Hey y’all. I’m new here but have done a lot of research on the web and this site and there is a bunch of good information but I’m at a loss. 2003 scag turf tiger, serial is 848xxxx, Kawasaki 23hp fd671d liquid cooled. Started at the end of last season, would lose power while mowing and pop and crack, and I just put it away and thought I’d deal with it this year, I assumed simple fix..not..so it runs fine but surges at full throttle , has always surged but always straightened up when the blades engage. Now as soon as I engage the blades it’s starts missing, and backfiring out of the carb, and kind of runs a little better if I throttle down while blades are on, but still not 100%, I pulled one plug wire off at a time and basically got zero change, also the unit has 2100 hours, to start with I pulled the coils and did a resistance check had one at 16k ohms and one at 20k ohms, so I ordered coils but just replaced the highest resistance one, (also the shipping was delayed on one so I just installed one and planned to do the other later), no change, removed and cleaned carburetor thoroughly, brushed all the jets out after soaking in carb cleaner, installed new fuel shut off solenoid on carb, I got the surge to go away at full throttle but still no power with blades on, still backfires. I changed fuel filter and blew out gas lines, adjusted the valves, and still nothing.i hooked up a inline spark checker and once I full throttle and engage the blades it seems like its spark cutting, I can see the light go from almost solid blinking to skipping once I engage the blades.I plan on rechecking the valve adjustment again this weekend, but I’m at a loss. I’m a mechanic by trade but don’t fool with small engines that much. I have owned this mower for 10 years and mowed commercially with it for 8 years. It’s been great, no major breakdowns, so I hate to give up, but all my other mechanic friends are also at a loss as well. Can anyone help? I know how hard it is to diagnose something without standing next to it but I’m struggling here. If anyone needs more info just let me know. Thanks in advance.