BernardW

Oh right, Mylar. Someone mentioned that and I'd forgotten about it, but if that diaphragm is indeed Mylar then I've got nothing to worry about. Four Stroker, yes you're right, no need to go nuts on it. I heard that the dia. itself was cheap and I believe that's one part that needs replacing after so many hours running, and a ring and paper gasket set sounds like good advice. Whatever turns out to be worth getting, it'd be a good idea to do it sooner rather than later I'd say.
BTW Hi again Ken! Only just got put onto the YS area here from some *other* forum (BernardW), and there's the same old faces, eh? Thanks for the advice as always. PS Actually *finished* that first build of mine just a few hours ago, ready for a taxi test tomorrow! Hoping for a maiden (flight that is) next week when I can book a day with the instructor I've been seeing. Can't wait! It's been a long project and I took my time trying to do my best, learning as I went, and along the way setting up my work area, getting tools, making stands, jigs etc. and learning a lot of stuff. Just over 6 months, I would never have believed it'd take this long. And I've done it on such measly amounts of money week to week it's a wonder I got the thing at all, but I'm finally there. I'll post a video soon as there's something to show.

Four Stroker

Well the front shaft seal in front of the bearing and the pressure seal on the carb barrel are silicone. The O-rings on the rod tubes may be silicone. It ain't just the diaphragm. Also the actual valve the the diaphragm operates is silicone.

Silicone safe oil will NOT hurt the engine if there is no silicone there.

BernardW

But it sounds like other than the diaphragm there's plenty of good reasons to use a silicon-safe oil?

Four Stroker

Why not ?

Here is what I was talking about. Notice that 06 and later has a silicone diaphragm on the regulator:

https://www.centralhobbies.com/ysparts.php?id=12

Note also that the part #74 is silicone. The plastic diaphragm #73 is probably what you have.

All O-rings may be silicone. Many are. Some maybe not.

MrShoes

My Feedback: (15)

I did a test on a diaphragm and regulator plunger I replaced on a 91AC. I placed both in a jar of motor oil for 2 weeks, no observable change. cleaned them, then placed them in a jar of gasoline for a week, only the silicone plunger appeared to swell only slightly ( I did not measure with micrometer, only visually). Neither felt like they loss any material strength. I was expecting them to melt in gasoline, but nothing happened. Having said that, YS in their Japanese manuals recommended using Klotz Original Techniplate as ARO. Klotz, on the other hand doesn't recommend using Original Techniplate as a storage oil.

Four Stroker

I did a similar thing. Mainly stained the diaphragm. Did swell some O-rings and seals. I tried some methanol 2 stroke oil once upon a time. It evaporated and the engine seized ! Have not tried Klotz.

But the important thing is that this guy figures out what diaphragm he has before ordering. Looks like 06 and later is the silicone circle.

BernardW

After '06, ah, I thought these engines had been out of production since before then. I'm only guessing that mine is 10+ years old, the guy in the store swears it's one of the last batches. I've not been able to find out when FZ91's went out of production, does anyone know within a year or so? Thanks for the link and info.
EDIT - yes it would be good to be sure of the type I should get. Is there any way I can do that from serial number or documents etc., or would I really need to dismantle it and physically have a look?

Four Stroker

Look at the edge of the regulator and see if you can see paper gasket A. The white diaphragm is very thin maybe 0.010".

Pattern freak

My Feedback: (1)