never saw one that wouldn't run.  If it runs 10 sec there probably aren't any problems with the motor.  OS has produced several different size venturis that will fit, some with a pretty large ID.  With the OS fuel nozzle supplied(same diameter as an OS needle valve) the venturi ID should be .25 to .26 or so.  They did produce some venturis as large as .281, which probably won't draw fuel well on the 25.

 Are you using the stock remote needlevalve?  If so, the fact that the fuel has to travel up to the needle and back down will cause some problems on starting.  Usually all the fuel drains back into the tank.  Even with a heavy prime the engine will often only run a few seconds on the prime and then die just as the fuel is starting to get sucked up  to the venturi.  The trick is to have your thumb poised over the carb and when the engines fires and starts to run briefly choke it to pull the fuel up to the carb.  The simplest fix is to bypass the remote needle and put the standard OS needle valve through the case, in the usual position.

remote needle..the only thing not checked was backplate seal..like I mentioned, its on its way back to the OS hospital

As I've posted before... ...one of our club members had two .15LA's on a twin, and one would quit after 2 or 3 laps, every time. The fact that it was always the inboard engine made it a bit exciting. Replacing the backplates with TT GP .15 units solved the problem. So, yes, it's an air leak! It's a lot less exciting, and a better use of your time at the field, to do this stuff before you put the engine in the plane, OBTW.  Steve

one more question ..can someone post the link for the conventional NVA for the OS 25?...I dont want to buy the wrong one..thanks!..Bob

Here's what I just found.  Found it by searching Tower for OS Needle Valves Assy.  I think the  NVA is used on all up to 46.

http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXCR49&P=ML

I've put this NVA in 2 LA 25's and an LA 46 and it fits both, works fine. Also, the FP Cover Plate they sell for the FP 20 will replace the plastic back cover the LA 25 comes with.

cheers,
Ken

No

cheers,
Ken

Jerry..thanx for the tip..but could not find anything on Stuka at all..they only go back 30 days for a search..maybe thats the problem. Do you possibly remember any of the strong points of his thread that really impacted running the engine better?..thanx again, Bob

Bob, Check Stuka again
Post titled "OS .25LA & FP" (I searched Subject only, "OS.25 LA", All Dates.)
Sept.29, 2002
By Mikey
I saved it, could send it from Stuka if ya cant find it, if your e-mail is listed.
Let me know if ya can or can not find it, it is there.
All mods and tips are there. His as mine did run great after changes. Got mine in a Barnstormer. I use a 9x4 Zinger, 9x5 on windy days.
15% Nitro, 24% oil 50/50. The 15 %Nitro is for my 5000' altitude.
Breakin time may vary, I did 2 min heat cycle. Two-4oz. tankfuls.

Jerry Bohn

Bob, Hows that for service hope it helps.

"OS .25 LA & FP Setup"


           After receiving a number of phone call and e-mails about how I was setting up the OS .25 FP & LA for the Primary Force I thought it would be a good idea to post it here. Please don’t think I know all there is to no about these engines but I have found a few things that worked really well.

OS .25LA: I used the engine in stock form and was quite surprised how well it ran right out of the box after a short break-in. There were a few things about the engine I didn’t like but the biggest thing was its lack of consistency (I am really fussy when it comes to engine runs but I believe that most sport flyers would be more than happy with the way the engine runs out of the box). I had to reset the needle before every flight and it was almost impossible get the same setting flight after flight. During some flights it would drop out of the rich 2-cycle and into a doggie 4-cycle for a few laps and then jump back to rich 2-cycle. In addition, the engine would go lean and almost quit in the second loop of the four-leaf clover (with plenty of fuel still in the tank).

I thought the rear mounted remote needle was the culprit and replaced the remote needle with a ST needle valve assembly (through the venturi). This was a little better but still far from what I wanted. I used different combinations of fuels, glow plugs, head shims, air filters, and muffler pressure with little or no change to the run. Completely frustrated, I switched to a tongue muffler and instantly most of the problems went away. The run was much smoother and more consistent cruising along at a rich 2-cycle but it still had a tendency to lean out in the four-leaf. Using a Bru-Line green air filter helped and almost stopped the leaning out at the end of the flight, but switching to a .250 dia. venturi (from Leonard at SSW) cured the problem completely. One other little change that I made was making a plug for the uni-flow vent that had a .050 hole drilled in the end to keep the engine from running a little fat (rich) at the end of the pattern.

Now the little .25 LA runs like a Singer sewing machine from the beginning to the end of the tank with a short power burst one lap before it runs out of fuel.

