Electrical problems 81 750F SS

[Sweedish 750]

Hello everyone

Just bought my 750 for this season and its been a roller coaster ride so far. Good times riding around in the sun mixed with memories of me trying to start the f**ker in the rain with leg power and pure anger.

So, its an electrical problem you say, and with my limited knowledge I would draw the same conclusion. But I'll give you some background and symptoms.

The bike has been in a non-heated garage for the better part of the last decade. When it was taken out to be sold it was fitted with a new battery and the carburetors were cleaned (that's what the previous owner told me, but we're acquaintances so I believe him). It ran well but after a while it needed pushing instead of using the much more comfortable electrical starter.

In the beginning it was really easy to push the bike to get it to ignite, but this changed later, maybe over the span of 4-5 rides, when it became much harder.

So I tried to charge the battery thinking that it was dead, which it probably was since the head lamp cut out when on low revs cornering. With a full battery it started after many turns with the electrical starter and finally pushing the bike. It only lasts around 2 days with the battery which means that it's not charging properly.  

I've charged the battery again and driven but it's still hard to start, so that's one problem. I've also checked the voltage on the battery which is 12,2 v when turned off, around 12-11,9 when in on mode but not running and 13,3 when running at around 4000 rpm.

It should be around 14 or a bit over, given what I've read so far on different forums, right?

Also I've checked the rotor to see if there's a problem, but it gave me a inconclusive result. It didn't give me a stable reading but varied between 0,1 - 2,0 ohms. I hope I used the correct interval. Don't know what this means since the video I've found only shows a rotor with no impedance. Might this be a problem with the regulator?

I'm looking for tips on what to examine/do next?

Thank you for your input

Semi-Happy 750 rider

[rich]

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.

[Sweedish 750]

Thank you for your quick reply, now I know where to start.
Would you recommend an aerosol contact cleaner as the best way of making sure that the connections are clean?

I hope I have some time this weekend to fiddle with the bike. The season is ending soon here in Sweden, otherwise I'm gonna have to wait until around April, since I don't have a place to work on my bike.

Not the best climate for motor bikes..

[LukeM]

Look for an electronics store like Radio Shack or similar.  Look for contact cleaner without any lubricant in it.  Easiest way to check is to spray a little bit on your fingertips and rub them together: if it's oily, you'll feel it after the cleaner dissipates. If you can find it, the Cramolin products are very good for cleaning and conditioning contacts.  I use D-5 in my electronics and it works very well.

If you want to go the cheap route, you could use isopropyl alcohol and a cloth, but it won't clean as well as the other stuff.

Luke M