what would cause a rotor to go bad?

CBrat750

CBrat750

New Member
Hey guys so I thought I had my 80 cb750 running great after I burned out the rotor and regulator for the first time. I replaced the RR, rotor, and stator. And then was able to rack up 300 or so miles over a month or 2. While on a ride to pick up another cb it died on me.

After running a few tested I found the rotor was cooked again...does anyone know what would cause 2 rotors to burn up. I feel like it is an odd part to go bad.
 
ADC is on the correct path. About the only thing that will make a rotor go bad (during "normal" use) is being asked to supply too much current. The demand for more current causes heat which damages the windings.

Two main culprits are too much power draw (aftermarket headlight, bad battery, too many accessories, etc) and/or a voltage regulator that is on it's way out.
 
Did you replace the rotor and R/R with new or 2nd hand parts, if 2nd hand did you test them after fitting and were they working to spec or did you just ride it? A lot the 2nd hand rotors on ebay are on there for a reason.
 
Rotor is similar to XS650, coil,of wire, carbon brushes to supply power and create magnet
Usual cause in my experience, someone fitted battery backwards and destroyed diode in regulator so battery voltage is being fed into rotor at all times (even with ignition off)
Some bikes just blow fuse, but, the early CB burns out rotor
 
AlphaDogChoppers said:
Weak battery that is sucking too much power, maybe?
Click to expand...

The battery is just about brand new. I got it maybe 2 months ago and less the 500 miles on it.

notlob said:
Did you replace the rotor and R/R with new or 2nd hand parts, if 2nd hand did you test them after fitting and were they working to spec or did you just ride it? A lot the 2nd hand rotors on ebay are on there for a reason.
Click to expand...

Yes the parts are 2nd hand parts. I picked up a motor and wiring harness on CL. So I tested the rotor checked out at 4.5ohm, the stator had continuity across all 3 yellow wires, and the RR was working correctly.

That is what is odd everything was working fine i put about 500 miles on the bike over about a month the dead mid ride. Oh only after market electical part i have is a phone charger. And i have the same set up on another bike for 2 years with out a problem.
 
If you attempt to charge a low battery with the bikes charging system the reg/rect and/or stator fails, not the rotor
The wire can wear through insulation if it's loose at any point, or, if would too tight
I'm 'almost' certain you can get rotor re-wound for around $90.00 (less if local vacuum cleaner repair man will do it ;) )
The two halves are pressed together, it's just a coil of wire in the middle

Doubtful you'll find any instructions for CB but Yamaha is similar enough that you could research that instead to see how it's done
New/modern coil wire has far superior insulation
 
crazypj said:
If you attempt to charge a low battery with the bikes charging system the reg/rect and/or stator fails, not the rotor
Click to expand...

If system voltage is low due to a dead cell, the voltage regulator will send more current to the rotor to compensate, possibly overheating it.

Not saying you are wrong, but I was using a marginal battery in my XS650, and it was the rotor that fried.
 
CBrat750 said:
the stator had continuity across all 3 yellow wires
Click to expand...

You should test each of the 3 yellow wires from the field coil for short to ground or the outer casing, If its a short to ground then that field is burnt out.
 
crazypj said:
Field coil is 2 wires, stator is 3 wires
Click to expand...

You had me thinking there Crazy so I went and checked, the DOHC CB750 has 3 yellows from the field coil and two a black and green from the rotor brushes.


Edit: just realised what you mean, I'm getting mixed up with field coils, rotors and stators. The test is still the same no short to ground from the 3 yellows.
 
The rotor is the field coil, you put power in it and get a magnetic field
The 3 yellows are stationary = stator
 
To sum it up ...
Rotor = continuity on the order of 4 to 6 ohms typical
Stator windings = again on the order of 4 to 6 ohms typical between any two and zero continuity to ground.
Dynamic test = approximately 75 volts peak to peak between any two stator wires , no load . (disconnected) from the regulator . Some systems will not provide field current to the rotor with the stator disconnected , in fact some field wires are part of the stator connector so ....
Field voltage should be at or near battery voltage when charging and near nothing when not .
As mentioned continuous battery voltage supplied to the rotor will kill it , the battery , or both .
Common battery failures as single cell failures result in more of a resistor than a battery that the regulator continues to feed voltage to the rotor that results in rotor or battery failure .
The regulator should close and send voltage to the rotor at anything less than about 12.2 volts and release before 14 volts .

Keep in mind that the older mechanical regulators were a bit of a black art and the stock rotors were marginal . There are two more services I know of for rewind/rebuild , http://www.customrewind.com/ and http://ricksmotorsportelectrics.com/rotors

~kop
 
I had a stator once, whose windings broke, probably because of vibrations. If copper vibrates it gets hard, if it gets hard it brakes.. I guess that can happen to a rotor too..
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light

    • Spread the Love

    Back
    Top Bottom