ORIGINAL: tele1974
Ran the 18x10 some more today and 4-5 minutes into the my second flight I lost the top end. Landed and the motor had tightened up pretty bad and it didn't sound very good turning it over. Went back to the 18x6 after a long cooling period and it was certainly much happier with that prop. I have the 15.5 on order. What the heck happened... I am thinking I got it way too hot. It wasn't air flow.
I would guess that you are running the engine too lean. It needs the high speed needle opened to 2 -2/14 turns if you are running a piped set-up as I have described here-in. Not sure what plane you are using....it must have a lot of cooling both inlet and even more outlet. If you are flying the EF Extra, baffling could help
Here's where I was when my engines were new[link=http://www.rcuniverse.com/forum/fb.asp?m=9536834]MattK's early needles[/link]
The needles now after pretty close to full break in are HS 2 1/8 open and LS 1 1/8. Pipe is at 28" to baflle straightline from the spark plug center. Running 90 octane at 50:1 BelRay MC-1 synthetic motorcycle oil. The prop is a 17x12 std blade but a variety of props will run at these settings. I tried the prop Bob Moulder lent me (15.5x 12W) and also Dave's 15.75 x 11, 3 blade. These all work okay on the set-up. The 18.1x10 apc is a good choice for break in and if you don't mind a little noise, will work very well on pattern planes.
The 18x10 should be less load than all of these props.