Well, I have a YS50st that has about 3/4 gallon through it. I am still a bit rich for break in, but I noticed today that I can now take the starting wand and just turn it over. I pulled it out of my T-Rex 600 and I can turn it over from the crank with the plug in, I mean nearly no compression. I am going to send it in tomorrow, but I have been flying airplanes for 20+ years and have never seen an engine loose compression like this if it had not been in a wreck (dirt ingestion) or had been run lean. After each flight I did the backplate touch test and I could hold my finger to the backplate as long as I liked, it was warm but not hot, I think that I was about 1 5/8 on the main running CY 30%. It had very good compression all yesterday when I flew it and the first flight today, but the last flight I noticed that the starter just flung it over and then turned it over by hand with the wand, it would NOT have done this yesterday or on the first flight. Any idea's? I looked at the piston/cylinder and there is no scoring. I am going to send it in tomorrow anyway, I understand that a ringed engine may not have a lot of compression until broken in, but I see no reason to suddenly loose compression during break in when run rich and not leaned out.

Sometimes if the engine is especially dry it will feel like little to no compression this is common.

Is the engine running properly with the same power it always had?


Another option is the glow plug. This is common on the 4 strokes but if the engine gets a little lean or way way rich...the seal in the glow plug stem can become a source of a leak. Also if you have a remote glow as many heli's use...the stem can have a mechanical load on it and it can actually break the glow plug stem loose from the plug and the seal will leak. If the engine is way too rich and the excess fuel that is not being atomized in the carb and not being burn properly in the combustion can put a load on the glow plug and also the glow plug stem. This is not very common on 2 strokes but can be on 4 strokes....The excess fuel pools in the head and then when the engine comes up against compression the chamber is not filled with a "mist" of atomized fuel its got raw liquid fuel in it. This can cause the compression to go way higher and blow that seal out of the glow plug stem.

In some cases the engine may run normally for a little while until the leak gets worse. eventually the leak will get worse and worse until the engine won't fire at all as there is zero compression.

Are you sure you are not getting some oil or residue up on top of the head from the glow plug.

Is the glow plug still tight in the head of the engine?

I would say the engine DOESN"T need to be sent in but that is your choice and you decision. I seriously doubt there is anything wrong with it. However I'm not looking at it and can't tell you.

I have very limited experience with the heli engines. However on the YS 4 strokes. just because they are low on compression doesn't mean that something is wrong. If the engine is not performing at the same level as before then its an issue to look into but the fact that you walked up to the model today and flipped the engine over and it didn't have compression doesn't mean its a problem. Put some fuel in it and start it up. Get it warmed up and then shut it down and see if has compression then.

Troy Newman
Team YS

It was bogging some today during manuvers that it was not boging on yesterday, I just chalked it up to poor collective managment, but after seeing how easily the engine turned over today there may be something up. I am talking about gragbbing the bare crankshaft and turning it over very easily with the plug in and the cylinder is pretty "wet". I was doing consecutive stationary rolls yesterday and it just went on and on. Today it bogged to the point where I had to stop in order to let the headspeed come back up, if you throw in a hard cyclic in conjunction with a hard collective it should bog, but just one or the other it should not. I am going to run downstairs just to check the plug like you said, but most heli engines are HARD to turn over. People are always looking for hi volt starters on the heli forums as even the 50's are very difficult to turn over. I have an OS55ax that is an ABN engine and it is cake to turn over compared to my 50 Hyper or the way this YS was yesterday.

I understand your point though, I have a Saito that seems to have no compression but it runs like a raped ape. The YS was deffinatly a bit weaker on my last flight, I just hope they have parts for it as it is so new. I am going to check on that plug now however.....


I do have a question for you though, do you recommend pinching the line and running it dry, or just kill it with the throttle? I know some brands want it done one way and other brands the other.

Joe
As I already said somewhere else.
You either have a cracked ring or here are some others:
Blown or loose Head. Does the head have a copper gasket? Check for loose bolts + check the gasket
Rear crankshaft cover loose..check bolts
Take off the muffler and check for bad or deep scratches on the exhaust port and spatially on the ring. You can see them showing as vertical silver lines. Very fine ones, less then a hair width are normal. Also check the piston for colour. At this early stage, the piston should not be a dark brown. If it is, then you are running the engine too lean.

Yes I assumed the head bolts were tight but this would do the same.

A damaged ring is possible but not very likely I have seen it once on a engine from running. usually if a damaged ring is there its from the installing it and breaking the ring.

I did this once. Broke the ring putting it in.

Troy Newman
Team YS