Topic: 1978 CB750 Charging Issues

[Owen.Cowsert]

The stock fuse box is known for having high resistance connections, and should be checked. Sometimes, the fuse holders can be cleaned, but sometimes the fuse box can get melted from the heat.

Since you mentioned it, I did notice some melted plastic on the bottom fuse connector in the box.

[PeWe]

That fuse box reminds me about a friends fault tracing on a CB550.
We scratched our heads until we saw a fuse mounted wrong. Between the posts for fuses. Very strange behavior that did not match the circuit diagram; D
The owner made a mistake...

Do not forget the ground cable from battery to frame-engine case. Good idea to remove paint to ensure good connectivity.

Melted fuse box indicate a need to inspect and clean all wirings/connectors.

[Stev-o]

The stock fuse box is known for having high resistance connections, and should be checked. Sometimes, the fuse holders can be cleaned, but sometimes the fuse box can get melted from the heat.

Since you mentioned it, I did notice some melted plastic on the bottom fuse connector in the box.

I'd upgrade to Hondaman's fusebox...

http://sohc4shop.com/catalog/product_info.php?cPath=23&products_id=48&osCsid=25ee24bfb8ef539bcdd4a2f83fecfbda

[1978 750F F2]

I have experienced the same issue and to me electrical issues are most challenging!
 
I discovered a peculiar issue while checking the voltage and amperage on the SuperSport charging system.
The bike has but 3530 miles, its relatively in new condition for a 1978; no corrosion on any parts of the electrical system.
Here are my findings using a multimeter:
1. ) Battery is fully charged--12.7 volts
2.)  engine running--1000rpm, volts 11.5 , amps 6.8 to 7.4
3.)  engine running--2000rpm, volts 11.5, amps 1 to 0.78
4.) engine running--3000rpm, volts 11.8,  amps 0 .74 to 0.78
5.) engine running--4000rpm, volts 11.8, amps 0.74 to 1.3
I adjusted the mechanical voltage regulator turning the adjustment screw as specified on page 15-8 of the Honda shop manual...  No change in voltage (adjusting to maximum and minimum voltage)

Please advise.
Phil

[scottly]

With the key on and engine not running, check the voltage on both the regulator black terminal and white terminal.

[HondaMan]

Something I often find on the 750 when rebuilding them for others is: the 5 'bullet' connectors under the left side cover (available with the sprocket cover removed) are burned and cooked. This causes low-charge situations, despite all good components. Look to see if yours has burned-dark jackets on those plugs/receptacles: the ones most often burned are the 3 yellow ones from the alternator's windings (the other 2 there are for the field coil, only occasionally burned). This happens mostly because they were originally zinc-plated steel connectors with a planned 10-year life - that was about 40 years ago - and the zinc was supposed to prevent the steel's corrosion. You can get identical versions in brass today from Vintageconnections.com: they are the 3.5mm size, and they have a most-excellent, airtight crimping tool for them, too. The brass ones won't die in 10 years: in most of the bikes I rebuild I replace these, every one. They have never suffered an alternator problem, either.

[Rayzerman]

I would take that a step further..... every old bike you know with older style charging systems has the identical problem of burned up stator connections on the three yellow wires, doesn't matter what terminal type or plating..... get rid of the teminals, period.  Strip, and either use bare butt connectors/solder/heat shrink or twist the wires together and solder/heat shrink. 

[newday777]

I have experienced the same issue and to me electrical issues are most challenging!
 
I discovered a peculiar issue while checking the voltage and amperage on the SuperSport charging system.
The bike has but 3530 miles, its relatively in new condition for a 1978; no corrosion on any parts of the electrical system.
Here are my findings using a multimeter:
1. ) Battery is fully charged--12.7 volts
2.)  engine running--1000rpm, volts 11.5 , amps 6.8 to 7.4
3.)  engine running--2000rpm, volts 11.5, amps 1 to 0.78
4.) engine running--3000rpm, volts 11.8,  amps 0 .74 to 0.78
5.) engine running--4000rpm, volts 11.8, amps 0.74 to 1.3
I adjusted the mechanical voltage regulator turning the adjustment screw as specified on page 15-8 of the Honda shop manual...  No change in voltage (adjusting to maximum and minimum voltage)

Please advise.
Phil

Welcome aboard the forum Phil
Is that a new to you bike? It looks great.
Follow Scottly and Hondaman's advice. Report back your findings and results.

[HondaMan]

The stock fuse box is known for having high resistance connections, and should be checked. Sometimes, the fuse holders can be cleaned, but sometimes the fuse box can get melted from the heat.

Since you mentioned it, I did notice some melted plastic on the bottom fuse connector in the box.

Yep, that one's in trouble!
The original chromate plating on those lower fuse clips has flaked off the tin underneath. This makes the clips heat up while they carry current. The short-term 'fix' for it is to take a steel wire brush and shine those contact back up. This will work for a little while, but today's fuses are not rated to run with vibration, so they will fail while you're riding even though nothing is actually wrong then.
Here's the skinny on the fuses: the OEM versions were the old SFE types, which are (were) rated for vibration while carrying current. The SFE fuse type was discontinued worldwide during the 1990s and the world's supply ran out somewhere around 2006 or so. So, even those who are selling "new fuseblocks" on the internet don't seem to know this, and those come to you with appliance-style fuses in them. Even worse, China has recently started marking appliance-style fuses as "SFE" on their fusecaps, increasing this confusion greatly since about 2 years ago.

I make a more modern version of those fuseholders: take a look at SOHC4shop.com to see what they look like and if you'd like one. They use modern automotive fuses of the ATC type, which you can buy even at the local grocery store now.