Bike wont stay running

Double check that you have not accidently hit the kill switch.

Yes occasionally a battery can put out enough cold crank amps to spin the starter and motor, but in doing so drags the voltage down so the electronic ignition wont fire.


a couple of warnings
If you check for spark make certain there is a plug in the lead and it is grounded to the engine, if not the voltage builds up in the coils until it can spark to something.
dont jump it of a bigger car battery, when the bike starts it can try to produce more amps than its supposed to to try and charge the car battery and stuff the reg.

sounds like the battery to me, have it checked by a sparky, they dont give you much warning before they **** themselves, I had to replace mine only a few months ago

you will need to try a new battery, unfortunatly sometimes jumping of a larger battery can spike the regulator, so when the new battery is fitted put a mulimeter across the battery, should read just over 12v engine stopped and rise to 13.5-14v with the engine running.

I dont know about using and tester for autos is going to give a correct reading for your problem i suggest you get a volt ohm meter and check the stator and regulator/ rectifier.

battery is loss

You need to completely drain and refill the gas - the gas you have is stale and doesn't have enough power to run the bike.
The battery needs to be fully charged - not just jump started. It wouldn't hurt to do an oil and filter change as well as a air filter and plug change.

You have to check the starter motor,prolly the brushes are gone . Also check the connections from battery to the solenoid and from the solenoid to the starter.

Good Luck.
Knight (Yamaha seca XJ750A 1981)

the floats in the carb are stuck. you can try lightly tapping on the carb with a screwdriver that should do the trick.