Ahh...ok, thanks 2slow...I think I see where our miscommunication was.

The manual says nothing at all about breaking in the engine in '4 cycle mode'. It refers to the fact that, initially, on first start, it will be running 'in 4 cycle mode' (which it describes as only firing every other upstroke of the piston...not exactly a '4 cycle', but hey, ok...I'll go with it). The manual explains this will occur because the engine is running VERY rich.

This is during the INITIAL process of setting the needle valve, and only occurs for a few moments, while you're closing the needle valve from its initial 1.5 turns open position...in other words, it only runs this way for a few seconds, on the first start.

When DR1 said not to break it in rich, I was confused, since the manual and nearly everything I've ever read on RCU specifically says that is EXACTLY what you do...run it richer than the max rpm setting of the needle valve. So i was wondering if he was proposing that the engine should be broken in by running it leaned out to max rpm the whole time *heh*. Didn't make a lot of sense at the time, but hey...i'm the newbie, maybe he had some tip I hadn't read before.

And yes, 2slow, I can see exactly what you're saying...the point of the breakin is, specifically, to allow components to expand and seat correctly...running it far to cool would, of course, prevent that from occuring correctly.

As it turns out, the OS muffler will bolt straight up to my Magnum XL 40, so I may just do that so i can fly the stick again today or tomorrow, and break the new motor in this weekend when I have more than an hour or so to kill.

Thanks again for the input, guys.