Greetings,

I'm having a lot of trouble getting the OS 15 LA in my Gnat to run properly. Regardless of all the things I've tried, it still lacks power. While I was breaking it in, it developed a leaky backplate. Since then, I've RTV'd the backplate and put in cap screws, so no more leaks there. It ran great for the first couple flights before that backplate leak. Is there a possibility that I ran it way too lean due to that leak during the breakin period? Would that have a long term effect of preventing it from running properly? I've gone through everything I can think of and tightened everything, and to no avail. My buddie's runs much faster and higher pitched than mine.

Any suggestions? What could I be missing?

I would appreciate any help that you guys could give me.

Thanks,
-Pat-

Hi,

I am familiar with that problem, I have four OS 15 LA and on every one I have to tighten the screws on the backplate quite often. But I have never experienced a large lack of power...
I suggest you check the needle valve for dirt and/or run (break-in) the engine with quite fat mixture on the lowest possible rpm (for 10-20 minutes).

I hope the engine runs better

Best regards
/Anders

You probibly didn't hurt the engine by having a couple of runs a bit off due to a leaking backplate. It's not good for the engine, but it shouldn't be the end of the world either.

You don't want to run these small ABC engines really rich for any length of time. Never run it "4-cycleing". Doing that losens the fit between the piston and liner, and causes a reduction in power.

The guys getting a lot of power from the LA's are putting on a smaller, cut down prop, and breaking them in set a little rich, but at high RPM. This creates enough heat in the liner to get it to expand properly and not wear down the piston.

When you turn the engine over by hand when it's cold, do you feel the piston grab at the top of it's stroke? Take the glowplug out, and rotate the engine by hand. The piston should grab at the top. If it doesn't, you either got a lemon engine, or you've worn the engine out already somehow.

Check the head bolts, make sure you aren't leaking around there.

You say the engine does't produce power, but does it run reliably with out going lean or showing signs of an air leak or anything like that?

I assume you're using the same fuel and plug as your friend? Did he take the baffle out of the muffler, and you didn't?

Lots of Gnat combat flying this weekend!

I tried the engine with a drilled out baffle, and could only get 16000 or so RPMs. Tightened up the backplate, and in doing so, stripped out one of the cap screws that I put in... dumb. So... now I'm faced with an already leaky backplate and a stripped out screw. Can I drill and tap to 4-40, or is that too big for the amount of aluminum in the casting?

Tightened down the head screws, and they weren't leaking. Tightened carb screws, cleaned air bleed hole and carb. I'll try some more things and let you guys know if I can get it up to RPM. A fellow at the field told me to make sure the exhaust flange hole lines up with the cylinder hole, and file it down if not. That sounds like a good idea, and I'll try that, as well as shortening my fuel lines.

Thanks for the ideas.
-Pat-

Hello,

Its sad that the engine is not working properly, because it is a very nice engine... I bought my first OS 15 LA four years ago and i have´t had any problems at all, except the leaking backplate. Have you tried to change the glow plug?

Best regards
/Anders

I'm fairly new to combat, and fellow combat buddy gave me this tip:

"After breaking in a new OS.15LA you may notice the engine will loose power. The screws on the backplate and on top of the cylinder head are prone to loosen up a bit. If you tighten these up you will regain the power again."

I personally haven't experienced this yet, so I can't tell you how applicable this tip is. But it can't hurt to tighten the screws on the cylinder head, if you haven't already.

Juice

Airmaster and Juice,

I have tightened/rtv'd/locktited everything I can think of, and have some cap screws for the cylinder head on order. This whole thing has not only me, but some others stumped as well. Everybody else I know has had great luck with this engine. The only thing I can think that would be causing a problem would be if one of the internal components was damaged... I'll have to tear into it to see...

Later,
-Pat-

Pat,
If everything is tight and sealed up then it is likely you have a poor piston to cylinder fit. A new set costs $25 from Tower. We have found these engines do not like more than 18% oil. We have also found that they do not draw fuel as well as larger engines and you have to make sure it is getting fuel easily from the tank.

There is more about the OS LA.15 on www.georgiacombat.com in the glow combat section. It covers break-in, and fuels. Good luck.

Are you using the stock plastic back plate or have you upgraded to the aluminum one? Also try some plumbers thread tape around the inside edge of the back plate.

Super67 and Lou,

I am using the stock plastic one. Where are the aluminum ones available from? I had heard some people were going with a .15FP backplate... is this a direct fit for the LA? I will try to plumbers tape method also, the RTV doesn't seem to be working too well, even with the cap screws and RTV I still get very little leaks. I am pretty much resorted to the fact that the engine may have gotten damaged during the breakin, as it probably got run really lean with those leaks. I will try the new piston/cylinder soon.... good stuff on the Georgia combat site!

Thanks,
-Pat-

Wrig0335,
The aluminum backplates are those used for the OS .15 FP. It is a tiny bit shallower and you will loose about 200 rpms with them but the gain in reliability is well worth it. Here a couple of folks have had great luck wrapping teflon tape around the backpate then reinstalling it. Socket head screws are bad on the backplate because of their narrow head diameter they can pull through the plastic. If you use socket head screws on the backplate put a washer behind them to spread out the load area. Good luck. By the way are you getting any bubbles in the fuel line? The OS LA needle valve assembly is excellent but it can leak.

What kind of propeller do You use on this engine?
I have flown combat in Europe for some years. The RPM's are limited to 17.000 here. I used MVVS .15 and APC 8x4 and it gave 16.400 RPM or APC 7x5 and gained a few hundred RPM's more.

Milos

Adding a picture of my blue Spit from 1999.

A good source of metal backplates is the Thunder Tigre 15 item. Incidentally every part except the case and head are interchangeable to the LA. The only reason the case and head aren't is because TT uses 4 bolts only on the head. Every part of the OS15 FP is interchangable to the LA 15.
Now as to the OS15 FP there are some differences they started out as iron/steel piston and liner sets and fully machined cranks and then at the last were alum/chrome and a cast crank just as the LA is.
Your best rebuild is a TT 15 P/L and Crank, which is fully machined and trued, in your LA. It is faster and smoother.
Dennis

Edited for spelling

Hi!
Being into pylonracing for many years I can give you some advise.
If an engine doesn't put out as much power as expected you have to take the engine apart and check for a worn piston to cylinder fit. The piston to cylinder fit is very important.
If the piston is so worn it can be pushed out trough the top of the cylinder liner it is totally worn out and has to be discarded.
Second... for best performance in every airplane it is vital that you use the best tank available and that is to use the Tettra line of Bubbleless fueltanks. 85cc (3 1/2 oz) and up is available.
These tanks are used by pylonracers world wide for their reliability.

Regards!
Jan K
Sweden