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91FZ quits

Old 06-05-2003, 04:26 AM
  #1  
cmassa
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Default 91FZ quits

Dave,

I've been having a peculiar problem with my YS91FZ which is in a UCanDo3D. It randomly dies. It doesn't matter if at idle, mid, or full throttle. It will fly a couple of minutes and then just quit without warning. No particular pattern noticed. It will always start up again right away with a single back-flip against compression. I have changed the plug, readjusted the needles/regulator several times, checked the fuel lines. I checked the regulator chamber for debris as well. It runs perfect otherwise - no hesitation, slight smoke trail, great idle. I checked the valve clearance which was slightly enlarged but not much. I adjusted them but haven't flown it again yet. I also considered it a heat problem and flew it without the cowl but the problem continued. The fuel filters are clean. The system holds pressure fine. I can't get it to happen on the ground.

This engine was in for service with you a few months ago (cracked air chamber) and worked great when it came back. Any ideas?

Thanks,
Chris
Old 06-05-2003, 10:47 AM
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Steve Campbell2
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Default 91FZ quits

This is stretching, but check the six bolts holding the back plate/air box assembly on. Mine keep working loose, which eventually causes a minute air leak, which plays hob with the mixture. It continues to run, but once it gets to a certain point in the "loosening" process, the engine begins to "hang" at low throttle and doesn't want to respond to throttle increases. In any other model but a UCD, this would have caused me to crash almost every time it happens. The UCD's amazing low-speed envelope allows it to drag it self around on the wing until the engine decides to spool back up. I too have played with the needles, thinking I had it too rich. Nope.

I have no idea why these bolts keep working loose. There is no excess vibration, the engine mount is solid, and the prop and spinner are balanced, and my thottle linkage is working properly. We have noticed (on the three UCD/91FZ combos at our field) that the 16x4W APC prop does load the engine down somewhat and affects throttle response. I'm heading down to New Orleans today to pick up a couple of 15x6 APCs. One of the guys has been using that one for a while, and his model has better throttle response without giving up much in terms of torque.

I've been flying this engine for a year, and it has been exemplary up until now. I'm not even sure I've got the diagnosis correct; there is much that can go wrong with these complicated beasts. I've been running four strokes for many, many years with success; but these YS engines have caused me some head-scratching.

When they're running right, nothing can touch them. That's why I still have my two. But there is a lot more to them than your basic OS or Saito.

Keep working the problem, Chris. You might need to adjust the regulator. But that's the last thing I would try. Give Dave a call; he's pretty good at trouble-shooting over the phone.

Steve
Old 06-05-2003, 12:44 PM
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cmassa
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Default 91FZ quits

Steve, I've checked all the bolts/screws on the enging and everything is snug. Mine doesn't "hang" at a throttle position, it just dies altogether. One second its flying along fine, the next the engine has stopped.

I agree with you about the 16x4w. I'm flying the APC 15x6 now and think its the best for this plane. It spins up fast and has plenty of thrust. The 16x4w and the 16x6 take to long to change rpm's.

Chris
Old 06-05-2003, 04:05 PM
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Dave Shadel
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Default 91FZ quits

Hi Chris,
Give me a call at 775-265-7523. Mornings are best after 7:30.

Thanks,
Dave

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