Bought a second-hand (I know - there's my problem right there...), OS 120AX. The piston ring was gummed up so replaced it with a new one, and gave everything a tidy up.

The engine was running fine. Now I get one flight out of it, and after that the engine will stop as I wind it up for take-off (this is after a cool-down period).

The problem gets progressively worse as we test the engine on the ground. It will start and work fine, then when you go to rev it for the randomth time, it will simply stop.

Have adjusted the needles, high and low, as per usual;

have tried different glow plugs;

had fuel filters in and out of the fuel line;

taken the fuel system apart and replaced tubing;

have blown out the carby

- nothing seems to make any difference.

My thoughts are that:

- it could be overheating inside the cowl (can't fly the plane without the cowl), but it doesn't really seem to be acting that way, and it always has a good opportunity to cool down between flights, so why not overheat on the first flight? And why is it stopping on take-off, when it is getting more airflow?


- it is sucking air, with the problem worsening when the engine is warm. But why would heat make an air leak worse? Would a dodgy head gasket cause this?


Has anyone else run into similar problems please? (please note - it is not the mixture adjustment - that has been tested by various experienced engine tuners at the field).

is t inverted? tank height ok?...have you tried n OS F plug, or a different glow plug?

Is it still not broken in after the ring change?

Originally Posted by Balsaworkbench

Is it still not broken in after the ring change?

That's my guess as well.
You need a full break in after a ring replacement, as the ring is the single part taking the longest time to fully seat, and a new ring in a used liner typically needs more time than a new ring in a new liner.

Originally Posted by 1967Brutus

That's my guess as well.
You need a full break in after a ring replacement, as the ring is the single part taking the longest time to fully seat, and a new ring in a used liner typically needs more time than a new ring in a new liner.

If the used liner is deglazed as Frank Bowman always instructed, the ring (one of his; fine grain Meehanite iron) should seat within 30-60 minutes. Steel rings will take longer in many cases. If the bore is glass smooth, that new ring may never seat (I have seen the latter firsthand before). I’ve found, especially with my SuperTigres, that the mixture has to be set richer than I normally would run it at and with a hotter plug until the ring seats.

I would see if dressing the liner a little bit and adding some extra oil for the first gallon helps.

Did you replace the bearings ?

Originally Posted by RCpilot7974

is t inverted? tank height ok?...have you tried n OS F plug, or a different glow plug?

It's on its side. I was pretty careful about tank height so that shouldn't be the problem.

I've never heard of an F plug. I've tried both 7's and 8's.

Originally Posted by Balsaworkbench

Is it still not broken in after the ring change?

Yes, that occurred to me. I did put a couple of tanks through it first, but maybe the ring isn't bedded in properly. I'll mix up some extra oily fuel and run it with a No 7 plug next time.

I didn't replace the bearings - they were turning ok.

Checked the head gasket, but didn't see any evidence of blow-by.

Bore looks ok.

However there is some carbon colouring on the piston crown, in a half ring on the hot side of the engine. Not sure if it means anything.

Rats, looks like I can't add the picture.

Get three tablespoons of first pressing castor oil, drink it.
Whilst you're on the dunny think of all the things you've done to the engine since you bought it. With the amount of time you'll spend in there you'll be able to figure out what's wrong.
When you finally get out the dunny fix it.
My thoughts are that the piston ring went back in the wrong place and/or the piston went in back to front. (IIRC, the piston is pinned, as is the cylinder liner.)
If the piston ring was gummed up then may be the carby fuel passageway is slightly choked as well and the engine getting warm adds some flexibility to the crud causing the partial blockage.
I'd strip out the carby completely and clean it, whilst it's off check the tapped holes in the carb spigot to see if the threads have been stripped also check the O ring between the carb and crankcase, make sure it's the correct one for the job.
I'd also check the exhaust pressure nipple is clear.
The only other things that I can think of is the tank fittings, if they are brass, nitro corrodes them and they can leak, if brass change for plastic or aluminium, also don't put your unused fuel in the plane's tank back into your fuel bottle, the 120 uses mucho fuel and the exhaust pressure will ensure that water vapour builds up rapidly in the fuel in the tank.
Finally, your on field experts haven't done too good a job, they seem to be as accomplished as us self appointed internet experts, much better if you do it yourself without many fingers twiddling.
FWIW, on my 120 ax, the idle needle has an effect on mixture control until the throttle is about 1/2 open.
My 120 ax has run as sweet as a nut, just as user friendly as the little 10 fps right the way up to my 160 fx, literally fit and forget provided there's no castor in the fuel, not even 0.001% and the fuel is not pumped back into the bottle at the end of flying, I prefer to let my engines run at high throttle until they empty the tank then a few drops of ATF and spin over with the starter.

I think you're trying to run it too lean. Try setting the top end so it runs really blubbery rich. If it dies when you run it up, it could be that the low speed needle is off. At idle, put your finger close to the exhaust and feel how wet it gets. If it's pretty wet right off the bat, it's set too rich. If the engine sounds dry and wheezy when you run it up, it's too lean. However; low speed needle adjustments are really hard to make until the ring seats.

Best thing to do is run it rich enough so it just flies and break it in while flying.

carl

>>Bought a second-hand (I know - there's my problem right there...), OS 120AX.<<

All the suggestions are only speculations. Here in lie your ultimate problem. You bought a second hand engine without taking it apart, cleaning it, replacement all the worn out parts except for the piston ring and expect it to run perfectly.

Disassemble the entire engine, clean out every part and rebuild it properly first. Otherwise you are just chasing your tail.

I had issues with a 120AX that finally went away when I changed the IDLE NEEDLE o-rings.
Just a thought.
Jim

Jim don't tell me that. Where am I gonna get those from???