SOHC.co.uk Forums > CB750
Starter Motor
mike the bike:
They're called brushes not bushes. I used to hear that all the time when I worked for Hotpoint.
atlsec:
Hi Guys ,
Stripped and rebuilt ,same problem .Even under no load the outer case gets very hot very quick . I suspect there is a field winding short as it registers virtually 0 ohms when measuring it .Its a pity as the rest of it looks in lovely condition .Just to let you know I am a qualified electrician so I know roughly what to look for . I could really do with trying another one just to compare the results . Thanks for all your ideas though .Cheers ,Alan
PS , Still on the lookout for a replacement starter to buy .
Bryanj:
I have 4 or 5 in my lockup but not sure if one is a 750 as i don't do much with those. Post a pic and i will try and compare
ginger_jim:
Cant you get starter motors rewound?
AshimotoK0:
Field is virtually short circuit anyway. Best way to check is connect 12v in series with an 18 watt bulb and the field winding. The 1.5 A series current, should magnetize the pole pieces . Check that both fields have approx. same magnetism. Identify with an ohm-meter the pairs of segments on the commutator relating to each armature coil, in turn and pass the 1.5 A(using the bulb) through each in turn and check for magnetism with a screwdriver on the laminations. If all OK with approximately the same magnetism, suspect the commutator is faulty or a sticking brush or brushes not contoured to the curvature of the comm. Clean up the comm as described by Trigger. If its badly scored or worn have it skimmed. The 'undercut' of the mica between segments of the comm should be more than 0.2mm (I think but will check). As Trigger says, clean out with a toothpick and cleaner.. also Check that there is infinite resistance between the copper segments and the armature laminations and also the field coils and the tubular casing. If you can find an old motor shop with a 'growler' they can check the comm magnetics but my method above works for me.
Bet you can fix it if you do all of above or at least convince yourself that it is indeed sha**ed but as Trig says, they are normally very reliable.
The original shop manual gave very little .. My 1968 CB250/350 manual was the best on it.
Below is the '77 CB750 offering from Honda but I would still use my method in the absence of a Service tester (basically used as an Ohm-meter anyway in this test)
I do have a spare motor but I won't let it go until I have rebuilt my K0.
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Ash
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