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