1980 Honda CB750K Won't Start Without Jumping

[calebmund]

Alright, so, everything is in, except the solenoid fuse(coming tomorrow), and I also checked my battery voltage, and it is still sitting at a steady 12.8V! So I'm pretty sure my battery is still good.

Here's to getting her running tomorrow!

[calebmund]

So I got it running yesterday, after putting the solenoid fuse in, and I took it up to my parents (a 45 minute ride), and then after a while I headed back home. I'm not sure all four cylinders are firing... I was experiencing a lot of hesitation, and a lot of backfiring. Couldn't rev it over 7000 rpms... And I was rarely able to even get that high.

But when I got it back home, it wouldn't start... Didn't even try to turn over!

Any thoughts??

[Re-run]

What voltage is your battery? Did the bike click or anything? Any lights?

Get the battery charged after testing. Start the bike and hook up a voltmeter and see what the volts are at. Remember, charging doesn't really happen till 2000 rpm.

[calebmund]

It clicked once, and then after that, nothing...

Do you have any idea why I'm not getting much power while riding? (Hesitation and backfiring)

[rich]

calebmund wrote

It clicked once, and then after that, nothing...

Do you have any idea why I'm not getting much power while riding? (Hesitation and backfiring)

When you're only running on the battery (It's not charging) there's not enough energy for the ignition system. I'll bet the missing, popping, and backfiring didn't occur on the first part of your trip. Only one the way home when the battery starting getting low.

Check your alternator rotor again. 4 OHMS or less and it's bad.

[rich]

Here's a good thread discussing electrical testing (with pictures!): http://honda-cb750-s.456789.n3.nabble.com/1981-CB750k-not-charging-the-battery-td3144722.html

[rich]

Also, think about getting some kind of gauge or meter mounted on the bike so you can tell at a glance if it's charging. Here's what I put on mine: http://honda-cb750-s.456789.n3.nabble.com/LED-Battery-Gauge-td3260407.html

[rich]

And... a few more tips from another thread on this topic:

The rotor should have 4.5 to ~ 6 ohms resistance. Clean the contact area (copper rings) with a pencil eraser and press the probes from your meter on there firmly to get a reading.

It should start pretty quickly with a fully charged battery. Is your choke working? Have you checked the spark plugs? If it idles and runs smoothly (with a fully charged battery) then the previous owner probably did have the carbs cleaned. Focus on the the plugs, coils and all the associated electrical connections; clean everthing up and make sure all the connections are good and see if that improves the stating issues.

Here's a great write up on checking the charging system from http://www.oregonmotorcycleparts.com/FAQ.html

Diagnostics for 1979~1984 Honda 750~1100 DOHC fours (and SOHC CB650)

The following information also applies to the CB550 Nighthawk but the resistance on the field coil (black to white wires) is
lower.  Honda has released an updated stator/field coil assembly for this bike (this is the only bike I know that uses this
arrangement) and I  recommend calling the Honda dealer for this information as the manual may be wrong.

The first test is to test the voltage between the red wire on the rectifier / voltage regulator unit (do not unplug it for this test) and
ground with the bike running.  If you're only getting battery voltage then you have a problem.  If you're getting over 15 volts then
the regulator is bad.

At first all those wires look rather intimidating but once you break it down it's rather simple.  Unplug both of the connectors and
use the test procedure above to test the rectifier part of the unit.  The red wire is positive and the green wire is negative and  test
them against the 3 yellow AC leads.

The next test is to test the voltage regulator.  Plug the small plug from the regulator / rectifier back into the wiring harness.  With
the ignition on but the bike not running and if the regulator is good, you should get less than battery voltage between the black
and white wires in the large plug from the regulator / rectifier.  You can do this test with the bike running but I find it usually
doesn't make any difference.

If these test have positive results the next thing to check is the resistance on the rotor.  Remove the alternator cover and check
the resistance between copper rings on the rotor.  There should be 4.5 ~6 ohms resistance if it's good.  Most of the time when
rotors are bad there will be either infinite resistance (completely burnt out) or less than 2 ohms resistance (internally shorted.)
Either way you're looking at getting a new rotor.  If the rotor checks out good the next step is to check the resistance between
the black and white wires (with the alternator cover installed.)  Wiggle and tug slightly on these wires during this test.  If the
resistance changes at all during this test or is different than the resistance between the copper rings on the rotor, the wires leading
to the brushes (black and white wires) will need to be replaced.  The wires to the brushes going bad is more common than one
ouw think

The next thing to check is stator.  I left this test for last because this is the least likely thing to go wrong.  There should be about
.5 to 1 ohm resistance between the yellow wires from the stator and infinite to ground if the unit is good.

[calebmund]

I'll be sure to check on these things you listed!

The only things is, I just got a new rotor from Rick's Motorsport, and got an ohm resistance of about 3.5-3.6, which after calling them, I found was normal, and good for their rotors.

Even with he bike running, though, my battery does not charge... Before I started my bike, the battery had dropped down to about 11V, and after starting, it went up to about 12.2V...

I also just put new brushes, as they were worn down past the scribe line...

Any thoughts?