cat's eye had an uncut corner (magnum, that's your bad!!). At the field I noticed that the engine did not suck fuel at idle (i had to prime it and start it with medium to high throttle settings)...so everything makes sense, since the corner of the cat's eye on the spraybar that effectively controls the idle fuel flow was blocked, and it only worked for medium to high throttle settings, so when I tried to lean the low end, the lsn went away of it's effective range because it was simply blocked.

with the factory settings on the carb, it means the low end needle is about at the middle of the cat's eye (at it's widest point), and with the carb at idle setting, should be able to blow air through it, which I wasn't able. so I decided to modify a xacto blade and tried to carb the missing cut on the cat's eye and succeeded...got the fine shape and I could blow air through it on the idle position, you could feel that the low speed needle was working now well, because when I gradually closed the carb while blowing, you could feel the progressive restriction.

I haven't had the chance this weekend to test the engine again, but I'm pretty sure I got it!

I just don't know what happens with this old O.S carbs, they tend to wear out somewhere you just can't see it, and they become impossible to tune how it should!. If I remember correctly, they don't even have a cat's eye on the spraybar, the idle needle is just a tube that slides in and out outside the spraybar and restricts fuel flow progressively.

The engines I have had with the problems we are discussing have been always in great shape, they have been through proper maintenance, new bearings, correctly set timing, giving the right power output, with o-rings that are good and sealing well, proper fuel, new glow plugs, fuel tank correctly placed and so on... so I insist that the problem lies on the carb.

Best regards,

Jorge

jaka

Hi!
I had an OS FS1.20 20 years ago and it was a very sweet runner! Had it in a "Old Tiger" model, plans published in MAN.
I don't think the problem lies in the carb,if it isn't such an easy thing to fix as a bad O-ring ofcourse.
The problem might be in old worn/rust ball bearings and carbon build-up on the valve steems. I have through the years helped many people with engine troubles like yours and it have always been something inside the engine that has been wrong, Worn piston/cylinder,rusty worn ball bearings, Carbon build-up on valves and valve steems, wrong cam setting, wrong fuel, bad glow plug and soo on, not the carb.
Have you dismantled the engine?

estradajae

Jan
I usually do engine maintenance for my mates at the field, and what I usually do is taking the engines fully apart, clean everythin very well, decarbon the valves, decarbon the head....put everything together, set the cam timing, set the valve clearance, take care of the o-rings and check the seals, I pay attention to the detail. I have done it many times for the last years and the engines work as they should after that.
I'm usually the one who helps people at the field tunning the engines and thanks to the knowledge gained here in RCU, almost always the engines end up well, except in the case we are discussing...

I wish it were just an o-ring or something similar, but no, it isn't, and the engine produces full power!

When I changed the carb, it behaved better (haven't tested yet with the fixed magnum carb ).

blw

My Feedback: (3)

+1

Thanks for chipping in with the help.

Cub Man

My Feedback: (50)

Guys mine is up for sale http://www.rcuniverse.com/market/item.cfm?itemId=907792