Will a bad rotor cause ODD running???

[bobber33]

Had a thread in the fuel section but this is no longer a fuel problem so thought i'd ask here.
Its a 80xs bobber, minimal wiring electronic ignition.

QUESTION - with the brushes out of my bike it runs perfect.

With the brushes IN it charges fine and runs great UNTIL it gets HOT(about 5-10 minutes) then the bike falls on its face, stops charging, spits, backfires, wont take throttle etc... if i pull the brush it smoothes right back out to normal???

Swapped out - Coil - Regulator/Rec. Same problem.
Tested all the parts and the rotor DOES test bad when hot, and good when cold - but i still don't understand why it will affect anything other than the charging system?
I have a new rotor ordered, but can someone explain how the rotor shorting will affect the running of the bike?
Or is there something else wrong besides my bad rotor?

[kopcicle]

Have you recently offended anyone by the name of Nanook ?

~kop

[richard]

There may be more than one thing wrong with the bike. If it were just your rotor the problem could be cause by thermal expansion of the coils on the rotor as they heat up. The heat weakens the coated copper wire and centrifugal force stretches the coils until any bare spots make contact with a metal surface on the rotor grounding it out.

The other thing that could be going on is that your idle mixture is not set right and once the bike is warmed up it’s too rich to run correctly, but Its not likely to stall the bike. In most cases it would just run bad and foul plugs.

You could have an obstruction in the needle valve of you float chambers, If that were the case it would run good at first but crap out under a load at speed.

If the diaphragms on you carburetors have holes in them they could leak more when they heat up or a plugged vent hole in your gas cap could cause it to act that way too in which case you just pop the gas cap and see if the bike runs again.

Really it’s just a process of elimination starting with a simplest things first.

[bobber33]

Sept 5, 2010 22:12:55 GMT -5 richard said:

There may be more than one thing wrong with the bike. If it were just your rotor the problem could be cause by thermal expansion of the coils on the rotor as they heat up. The heat weakens the coated copper wire and centrifugal force stretches the coils until any bare spots make contact with a metal surface on the rotor grounding it out.

The other thing that could be going on is that your idle mixture is not set right and once the bike is warmed up it’s too rich to run correctly, but Its not likely to stall the bike. In most cases it would just run bad and foul plugs.




You could have an obstruction in the needle valve of you float chambers, If that were the case it would run good at first but crap out under a load at speed.

If the diaphragms on you carburetors have holes in them they could leak more when they heat up or a plugged vent hole in your gas cap could cause it to act that way too in which case you just pop the gas cap and see if the bike runs again.

Really it’s just a process of elimination starting with a simplest things first.

thanks for the input all...but...

This is 100% not a fuel issue.

It runs just fine without any charging system?

I mean instantly....
if i disconnect the brushes, or the plug to the alternator/rotor it smoothes right back out to a perfect running bike.??

[bobber33]

OH, but keep in mind it runs just fine with everything plugged in for the first 5-10 minutes.
Then as it starts to run like crap - if i unplug the alternator while it's running it instantly runs perfect again.?
So I'm back to a problem with the charging system?
My only thought is that somehow it is affecting the coil output to the plugs causing it to misfire...

any other thoughts?

[pamcopete]

bobber33,

Well, it would help if we knew what year / model bike you have, but if it is a '80 to '83 model with TCI ignition, then your problem is a weak pickup magnet on the rotor.

You can fix this problem by JB Welding a very small magnet of the same polarity on top of the magnet in the rotor. Be sure that the new magnet clears the stator case.

Your new rotor should also fix it for the same reason, but these rotors are getting older and if the rewinder does not replace the magnet, you have a 30 year old magnet that has lost much of it's original magnetism, trying to drive a 30 year old TCI box.

[bobber33]

Sept 6, 2010 6:34:41 GMT -5 pamcopete said:

bobber33,

Well, it would help if we knew what year / model bike you have, but if it is a '80 to '83 model with TCI ignition, then your problem is a weak pickup magnet on the rotor.

You can fix this problem by JB Welding a very small magnet of the same polarity on top of the magnet in the rotor. Be sure that the new magnet clears the stator case.

Your new rotor should also fix it for the same reason, but these rotors are getting older and if the rewinder does not replace the magnet, you have a 30 year old magnet that has lost much of it's original magnetism, trying to drive a 30 year old TCI box.

Thanks,
Yes as stated at the top its a 1980 with electronic ignition.
My thought was the pickup coil was somehow getting screwed up.
But WHY when there is no power to the rotor does it work fine?
Is the magnet for the pickup in the rotor shorting when power is applied?

[pamcopete]

bobber33,

The rotor has an electromagnetic field when power is applied to the brushes. That magnetic field interferes with the small permanent magnet used to trigger the pickup coils and TCI. When no power is applied to the rotor, there is no magnetic field to interfere with the small magnet.

Here is a pic of the rotor showing the little magnet for the TCI:

[bobber33]

That makes sense.
Thanks again.
I'll post results after i get the rotor swapped out.

[bobber33]

posted pics of the build here..

xs650forum.proboards.com/index.cgi?action=display&board=projects&thread=2123&page=1#8606

[bobber33]

Sept 6, 2010 12:12:51 GMT -5 pamcopete said:

bobber33,

The rotor has an electromagnetic field when power is applied to the brushes. That magnetic field interferes with the small permanent magnet used to trigger the pickup coils and TCI. When no power is applied to the rotor, there is no magnetic field to interfere with the small magnet.

Here is a pic of the rotor showing the little magnet for the TCI:

Finally got the rotor changed and same problem... which leads me to the pickup coil being messed up by the magnetic field of the rotor when hot...?

[650leo]

I had a rotor that would quit working as it warmed up. At room temp it ohmed at 5 ohms, at running temp it would read 8.5 ohms. At 8.5 ohms it stopped charging.
Check the ohms after it gets warm.

[fiineliine]

Very interested in this. I have the same problem but not heat related. Bike runs terrible-won't take throttle , occasional backfire, etc. but charging 14v. Take the brushes out and it runs perfectly. sounds like the same disease. 1981

[richard]

Ok so a short in any part of your electrical system could drag your battery voltage down and cause it to run bad.

A rotor in you charging system draws a lot of power. 4000 ohms is a lot. The best way to sort it out is to check the resistance on your rotor and your stator. Anything with windings can have a short that only shows up when electricity flows through it. They have a thing called a meg-ohm-meter that is used just for checking that on electric motors and windings. Remember when you are checking windings you also have to check to make sure they are not grounded to the metal in the rotor its self. Rotors tend to fail more because they are spinning and hot at the same time. They see the most stress and centrifugal force trying to put those copper winding apart. The only thing in them that separates the copper wire in them is a varnish coating

The Diodes that turn AC to DC can also fail with heat, but you would see that in a drop of voltage. Diodes tend to just quit all at once. They either work or they don't.