81 xj650 when i go to start voltage goes down to 10.75

battery is fully charged. spark is decent. when i go to start it the voltage on battery goes to 10.75 .... if i use starting fluid to get going than its fine to restart once warm.. and the voltage while running is almost 13.4 so i guess the charging system is working. but need a little help on drop in voltage while starting from cold......any ideas?

 

I believe the start voltage can drop to as low as 8-9 volts without a problem so your battery should be fine. If you are having cold start-up problems without starting fluid, I would suspect carb issues. More specifically, your enrichment (choke) system isn't working properly. Time to pull the carbs and do some cleaning...

 

Batteries have an internal resistance, also called impedance. Although batteries are usually considered a constant voltage source when analyzing circuits, their terminal voltage is dependent on load and can be modeled with the equation V_terminal = V_OC + IR, where I is negative when discharging, and R is the impedance. V_OC is the open circuit voltage, as a function of state-of-charge (SOC), and is what you measure when everything is off. V_OC drops as the SOC drops (as it discharges). Keep in mind that you can't actually "see" SOC or V_OC when I /= 0, it has to be estimated.

When you start your bike, the starter motor draws a lot of current, and that is why your V_terminal drops. I think there is an actual spec on the drop when starting, but I'm not sure what it is.

To determine what your charging system output is, you need to check the terminal voltage while the bike is running and rev to 2500 rpm. You should see 14.5 +/-0.3V. Rev higher (up to 5000 rpm) and the voltage should stay in that spec. If it does, your charging system is healthy.

 

My thought is that while the battery may be fully charged, it may not be a strong battery anymore. So.......

How old is the battery, and how long have you been using it? Is it a water-filled battery or is it an AGM battery or gel battery?

Sounds like a weak battery to me. Also, if you're having the issues on colder mornings, there's a combination of weakened older battery coupled with low temps.

Been there MANY times with my 650 when I had it. For cold mornings, I planned ahead and put the bike up in the shop before I went to bed. In the times it wouldn't start, I'd quick hook up the booster, start the bike and ride on my way. Afternoons were fine, since the temp would be up, and the sun would keep the engine warm, too.

Keep at it, and you'll find what the bike likes............

Dave

 

Thanks guys for all the great ideas! battery is new , water type. problem is when starting from cold. cleaned the carbs for the 5th or sixth time...lol pulled every jet. emulsion tube , top below diaphrams. needles and seats. you name it ive cleaned it. i was wondering if i go 1 stage hotter on the plugs would that help. now i have bp7es and then go to bp6es. any thoughts?
once i do get bike started tho and warmed a little... no problems re-starting at all. cleaned pet cock again and is functioning properly. driving me nuts!! lol

 

also, at what voltage drop will the coils not produce a spark or erractic intermittant? just wondering..

 

How certain are you that the tiny enrichment jets are clear? Those orifices are very small and can easily get plugged. Sounds to me like your problem is still fuel related if it starts with starting fluid. If it's not the enrichment jets, then the mechanical linkage or brass plungers might not be functioning correctly for your choke circuit. All four plungers must rise and fall the same amount (synchronized) with each other to function properly. The simplest explanation for a problem is often the correct one (Occam's Razor).

 

It depends on the RPM when you took this reading.

The system should be putting out 14.5V at anything above about 2000rpm. If it's not, then your charging system is NOT "healthy."

Another possible cause would be a starter that's so gummed up it draws excessively when operating.

 

cleaned carbs again. started cold. not much difficulty. sync'd carbs. didnt have to do much tho. they were all pulling about the same amount of vac. thanks for all the advice.. helps to talk it thru with folks on these old machines..

 

As Fitz said - you should be getting somewhere near 14.5v @ 2000 rpm. If you're not there's other issues -- primary to check are the brushes in the alternator, and the state of the copper rings they ride on on the stator. On my bike, the brushes were fine, but those rings needed a good cleaning/polishing -- brought the output right back up to normal.