Members who have donated towards server costs
Join Date: Mar 2006
Location: Kyle, Texas
Your most likely problem is a weak battery. If it is a wet cell with the drain tube and covers you add water to, you will need to replace it with an AGM Maintenance Free battery. On the older bikes, it is not unusual for the wiring to be weak going to your coils so I'd at least check your voltage using the procedure below in my sig for the coil mod.
These bikes have some known weaknesses right from the factory with the wet cell battery and such. If you still have your goat's belly (pre muffler) and the regulator/rectifier is still mounted over the GB and under the battery box, the heat it needs to disperse gets combined with the heat from the GB and it stays really hot. Most of us have moved our R/R out from under there to a simple mount we make up from hardware store pieces, to get it out where it can get air flow and run cooler. (that mod is also in my sig) Then, most of us will purchase NGK Iridium plugs for our bike to replace the old ones. The iridium plugs seem to have a more consistent hot spark than the stock plugs and last many times longer.
The backfiring on trying to start the bike when it is hot is the main symptom of weak spark, which causes a rich fuel condition and this rich vapor tries to ignite with the weak spark and doesn't fully ignite, but when the exhaust valve opens to force out the exhaust, this vapor hits some fresh air and BAM! You get that "wake the dead" backfire. I did the R/R relocation, and the coil mod at the same time (coil mod was mainly to see how it worked on the 750, as I had done it previously on a couple of KZ's but no one had done it on the 750 so I decided to try it and it works great) so I don't know which one had more effect on the hot start issue I was having, but it was cured and I never had the problem again. I already had an MF battery at the time. In some cases, the MF battery cures the issue by its self, but others have had to do more, including rebuilding the starter motor.
So, start with the battery, then check your coil power wire voltage, get that r/r moved when you can, and plan on some iridium plugs asap. If you still have issues, you may have to mess with the starter, but I feel pretty confident you'll eliminate the problem before you get to the starter.