Electric starting problems - 82 cb750

[gandrolok]

Hello! I bought a cb750 and was enjoying it, taking it on a 2 hour, 110m trip yesterday. That was the longest ride I've taken so far(the other handful of them so far being less than 5-10 miles). I was at thruway speeds(65mph) and didn't stop.

When I got to my destination(2 hours away from my tools and comfortable garage), my bike started less quickly. The solenoid clicked slower and slower and the last time it started, it ran strong to where I was able to park it at my friend's house. It ran great for the 2 miles I took it from where we were to the driveway where it's currently parked, so I think the problem is electrical and either a bad starter or solenoid.

When I try to start it now(new battery from 2 weeks ago), the lights dim and I get one single click. Nothing mechanical seems to move(as in the starter or engine), at all. After scouring the internet for possible solutions, I tried manually activating the starter via screwdriver arc'ing the positive and negative terminals on the solenoid.  There is a strong spark, but still no mechanical movement.

Does anybody know what's wrong? I'm stranded up a creek without a paddle, so to speak.

[gleado]

check battery voltage first.  should be around 12.6 volts.  If not either your battery is failing(yes I know it's new), or charging system is not charging, or batt is draining with key off.   if that's good are connections clean?  Fuses?  Sounds like a starter to me but check easy stuff first.  If you can't use a meter find someone good with one.  If you have to pull the starter, it may just need a little lube on the bearings/bushings.  Worth a shot anyway-let us know what you find and good luck.

[sgtslag]

Actually, it sounds like your charging system is not working, so your bike was running off of the battery's current, only.  In other words, check the battery's voltage (as stated already, it should be 12.6 Volts, but I guarantee it isn't anymore); charge the battery to full, then try turning it over.  With a fully charged battery, it will very likely start, as normal.

Once it is running, put a voltmeter across the battery terminals, and open the throttle to run the engine up to 3,000 RPM.  At 3,000 RPM, the charging system should be pumping around 13.5 to 14+ volts into the battery...

If you get less than 13.5 to 14+ volts, with the engine idling at 3k RPM, then it is your charging system which is failing.  There are several possibilities here:  the Rectifier/Regulator may be blown, or the alternator is bad, or there is a wiring problem.  To properly troubleshoot it, I would recommend following the guidelines within the FSM (Factory Service Manual), found on the Home Page of this site, as a free download.

Take some time to read up on the troubleshooting process, then give it a whirl.  Get back to us with what you find, and if you have questions, we would be happy to help out.  Cheers!

[gandrolok]

Thank you so much for the advice! I'm just a young enthusiast trying to get started, and I have limited experience. This is my first real bike. It's super helpful to hear from you both.

So I got a ride to Home Depot and bought a multimeter. We got back and checked the voltage on the motorcycle. It was 10.40V when we got back. We waited 2-3 hours and checked it again. It was 10.38V, so it was definitely dropping a little bit over a while.

Hooking jumper cables up to the friend's car and beginning the jump, we measured a boost to 12.70V and it evened out about there after a few minutes. Closer to the boosting car's battery it was 13.8V. At this point I was able to start the bike. The relay clicked and the starter turned and the bike started right up. It seems like everything in the starting line is good.

We removed the jumper cables and monitored the voltage. At 3,000-3,500 RPMs, the voltage remained about constant at 11.8V. When the RPMs dipped down(to idle around 1000-1500RPMs), the voltage dropped pretty quickly, maybe .01-.02V a second. We didn't measure faster than 3500 RPMs.

When I shut the bike off, the voltage on the battery dropped to 11.51V. It dropped to 11.4V before I disconnected the negative cable. The voltage, after the negative cable was disconnected, stayed about the same over the 1-3 minutes I watched it.

Is there a leak of current somewhere? Is the alternator underperforming? Bad battery? Both?

Fortunately, I should be able to ride it the 100 miles back home to a garage where it's safer and easier to work(tools!).

Thanks for all your help!!

[sgtslag]

The charging system is NOT working...  You will need to fully charge the battery BEFORE riding it the 100 miles home...  The battery's power is all you have to run the engine on.  Once that power is gone, you have nothing to fire the plugs.

The battery is being deep discharged -- this will shorten the life of the battery, dramatically.  You need to get a trickle charger on it, to slowly, and safely, bring it back to full charge.  I recommend a Battery Tender Jr., available at most automotive stores, and DIY stores, also can be ordered online, all around $30.  Hook your bike up to it every time you shut it off, in the garage, and it will likely double the life of your battery (will work on any type:  flooded lead-acid, Gel Cell, or AGM).

Download the FSM, as discussed, previously.  Then read up on the charging system, and do some troubleshooting.  Cheers!

[gandrolok]

Well, we got the bike 100 miles home. It took one breakdown(about 40-45 minutes after leaving) and sitting under a thruway bridge before help arrived with jumper cables. Two charges later(the second we did as a precautionary jump at a rest area), the bike is safely in the garage.

The battery has been sitting on a trickle charger since I got it home again.

Once again scouring the manuals and internet for charging system defects/troubleshooting/known problems, I have decided that my alternator rotor is bad - the resistance between slip rings was less than 1 Ohm, consistently.

After looking around for a suitable replacement or solution, I've found this: http://www.ebay.com/itm/Honda-cb650-cb750-cb900-cb1000-rotor-rebuild-service-/180896474621?pt=Motorcycles_Parts_Accessories&hash=item2a1e4525fd#ht_1871wt_1156

I'd like to give it a try. (Thoughts?)

HOWEVER, when I went to remove the rotor, I absolutely have tried everything in my garage to get that darn bolt off, and I simply can't. I've tried hammering, wrenching, air impact wrenching, manual impact wrenching(old school with a hammer), PB Blaster. I've shifted to fifth gear and braked, and the clutch just slips(another problem?).

So I'm giving up for now and hopefully riding the darn thing to a professional mechanic tomorrow.

[robs750]

You can also check with Race Tech for electrics .............


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