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Motorcycle Repair/battery drain on 750 vulcan


Hi. I recently bought a 1991 vulcan 750. It has trouble starting at times but when it does it warms up normally and runs smooth once the choke is disengaged. Unfortunately after a couple of rides the bike then struggles to start and the battery drains. I don't know if it struggles to start because of a battery drain or if the battery drain is a result of something else not allowing the bike to start. I bought anew battery but with the same results. Any thoughts?

Hi Dominic,

There could be a few different things happening there with your bike.
One is making sure the battery is a good one and has been fully
slow charged. A full battery will read 12.6 or more dc volts
with the ignition key off.

When starting it will drop to about 10 volts but come back up
immediately if not run down too low.

Another possibility is if the bike is not charging the battery
or you have been taking short rides which will not charge
very much on a motorcycle.

All the above checks can be done with a common multimeter.

It should read above 14 volts on the battery terminals
when the engine is above 2000 rpm.
Any less indicates a possible problem with the alternator
stator or regulator.

Another possible battery drain is just a worn or dirty starter motor
that is dragging or has bad brushes or bushings. The battery voltage
test while cranking will show the voltage drop there.

The connections on the starter relay should be tight as well as battery cables and
battery termnals. Both ends of the battery cables should be checked as
a poor ground to the frame could draw excess current from the battery.

If you want to see if there is a large drain on the battery just disconnect
the negative battery cable. Put a test light between the battery negative
post and the loose negative cable end. If the light comes on there
is a drain. Do this with the key off.

Let me know any results of the above checks and I can advise you further.


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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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