My rock solid dependable YS140L stopped in flight two weeks ago. On disassembly, couldn't help but notice the failed connecting rod, damage to the aluminum piston near the wrist pin, the piston ring (came out in pieces) and the piston sleeve was scored. Replaced all these with new parts after thoroughly cleaning crankcase internals. Re-assemble with leak proof gaskets. Engine started immediately and operation back to normal, or so I thought.....

Engine starts right up when cold and produces full power (16 X 10 APC @ 9,200 rpm @ 85 deg. F, 100 ft. above sea level) at full throttle. The problem is, when engine warms, it loses power. Then, it's hard to start up. When it starts, it surges, then clears but only achieves partial power at full throttle, yet idles normal @ 2,200 rpm. When hot, it won't start at all. I'm running Cool Power 25% with added syn oil. YS 4 Glow plug okay. Check valve okay. Inline fuel filter free and clear. Compression very good. Best its been since I've owned it. No internal header or muffler obstructions. Disassembled regulator. Diaphragm, plunger, plastic end cap, spring, o-ring all okay. Disassembled head to check the valves, springs, seats, etc. Cleaned the valves and lapped in with jewelers rouge. Haven't run the thing since confirming regulator internals and the valves were okay. Waiting for the weekend. Gotta work and it gets dark early now.

Guys, have I overlooked something? Could it be a setting problem? Any recommondations besides sending it off to Gardnerville?

Steve

Are you 100% sure that you got the timing markings in the correct position? Just trying to guess.

Vicente "Vince" Bortone

I had a couple of L's do this: "The problem is, when engine warms, it loses power. Then, it's hard to start up. When it starts, it surges, then clears but only achieves partial power at full throttle, ... Does it also smoke a lot? Mine did. This happened when the engine was getting worn in both cases. One case required a new piston as well as some parts up there -- the piston was badly scored on the skirts from rocking. In both cases the engines were sent in for service so I don't have a good handle on exactly what to do. I have heard from reliable places that as the parts heat up "stuff" starts blowing by the ring/piston and causing the symptoms you describe and what I saw. I finished a contest with an engine like that by continuing to use Wildcat YS 20/20 fuel and switch from the 16X10 to a 15X12W. It took some of the load off the engine and I was able to fly it. I flew the rebuilt engine part of the next season with that 15X12W and it seemed quite happy -- granted I had to relearn the throttle positions but I think it was a better combination.

In the 2 or 3 weeks leading up to the contest problems, I think it was telling me it was in trouble but I didn't pay attention. At initial run up I go up slowly on the throttle until it is wide open -- stay there for maybe 5 seconds. I started noticing a slight drop in RPM after 4 sec. or so -- then it started happening a little sooner and with a slightly bigger drop. I'm thinking maybe it's a litle rich so I leave it -- rich is good, safe, etc... I think it was the fit loosening as it warms up (gets hot).

You might also post this question in the engines/manufacturers thread. Dave Shadel is pretty good at answering.

I just got another L back from Gardenerville but will probably have to wait til next spring to try it out (Minnesota...). Same symptoms, long in the tooth...the engine, not me.

Good Luck, Tom

Yes, 101% sure.
Steve

I have a 140L, that had a crook piston, and would do exactly as you say.
It seems the piston would distort ever so slightly when it got hot.

Maybe try another piston. My fix was exactly that, a new piston after the first 15 runs.

All was well after that.