Dempsey's Moped Repair Handbook says that a moped engine needs around 90psi and that 130psi is overcompression likely caused by carbon deposits. So 120psi seems high compared to that information.
I'm looking at manuals and seeing compression ratios of 7:1, 9.2:1 (Puch), 8.4:1 (Peugeot), 8.5:1 (Tomos A3), and 8:1 (Minarelli). What I'm reading about converting from those ratios to psi is a little complicated/over my head/depends on a lot of factors I'm not going to include, but most sources are saying multiply those ratios by 14.7 to get a ballpark range. 7*14.7 = 102, and 9.2*14.7 = 135, so I'd say that the manuals appear to be recommending anywhere from 100-135 psi stock, or maybe a little lower than that since the 14.7 is (I think) for an ideal and leak-free engine. Regardless, my mopeds seem to back this up -- I've got a stock Puch with 140psi, and a stock Tomos with 120psi. I think the number in the Dempsey repair handbook is low if it's saying you only need 90, and it's wrong (or at least, not entirely/always correct) if it's saying that you've got a problem if your compression is over 130. Just my opinion, though. -- Mycider 22:49, 9 October 2010 (EDT)