No start when hot

My 82 KZ750 LTD recently (late last season and this season) began having a problem where it occasionally won't start when hot under its own battery. It cranks fast and long and the battery doesn't show any signs of weakness but won't start. If I jump it with a car battery it fires right up and then it works fine on its own for a while - could be a month, could be the next stop. It did it Sunday and while I was waiting for my wife to come give me a jump I did some troubleshooting. I have gas in the carbs. I did notice that I had no spark on the outer two spark plugs (1 and 4)showed no spark with I placed a screwdriver in the cap and near the engine block. When I connected the car battery to the bike - zoom - it fired up and ran fine. And I could not duplicate the problem. I am thinking either a weak coil (though it appears to be on both sides) or possibly high resistance 25 year old wires. Does this seem reasonable? Any other suggestions? Other than this one quirk the bike runs perfect. Starts good, idles good, accelerates well. Throttle fallback is a little show after high speed, but not terrible. Sorry for the long post.

My 550 had a simalar problem , I found a plug under the tank that was not pushed all the way in.Might be worth a shot.

GP550KZ,

I wasn't thinking about that, but I did remove and reseat each plug while troubleshooting. Didn't make a difference. I did forget to say that number 1 and 4 are running verry slightly lean I think. Just a hint of grey.

There's folks on here that have forgotten more than I'll ever know about bikes. My thought would be if it fires right up with a jump but the battery seems strong it sounds like a bad connection somewhere maybe to your coils. I only know my 900 but on it one coil fires cylinders 1 and 4 and the other coil fires 2 and 3. You might first check you've got good connections on your coils, or check into the coil mod to directly wire the coils into the battery with a relay. Look in the Electrical section for the wiring diagram. Good luck with that.

Sounds to me you have a bad electrical connection going to a coil.With the loose connection it will be ok until it gets hot (bad connection=heat=high resistance=no connection)The factory bullet connectors are goodbut tend to open up a little and you will lose your connection.

Good luck

Thought I would update this. I found a solution to my problem (or at least a fix). I checked the gap on all the plugs and found they were all around .034". I re-gapped to .027 and so far it hasn't failed. I am guessing that I only postponed the real repair, but maybe this will get me through the summer!

I had GS 550 a few years ago with the same problem, i checked and re-made all my earth connections on the ignition circuit and sorted the problem without buying anything... try that...

I only want to throw in here that I hope the donor vehicle for jumping wasnt running at the time. Ive burned 2 regulators doing this, before I knew it was bad to jump a bike from a running vehicle.

Start with the basic. Test the battery and make sure its good. Mabey buy a new one.

This is kinda off topic, but seems like a good spot. I have read several times here that jumping from a running car is bad and I don't understand it. A car regulator puts out roughly the same voltage that a bike does - mid 14s at high idle. And the bike is only going to pull the current it needs to start - just like from its own electrical system. The advantage from a car battery is it has a great deal more reserve so it won't droop the voltage under the load of the bike starting. Is it due to a spike that might generated when connected? I did jump it from an idling car, so I would really like to understand this.

This is kinda off topic, but seems like a good spot. I have read several times here that jumping from a running car is bad and I don't understand it. A car regulator puts out roughly the same voltage that a bike does - mid 14s at high idle. And the bike is only going to pull the current it needs to start - just like from its own electrical system. The advantage from a car battery is it has a great deal more reserve so it won't droop the voltage under the load of the bike starting. Is it due to a spike that might generated when connected? I did jump it from an idling car, so I would really like to understand this.

This is kinda off topic, but seems like a good spot. I have read several times here that jumping from a running car is bad and I don't understand it. A car regulator puts out roughly the same voltage that a bike does - mid 14s at high idle. And the bike is only going to pull the current it needs to start - just like from its own electrical system. The advantage from a car battery is it has a great deal more reserve so it won't droop the voltage under the load of the bike starting. Is it due to a spike that might generated when connected? I did jump it from an idling car, so I would really like to understand this.