New Bike Owner with charging problem

Sbartram91

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Howdy ya'll,

I am now a proud owner of a 1981 CB750F Super Sport. This is my first bike. Literally bought it yesterday. Riding it back from the guy I purchased from yesterday and the battery died on the freeway. I was able to make it home and I left the battery on the tender overnight. The previous owner said it was a good battery and relatively new. Can fire the bike up, and using a multimeter I confirmed that the battery is not being charged, regardless of the RPM. Battery terminals appear clean, and I haven't identified any bad grounds as of yet. Is there something I should be looking at before I test the stator/voltage regulator? If I was to test the charging system for proper function, how would I go about doing that? Unfortunately I do not have a manual for the bike. I'm relatively new to electrical so any input would be much appreciated! Also, happy to be a new member of this forum! :D
 
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amc49

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The rotors commonly go bad on them and if driven long enough like that they then blow the regulator out too. The rotor on an early DOHC is the weakest most unreliable part on the bike.
 

Sbartram91

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The rotors commonly go bad on them and if driven long enough like that they then blow the regulator out too. The rotor on an early DOHC is the weakest most unreliable part on the bike.

Thanks for responding. This is good information to know. Definitely going to check out those documents to get a better idea of what is going on. Appreciate the help.
 

amc49

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Be advised the rotors can check out as good and not be. The windings move to contact and short at higher engine speeds and then separate back out at slower. And any combination of that up to zero charging at all.

At some point past curing that you'll need to go over the valve clearances using .004"-.006" as the set range rather than the service manual figure, which can and will burn valves if used. Most of the bikes never had it done because you have to buy shims to do it and why they run like crap. A compression check will show it quick.
 

Wez_

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Don't scare him with valve clearances just yet :D Remove the right hand side cover housing the alternator. 3x 8mm hex bolts. Clean the rotor with brake clean and dry. using a multimeter, check the resistance between the two copper ring faces. You'll have to google the spec, but it's something like between 4-8 Ohms. The dust can also prevent charging. There is a place Rick's Motorsports here on the east coast. They sell vintage electrical components. You can send them your rotor and they will test it for free if you feel it has the invisible failure.....

Before that, you have to do resistance checks on the regulator/rectifier and stator. There is a check on the AC and the DC side of the reg/rect. The device takes in AC voltage and converts it to DC. I had a failed regulator/rectifier on the AC side. One of the diodes was failing the resistance check. Bad connections is usually the culprit 9/10 times. You go to prbe the regulator rectifier harness and a wire falls out. there you go. But also, Ive learned that charging failures cause chain reactions. Usually two components fail and you'll be chasing your tail thinking it's just one failure. Also, you'll need a charged battery all the time for tests. Resistance on the stator needs to be checked also. Start youtubing the measurements. Go get a digital multimeter that has the diode test, an audible chime for continuity and one that can measure at least 10 mV or 0.01V for precision. Some meters cannot accurately measure small signals.
 

amc49

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You absolutely CANNOT check a regulator out to positively 100% know for sure it is good unless you have a known good one to replace to test that way. There are simply too many ways they can fail and not all are positively discernible. You can't diode test either other than getting an all good reading on all 6 together, anything else is a fail. The diodes are banked in the install and the only way you can check them is to have them all detached from each other to test separately, something of a problem there. When your voltmeter has a diode function it is only intended to allow 'burst' or crossover volts to activate one diode not multiples. So the output can be misleading.

A 'charged' battery may not do spit if it is still no good. It must be well charged and over 12.8 volt and then let it sit 8 hrs. and it must still be between 12.3-12.8 volts or it is damaged and can affect your alt outputs.
 
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Sbartram91

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Totally forgot about posting in this thread. Been so busy with work and life. Appreciate all the help! Sorry it took me this long to get back in here. So, update!
I replaced the rotor as it was reading bad. Installed it a couple weeks ago and now the bike is getting a proper charge. But now I have run into a different issue. So the bike turns on and runs but the bike dies under load and dies at idol. I can get the bike to run and ride if the choke is pulled out halfway but as soon as you push the choke down it dies. Also replaced the brushes on the stater. I suspect a fuel issue? Any ideas! Thanks y'all!
 

amc49

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You are now at the valve clearances issue spoken of earlier. Compression check before you go any further, or all work may be wasted. The choke issue hints at carb troubles which these have in scads too. If the engine is dying at idle due to compression though you can put 10 sets of carbs on and problem is still there.

40 year old engine, don't make the mistake so many do all day long of thinking it's in great shape, most aren't.
 

Sbartram91

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You are now at the valve clearances issue spoken of earlier. Compression check before you go any further, or all work may be wasted. The choke issue hints at carb troubles which these have in scads too. If the engine is dying at idle due to compression though you can put 10 sets of carbs on and problem is still there.

40 year old engine, don't make the mistake so many do all day long of thinking it's in great shape, most aren't.
Thanks for the reply. I have Sunday and Monday off work so I'm definitely going to work on the bike. I'll refer back to this before I mess around with it though. Thanks man.
 

amc49

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Bear in mind that whatever you get on the valve clearances take off .002" (.05 mm.) It isn't really there...........it disappears in the cam cap clearances because they are bigger.
 
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