Answer the best I can.

Decoke the cylinder head/exhaust valve?
Depends on the fuel you choose. There is one fuel here in the USA that will carbon up headers and the exhaust so bad that in 50-75 flights the engine won't run well. I don't recommend that fuel. I used to use it but they have inconsistent choices in the oil used and things are always changing. On my engines I only use the Cool Power 30% heli. I would say the exhaust valves get a light coating and about 500 flights is when I clean them off. Its not bad at all. In fact It probably could stay there as the coating doesn't build up.


Change the piston ring?
Power change is when I do the piston ring. Depending how its run this is usually 250-300 flights. Not a new piston or sleeve just the ring. $20


Change the bearings?
Depends on where you fly. If its grass field then you will be picking up dirt and debris all the time. Use and Air filter and it will help but not eliminate the bearing issues. If you fly on grass or dirty fields then change the rear bearing every time you do the piston ring 250-300 flights. Do the front bearing about every other time. I fly on pavement almost all the time, so I can go 500+ flights on the bearings generally. If they show signs of roughness then they need replacement.

Change the rear disc valve?
This one is a funny one as I rarely have changed the rear disc valve. But YOu need to look it over and if it has scratches or the gap is too big this will certainly hurt performance. If it gets too bad the engine will not run properly. Again grass or dirty fields will affect this one too. I have one engine that has over 1000 flights on it and the original rear disc valve

Change the pump diaphragm and/or pump piston or cylinder? Only when its bad....Its been rare I think I have replaced 2 pumps in all the time I have Dingo engines. (4) 140's, (3) 160's and now (2) 170's I have been running them exclusively since 2001 and just not had problems. I think one of those pumps was bad from the beginning and the engine didn't run right out of the box. A new pump solved it.

Replace the con rod?
Only when it gets worn. Check for play on the piston wrist pin, and also for play on the crank pin. If its got play at either end it needs to be replaced right away. Again this will depend on how nice you have been to it. If it has been run hard and detonating the con rod will be worse shape sooner. Play with it nice and it will lost a long time.

Do you keep replacing parts or have you ever reached the point where you consider it more cost effective to scrap and buy new? Usually unless the engine has a failure and a parts breaks its much cheaper to replace parts. Yes if you go the entire rebuild it will be expensive...piston, ring, sleeve, bearings, cam gear, rear disc valve, orings and gaskets it will cost you up around $300 US. But thats only half the price of the engine and when you do that its a brand new engine basically...so you just got a new one for 1/2 price. Its only when a failure happens and you need a new head or new case in addition to the parts above that I look at just getting a new one.
I usually just end up trading up to the latest engine and pass along the older ones to guys that are need of a motor that is good but don't want to buy a new one. I still have a 140DZ that I have owned since 2002 and a 160 that is still in service since 2003. They both are good engine and can run tomorrow. The 140 is in a box but the 160 is on a model.

Signs of when to do repairs....Usually its the power drop thing. The engine will start to fall off in the power department at about 250-300 flights. When I tear it down to do a new ring I look at all the parts. Look for wear on the cam, the con rod, and all those things that can break. If it needs the part I stick it in.

I can't say this enough....fuel choice and how you run them makes all the difference in the world. Run them in the right RPM range they make huge power and the right fuel means the internals are well lubricated and don't get carboned up.

Troy Newman
Team YS Performance