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have a 81 XJ750 Seca. Starts up, when it drops to idle after it warms up, all lights fade and eventually bike dies. lights brighten with higher revs. can start bike again no prob but same thing happens. is it the batt or what? batt is 2 yrs old. this may be a dumb question , but i dont know much about this stuff, and am working with a college students budget so i cant waste much cash. thanks any advice would be greatly appreciated.

noobtacular.

Tim2005 wrote:have a 81 XJ750 Seca. Starts up, when it drops to idle after it warms up, all lights fade and eventually bike dies. lights brighten with higher revs. can start bike again no prob but same thing happens. is it the batt or what? batt is 2 yrs old. this may be a dumb question , but i dont know much about this stuff, and am working with a college students budget so i cant waste much cash. thanks any advice would be greatly appreciated.

noobtacular.

Try adjusting your throttle to keep the bike idling at slightly higher rpm's. Also are you using the choke until the bike is warmed up?

yes i am using the choke , after its warmed up i take it off. I allready adjusted the idle, its at 1.5 k, id rather not go any higher.

I think it's the battery. If you live in the states, you can take the battery to an AutoZone (or similar) and they test the battery and charging system for free.

I'm pretty sure it's normal. The fact that your lights brighten with throttle means your alternator is working. My nighthawk states in the manual the the RPM has to be 1700 or above to begin charging. This is why an idling bike runs it's headlights so dim. It;s using the battery. If you continue to operate it at idle for extended periods, you'll drain the batt. I think most bikes operate like this. They have very small alternators compared to that of a car and rely alot on load sharing so to speak between the alt and the batt.

Also, as far as the shutting off problem. Check for a vacuum leak. This is easily accomplished by using a propane cylinder (not lit!) and moving it around all of your hosses and gasket areas. If you notice an increase in idle in a particular area, then that area is pulling in propane. Thats where your leak would be. A little dangerous, but works for me. Just do it in a vented area and you should have no problem. But that shutting off prob really does sound like vacuum. Engine just can't cope with the extra vacuum requirement at idle. I could be something as simple as a hose disconnected.

jetmech727 wrote:I'm pretty sure it's normal. The fact that your lights brighten with throttle means your alternator is working. My nighthawk states in the manual the the RPM has to be 1700 or above to begin charging. This is why an idling bike runs it's headlights so dim. It;s using the battery. If you continue to operate it at idle for extended periods, you'll drain the batt. I think most bikes operate like this. They have very small alternators compared to that of a car and rely alot on load sharing so to speak between the alt and the batt.

for some reason i think my headlight dims when i rev the engine, at idle the headlight is brighter. i never understood why.

jetmech727 wrote:The fact that your lights brighten with throttle means your alternator is working. My nighthawk states in the manual the the RPM has to be 1700 or above to begin charging. This is why an idling bike runs it's headlights so dim. It;s using the battery. If you continue to operate it at idle for extended periods, you'll drain the batt.

Never knew that. Hmmmm, learn something new every day

I would agree that its probably a dying battery. On yamahas espacially, I think the field winding is excited by drawing current from the battery. As a result, the alternator will generate additional power only if the battery had enough juice to start with.

If the battery doesnt have enough charge, then your magnetic field is weak... and hence the alternator cannot generate enough power to both charge the batt and run the bike... hence it dies out.

As for the dimming of bulb with higher revs... this indicates that some circuit is drawing more current than it should.. esp at higher revs. Probably check the regulator in this case... could be malfunctioning at higher revs (thus higher input voltage/current).

Even new(ish) batteries will die if not taken care of over the winters.

Bad battery, just for the heck of it check your brushes on the alternator. The lights get brighter when you rev it indicates the charging system is working. Batteries are cheap.

Good Luck

so, another noobie question, if my battery is totally dead, would i even be able to start the bike? i have a kick starter so... i was hoping...