At last count, I now have eight of these, bought new in the box at bargain basement prices of about $50.00 each. I love the way they run, but when I first started using them was extremely frustrated with the run quality. It was all over the place. I tried extra head gaskets, different venturies, hemi heads, uniflow, atmospheric vented tanks, muffler pressure, tongue mufflers, standard mufflers, custom lightweight mufflers, high pitch props, low pitch props and stuff in between. Some of these worked...kind of...for a while and then it was back to running inconsistently. Finally, ready to give up, I read the manual...again, only this time I followed the directions....and used Cool Power all synthetic with low oil content. The difference was like night and day. The runs immediately became consistent and I could now start dialing it in. Internally, I run the engines dead stock. No extra head gaskets, no dremel work. The only modifications I make are 1) install conventional Venturi/spray bar assembly through case. Venturi bore is .272 with .156" diameter spraybar (Jim Lee sells a true Venturi for this with the same choke area and it works great if you aren't comfortable drilling a hole through the case for a spraybar) and 2) machine the outside of the head round and sand off the fins on the back of the case to match. It saves a little weight and in my opinion looks better (not having so many of those goofy fins). At our field in the foothills west of Denver (6000 ft. elevation) I usually run Xoar 11 X 4 and APC 11 1/2 X 4. When I go to Tucson, I switch to an APC 12 1/4 (cut down to 11 1/2) X 3 3/4. I run Cool Power 10%. Launch between 10,000 - 10,200 rpm. My engines have well over 300 flights and are easy starters with great compression. Frankly, with all the bad mouthing regarding these engines I've been fortunate to get some really good deals on them. It's to bad production was discontinued on them. In my opinion, Horizon Hobby screwed up by marketing them as an introductory type engine with the rear needle valve assembly and crummy venturis. I think they would have been more widely accepted with a good Venturi/needle valve assembly.