I also have issues with my old keep off 4 , it's perfect from maintenance point of view, no air leakages and new biella is installed also front bearing. I'm getting a good tune in 10 minutes on about 105 Celcius and then after 15 minutes of run on the track i get too lean low end with temp rises 115-120 Celcius. Car is not drivable with lean low end. When i go to rich it out temp is dropping below 100 Celcius and flame out because of too rich low end. This phenomenon is end of life of piston and sleeve, this is how more experienced racers explain to me this issue. Compression is vary after certain time running or if temperature rises a certain point. I think is reasonable after 60-80 liters of consumption and something to expect.