another 'priceless' YS moment

[Koenig, Tom]

Hi all,

I just wanted to send out a 'heads up'  about my most recent DZ experience.

I'm not one to whinge too often, but my recent experience may just save
someones model out there..or even themselves. I have been a most dedicated
fan of the YS machinery fo many years, but even I am getting pushed to my
limits.

I just fired up my brand spanker DZ 140. All went well, the usual little bit
rich etc etc. After some gentle tweaking here and there I ran it up to full
noise ( still rich and definitely not peaking out). All up, I ran it on the
ground for a grand total of about  one and a half minutes. I then ran it up
to full power once more just briefly, to check all was ok. As I throttled it
back it went to about 1/3 noise when it promptly disassembled the mount and
whole nose area of my plane. I quickly picked up the model and shoved it
into the ground ( ala whipper snipper style) to stop the engine as it was
about to explode. It was the meanest , fiercest detonation I have ever
experienced. Infect it was the only detonation I have experienced since the
dark ages of the OS 120SP!!! I thought this thing was going to take out
everything and everyone in the surrounding area.

Now before you all jump to conclusions...I did everything right. After I had
calmed down and was somewhat coherent again..(three days later!! ) I decided
to check out the engine. This time it would not start but for a short burst
and die out smoothly...just like a nice DZ is supposed to.

Anyway this story is long enough.....but ....the culprit was the diaphragm.
I pulled this little sucker out and it was never fitted properly. You could
actually see where the imprint had never made the seal around the housing
edge/screw hole. The result was that as the diaphragm was doing its minutes
worth of work, it pulled itself in from around the screw hole and the engine
got the biggest gulp of air with the fuel and hence promptly blew the nose
apart on my plane, spinner, prop etc. The diaphragm looked like it had 5000
flights on it, it was completely butchered into position. 
I think the assembly worker must have been in a hurry to get home that day
to have his plate of Sushi.
I have since checked out another brand new (never run DZ) and the diaphragm
there too, was NOT fitted properly.

Guys...BE CAREFUL!!!!

In my opinion all Yamada needed to do was increase the 'land'area
surrounding the pump diaphragm by about half a millimetre to ensure proper
grip and no chance of a misalignment causing leakage.

I wonder wether this is the 'point of origin'for some of the more serious
faults some engines develop?

I have since replaced this diaphragm myself ( very, very carefully,being
careful not to twist it as it goes into position) and sofar it appears to be
running like a swiss watch........
Then again...after replacing all the bits it destroyed, and repairing my now
very average looking nose area of my plane...it might have been cheaper to
just buy a Swiss watch!!
I cant wait to see the bill from the therapist (which I now need due to all
the snide comments from the OS two stroke brigade!!!) But seriously guys..it
is an area of concern.

I apologise for giving the engine 'bad press' but it truly frightened me!.

Tom
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