Suddenly won't rev at all

KJ790

KJ790

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Today I ran into an interesting problem. My '81 XS650 had not been charging the battery and I discovered that the rotor was bad. The bike sat in my garage for about 5 weeks while I waited for time to fix it. I bought a new rotor and put it in yesterday and everything seemed to be good. I took it for a quick spin today and everything seemed fine, rode about 10 miles, revved through the RPM range fine. Stopped at home for about 15 minutes, then went to leave again and as soon as I pulled onto the road and gave it any sort of gas it just started to break down and would not rev up at all.

It will idle perfect, but as soon as you touch the throttle it starts to sound like crap and will not rev past 2,000 RPM. I drained the gas out of the tank and carbs and put in fresh gas, no difference. I checked the cam timing, no difference. I pulled out each spark plug one at a time and ran it and it did the same thing on each cylinder.

Any ideas what it could be? Since both cylinders are affected, I find it hard to believe that it could be a carburetor issue (though it's not impossible). I will pull the carbs out when I get time and clean them out, but I doubt that is the problem. It seems to me like it is an ignition problem. Only bike I've ever had act like this was a newer bike with a CDI ignition and the CDI went bad. I don't really know where to start first with this one, any help would be great.
 
KJ790,

Yes. The reg / rect is part of the charging circuit for the battery. It is not needed for the ignition, however, you can only run the engine for a couple of hours without the reg / rect. This is for a test only, it is not a fix for your problem.
 
Alright, just unplugged the rectifier and it revs fine now. Am I correct to assume that a bad rectifier is the problem?
 
Thanks pamcopete! I knew that you meant just for a test, I just wasn't sure if the ignition ran through the reg/rec or if it was just the charging circuit. Thanks again for the help!
 
KJ790,

Well, you said that you just installed a new rotor, so the problem could be a weak magnet on the rotor that is used to trigger the TCI. You can buy a magnet at Radio Shack and epoxy it over the trigger magnet on the rotor and see if that fixes the problem. Be sure to match the polarity of the magnet on the rotor.

The other possibility is that you have a dying TCI box.
 
when my magnet went south I only had issues with accesories such as turn signals, breaks and high beam
But the magnet is such a simple and cheap thing to start with
 
Well I guess it is easy to try a new magnet. If it were the magnet, what would be the reason for it running fine for a 10 mile ride, then not working the next time I started it?
 
Well I epoxied a new magnet over the one on the rotor and it ran great for 11 miles, then started doing the same thing again. I brought it home, pulled the left cover off and made sure the magnet was still in place. I cleaned the pick-up for the TCI and tried running it, but that made no difference. Does this mean that my TCI is dying? Is there anything else it could be before I go buy a new TCI? It just seems odd that this never happened until I changed the rotor.
 
I had a similar issue with my 81 and it was the TCI. I ended up swapping for a Pamco setup and have had zero issues. Starts and runs great...Something to think about instead of sourcing a new TCI box if you end up needing it.
 
kj790,

Well, you can't buy a new TCI they don't make them anymore. The distance you are able to ride before it acts up is about right for the battery to have been recharged after starting. Past that point, the alternator provides current sporadically in response to the load. This affects the current through the brushes, so I would suggest cleaning or replacing the brushes and cleaning the slip rings. The brush length should be 3/8 of an inch, not 1/4 inch that you see in the manual.

While you are at it, check the battery voltage at 3,000 RPM after the battery has had a chance to recharge. If the alternator is overdoing it, you may have a higher than normal current from the alternator that could affect the TCI sensor.

But, of course, I like smokeys solution better :D
 
Thanks guys. I know that they do not sell new TCI's, but I was hoping I could find a good used one for cheap. After looking at the price of used ones I will probably end up going with a Pamco unit, at least then everything will be new. The brushes and the rotor are brand new as of 2 weeks ago, and I cleaned everything while it was apart so I know that it is not a dirty connection in the alternator. The battery was charging fine at 3000RPM when it would rev that high (I checked it after initially changing the rotor).
 
I've tried two other TCI's and they all do the same thing, so I don't think it is the TCI itself. Any other ideas? Could the new rotor itself be causing this issue?
 
pamcopete said:
While you are at it, check the battery voltage at 3,000 RPM after the battery has had a chance to recharge. If the alternator is overdoing it, you may have a higher than normal current from the alternator that could affect the TCI sensor.
Click to expand...

If this is the case, what can be done to get the current back down to normal?
 
The battery is brand new and is fully charged. I just hooked my battery charger up to it to see if charging the battery for a while would make any difference, but it did not. I cannot check the voltage when I rev it b/c it will not rev above idle. When I unplug the reg/rect. it runs fine... Any more ideas?
 
Does the battery store 12+ volts? Mine wouldnt even after a full day of charging, kick the bike and it would idle touch the throttle and the sputtering began and the volts dropped steadily 10.5 to 7.xx. Do keep in mind that I have a pma and a pamco
 

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