Listed below are the suggested changes and/or parts needed:

.250” i.d. Venturi.
S. T. needle valve assembly.
Tongue Muffler. Modified RSM or SSW
3 ounce Uni-flow tank (1” x 2” x 3”) Brodak or GRW.
Props: APC 9” x 4”& *10” x 4” (*trimmed to 9-1/2”).
Grisch 9” x 4” Black Magnum
Bolly 9-1/2” x 4-1/2” (re-pitched to 4”).
Glow Plug: S-300 Sonictronics or Thunderbolt R/C.
Fuel: Sig Champion 10 or 15% ½ synthetic & ½ castor.
RPM: Set at 10,2 to 10,500 on the ground depending on your prop choice. It should be braking back and forth from a 2-cycle to a 4-cycle (more 2 than 4) and as it warms up it should be in a rich 2-cycle (by the time you reach the handle). After launch, the model should speed up during the first lap and as the engine cools a little you should hear it back off a little into a rich 2-cycle.

OS .25 FP: This was really easy to setup because the above modifications worked perfectly on the FP. There was no need to plug the boost port or angle the intake ports on the FP. It ran really sweet and I think it even has a little more power than the LA. The FP liked the Bolly 9-1/2” x 4-1/2” with the pitch set at 3.75”.

One other thing that I will point out is I live at an altitude of 5000’ so some of the changes and settings maybe a little different at lower elevations. When I travel to contests (at lower elevations) about the only changes I make to the engine is adding a Bru-Line green air filter and if it is really hot I switch to 15% nitro fuel.




 
 

Bob, Your welcome. Glad I could help. I'm not the greatest flyer, I've received a lot of help from others on the forums in the past 12 years, glad I could contribute for someone else.
By the way, I use the stock OS muffler, runs good for me.

Jerry

No pressue, no restritor on uniflow line intake, never had any problem with end of runs. Tried restictor, made no difference that I could tell. Mikey may be a bit more particular than I am.
Feel free to ask as much as you like. Engine gurues answered all mine till I was very happily satified. They were concerened that I really understood what they were telling me. So I will help anyone who asks a question, no BS, if I don't know it from experience, I'll say so.
I think it's hard to troubleshoot engine problems on the forum for others, with my limited knowledge. Same engines run different for different people. Must have to do with fuel, props altitude, plane used on and luck.  My hats off to others who did it for me.
My main concern is you got Mike Pratts info first hand and I verify it made a world of difference for my OS25LA.
Good luck with engine, Feel free to ask away.
Now to get my 40LA to run as good as my .25LA and .46LA, seems like there different animals. But, I'll get it sooner or later. Thats part of the fun unless thats the only engine ya got and ya want to fly soon.

Jerry

I just received My OSLA25 from Tower hobbies for repair...it was new and did not run worth a C%^p. Anyway, if anyone  wants to know the outcome, they replaced almost EVERYTHING but the crankcase.Maybe it was a total dud, or they do this for good measure on all returns. I will be trying it out tomorrow, and we will see.

My guess is that they trouble shoot an engine like a typical home car mechanic (not technician) did 30 years ago. "It needs a tuneup." "And spark plug wires, rotor, and cap." Still won't run right. "And coil." "The ignition switch must be bad."  Hey, that reminds me...did you ask for all the old parts? You should be able to get them back, buy a crankcase, and put together another engine. Or maybe that's what the dude at Tower did...after throwing 'em into his lunch box.

Remember the 3 rules: Fuel, spark, and compression. If you have all 3, your engine will run, unless you put the sleeve in backwards, or the backplate leaks. The cylinder is pinned on those OS's (fine idea!), so I'd bet a dollar that the backplate leaked, or you didn't have those 3 requirements working for you. That would include a blocked spraybar, spraybar not pointed the right direction, duff glowplug, or maybe a honey bee down the venturi.  Steve

Sorry for the long delay posting re: la25....got it back TOTALLY rebuilt..installed the alum backplate and conventional NVA and bench ran a break in.....Now, this thing is super!..put it on a Flight Streak and that combo is VERY fast....at least for 60 yr. old me. I would reccommend taking all the the plastic off the engine and go with what I did...thanx, Bob

Hi Bob

On my LA's the plastic backplate per se was never the problem, only the poor excuse for a remote needle setup had me fuming. The fuel pickup nipple on the remote needle assembly (from tank to needle assembly) is positioned entirely incorrect for anything save some unusual R/C setup. It should point straight back at the very least, not sideways! Also, some of them leak!

Anyway, I removed the offending appendage on all of my LA's and just run the regular front needle setup. I'll try and include a pic of my modified backplate sans the remote assembly.

Best regards, Les