I rebuilt this thing a year and a half ago and just got it all back together in Oct. I ran about 12 gallons of 25:1 through it going by the manual for breakin. I don't think it has 20 hours on it. It ran great up to the last time out a couple weeks ago and seemed to sputter at idle quite a bit. I took the plugs out to check them and they were clean. The electrode was black, about like gloss black but the rest of the plug was clean like new. This was both plugs. The first time I checked them after getting it running they were nice and tan. Would running at idle VERY rich for an hour or two have cleaned them off like that( I had the lowspeed screw out 4 1/2 turns for some reason) or do you think I blew a head gasket? I hadn't re torqued the head yet. Thanks.