OS 25 FP engine problem

[Gareth Jones]

I have a Multiplex Skycat fitted with an OS25 FP engine.  My son and a friend bought it at the Nationals  swapmeet 2 years ago so the engine's history is unknown.  We have flown it a number of times and the engine has always started easily and initially ran well.  Lately however it has been difficult to get it to run at full power.   It runs well up to about half throttle but beyond that, although it initially seems to reach full power, after a few seconds or a minute or so, it will gradually reduce rpm and eventually die.   I have tried adjusting the needle valve,  and air bleed valve  but to no avail.  At the moment, the best setting seems to be with the needle valve about two turns open and the air bleed  set with the screw end just covering about half the hole in the vent.   I cannot see any bubbles in the fuel lines and the tank is pressurised by a connection on to the exhaust silencer.  Does anyone have any ideas what the problem is and what the solution might be?

[Tim Mackey]

Could be the plug. Adjusting the air bleed screw will have no effect on mid to high range revs, so get it idling and picking up cleanly, then leave that one alone. Could be dirt in the carburettor, or just mixture  too lean, and it is overheating and dying. I would change the plug first..... as it did run fine, and then suddenly changed.

[Mark Lubbock]

As well as the things above, check the clunk in the tank-I had similar problems with a model of mine-the clunk had gone to the front of the tank after a less than perfect landing(!), kinking the fuel line & restricting the flow. It passed just enough fuel for slow running, but not enough for high speed.

[Tim Mackey]

I normally insert a length of plastic / brass tubing along a section of the pick up tubing, to stop it bending over on itself, but still allow movement.

[Mike Bell]

I have recently had a similar problem with a heli engine that would run for a minute or so then just sag.  In my case the problem was that the piston and liner were heavily varnished.  I use fuel which has half castor and half synthetic oil. The castor oil can deposit a varnish like layer in the hot zones of the engine.  This affects the lubrication between the piston and liner which causes the engine to over heat and grind to a halt.  To check for this, take the silencer off and look down the exhaust ports. As you move the piston up and down does the piston look shiny silver or is it stained a reddish brown?  If it is the latter then it could need taking apart for a good clean.

[Former Member]

[This posting has been removed]

[Gareth Jones]

Thanks to all those who offered me some advice on the above problem. 

I thought the most likely cause was a fuel problem so I dismantled the needle valve and discovered a small amount of 'fluff' in the valve.  This prompted me to remove the cowling over the tank so I could remove the tank and flush it out.  This revealed that the clunk was positioned right at the back of the tank.  With the fuel pick up pipe lying straight backwards the clunk inlet was almost flush with the rear wall of the tank. 

Having rinsed the tank and repositioned the clunk about 5 mm from the back wall all is now well again and I flew the aircraft a couple of times at the weekend with no problems in achieving full power.  Thanks again everyone.

[David Martin 2]

Glad you've sorted it Gareth.

 Just something else to look out for in future with similar symptoms is if the pressure pipe has hardned slightly on the exhaust pressure nipple (due to heat), you will loose back pressure and your top end power will be lost.

I had this on my OS 55 AX and stupidly removed the spinner, prop and cowl to get to the engine, couldn't find anything so I thought I had a tank pick up problem so removed the servo tray and slid the tank out only to find that was ok too...

The end of the pressure pipe is external on the model and could have been re-ended in 2 seconds..!! I only noticed when I was reconnecting it back up after the tank and everything else was back in...I need to re-end this pipe about every 50 flights or so.