I have been driving long enough that I am starting to play around with the tuning on my kart (stock Honda CR125, Keihin PWK, pump around). I have been running a 180 jet for the past 3 years! The kart has always run well and has been very durable. We do about 8 race weekends a year which for us works out to about 10-12 days (some sprint, some road race). This weekend I dropped down to a 175 jet which according to some should have still been on the rich side. We run 110 Sunoco race fuel mixed at 20:1. On the long front stretch, at WOT, I would start to hear and occasional “pop” from the exhaust. I would also hear the “pop” anytime I was at full throttle for more than a few seconds (always as the revs started to get to around 11K or so. This caused me to short shift but there was not much I could do on the straights. I can’t really read the plug (it always looks the same to me!). I pulled the head to check the piston. The piston surface (new before this race) is a light tan and feels like sand paper. The is one tiny ding on the surface of the head and some very tiny marks around the flat portion of the head. Otherwise the head is very shiny and clean. Does this all sound like the 175 was too lean?
20 to 1 is a rich mixture (even for road racing) and may even be contributing to the speckling your seeing on top of the piston. Think that comes out to 6oz per gallon, go to 4.5 or 5 oz. Running a rich fuel/oil mixture = leaner in the combustion chamber. The popping your hearing is common with a lean condition.
1999 kit motor or 2001? What pilot and needle? I’m guessing this was at Buttonwillow?
This was actually at Summit Point, WV. I don’t know what pilot I was running as I have never swapped the pilot. I was under the impression that the pilot only affected the mixture at part throttle (I’m almost never part throttle at SPR). The popping only occurred at full throttle on the main straight. Needle was DGJ. I think 20:1 is the spec ratio for our club. The motor is a ’99 but I built it from an old dirt bike motor (completely rebuilt including the 6 speed). I did notice that I am missing the rubber boot that goes on top of the carburetor around the cable. Not sure how much that would affect it.