Piston Slap??


New Member
Nov 30, 2018
I bought my 66cc engine about 2 years ago, and just got it working and running a couple weeks ago. When I first tried to install the engine and get it to run, it ran for a few minutes and then stopped, and I couldn’t get it running again. I now think that it was just an air leak, because when I finally started working on it again, about 2 weeks ago, it started up just fine after tightening everything. I guess I haven’t followed the break in procedure to a T. I think I’m running the correct oil mixture, but recently I started it up, and started to hear a fairly loud "knocking" when at low rpms and under load. Doesn’t make the noise at higher rpms, or not under load. Anyone have any ideas?

thanks in advance for the help!

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
up north now
Break in procedure? haha.

Mix your fuel/oil at 40:1 and ride it as you normally would. That's the "break in" for these. Various throttle and loads, don't "lug the engine down" and don't scream it until you are sure you have everything tight and pointing the right way.

I doubt if its piston slap. It could be a wrist pin or bottom end going, but that's doubtful. You may be hearing something else making that noise. Ride it, keep an eye (and ear) on things and find out WHERE it's coming from.