Dave,

My YS 120 (very old) came with just a curved pipe exhaust, and I want to move to something quieter.... do all 120's have the same exhaust threads?

They do. All YS 91 thru 160 use a 13x1mm thread in the cylinder head. Try www.pspec.com for a muffler.

will do.. I haven't been too happy with Pitts mufflers on 4-strokes, so I think one of their bullets might work just fine.

One other quick question before I get my drill out and ruin something.. I've seen other posts where you're recommended drilling a 1/8 hole in the throttle inlet to assist with idle smoothness... does this hole go in the large side of the throttle butterfly? I just want to confirm first.

Don't open up the airbleed hole unless it idles too rich. Then only drill in .005" increments until you get it right. 1/8" is way too big a step from the stock hole size in the barrel.

Thanks.. glad I asked.. it does idle a little rough on the test stand, but I'm not sure that its not a simple tuning issue. However, there's not a lot to adjust for the low end on this plane.

Thanks again for the advise.

You can try leaning the regulator 1/8 to 1/4 turn.