flyingdaemon

My Feedback: (1)

I recently acquired an almost new YS120SF, but it seems to be overheating and loosing power after a few minutes, so I took it apart to check everything. I was running Wildcat YS20/20 fuel. So far I haven't found anything but a nice clean engine although, like another post in this forum, I do think the compression is a bit low but that may be because I am comparing it to my two strokes. My prime guess is that it was running too lean which might be a problem with the regulator. If I richened up the needle valve, it would just bog and die but still seemed to get too warm to touch. I didn't have the cowl on and the exhaust has a new Slimeline Pitts muffler. I haven't tried it again since tearing it down and putting it back together, but I haven't really found any indication of a problem. One thing I noticed, when I was puttng it back together is that the piston is NOT actually at TDC when the marking on the drive washer is aligned with the casing mark. Actual TDC of the piston doesn't occur until slightly past this point. Is this normal? Any other thoughts?

Dave Shadel

No problem with the timing, but make sure the punch mark on the cam gear is vertical when the piston is at TDC.
You didn't mention prop size, what is it?
The muffler you are using can cause the engine to run somewhat hotter as well.

Let me know,
Dave

flyingdaemon

My Feedback: (1)

Dave; Prop size was 16x6 APC. The timing gear "dot" is almost at the bottom but in a straight line with push rods when the timing mark is aligned. When the piston is actually at TDC, the dot is straight down. Someone loaned me a manual for an OS and that seems to match their instructions too but I didn't have an actual OS to compare the timing mark to the actual piston position.
I read you comments in a previous post about the piston ring gap but I'm not sure if you meant the gap between the ring and the piston sleeve or the gap in the ring itself. From the previous question/response, I am guessing the modeler was refering to the gap in the ring (and I think you meant the ring to sleeve gap) which is probably quite large compared to the clearance with the piston sleeve. In either case I don't really have a means to measure it and the engine seems too new to need a new piston ring.
Do you think I am on the right track with respect to being too lean? I think someone at the field has one of those remote thermal sensors. What should I expect for temperature. Also, what RPM should I aim for with the 16x6? I'm going to try to set it up again this weekend on the bench to see if anything has changed. Oh yeah, just in case you are wondering, there is lots of pressure in the tank and it doesn't seem to bleed off. Even after sitting for 10 minutes there's plenty of pressure in the tank.

Thanks for your help.

Dave Shadel

The cam gear punch should be straight UP (cam lobes pointing downward at an angle) with piston at TDC. It sounds as though the cam is 180 degrees out of time.
The 16x6 is likely too small, but shoot for about 8200 to 8500 rpm on any size prop and you'll be right.

flyingdaemon

My Feedback: (1)

Thanks Dave; Obviously I need to take a closer look at that. Whould you recommend a 16x8 prop then or something else. The motor was mounted on a Midwest Giles 202 , but I hope to put it into a Dave Patrick Ultimate - if that helps make a suggestion.

Dave Shadel

The 16x8 should be better, or maybe a 15x10. Try for the rpm suggested with whatever you choose.