x 300 dead at key

Hi to all and ty in advance for any and all that can help

well were do i start was doing service on x300 found out battery was dead and the carburator had some issues with build up so i cleaned carburator and charged the battery with 4 ohm load from a professional charge system so when i was finished with the unit started to pack up when i was ready to finished testing unit for driveability had to shut it off to move it out of basement when i went to go start the unit would not start had no power to the hour meter and was at loss checked the fuses and the key switch checked everything with power probe also checked all of the wires coming off the key and the switch itself but still no start i thought it might be the ecu i didnt want to go buy one until i was sure -with the cost an no return policybut once again every one i called said that it was not the issue i had called a few of the dealers in the area but no one could seem to figure out what could be the problem now im here awaiting answers or any advice.

If you put the leads of a volt meter across the battery, what is the voltage. Now turn the key to start on read the voltage. What is the voltage. You can have 12v at the battery but when you apply a load can drop to zero.

ok just an update from this morning i went threw the hole unit once again 2 hours and came to find out few issues going on first was that the ground wire kill switch for the rectifier was disconnected which i did not do! second the 15 amp fuse that i had tested 3 times that read good was reading good because the wires in the back of the fuse panel were loose and were cause a false reading third after replacing the fuse and making sure battery was charged it started and came to find out that that black wire had also caused the unit not to be able to shut off that wire goes to the ecu which i think was cause of this hole problem to begin with which some one had disconnected (cough jd dealer) but it was badly connected wires back side of the fuse panel ,fuse ,& bad ecu had to be replaced.