Bike won't rev past 4000 RPM

My 1980 XS with Stock ignition and fuel system is having trouble revving. I can't go past 4000 rpm, whether I am cruising down the road, or if its in neutral in my driveway. It almost feels like the bike cuts off temporarily, like when you bump start it, but it won't start. But as far as I can tell, it never really turns off. Just sort of feels like the engine can't fire fast enough or something. I pulled the plugs after i noticed the issue on my ride today, and they look perfect.

I've read a bunch of posts, but i was just wondering if someone could give me a few simple steps to get started in the right direction to figure out whats going on. Is there a "most common" cause for this kind of issue? Or what do I check first? Thanks.

diab,

Try unplugging the reg / rect. The plug is under the left side air filter cover. One of the most common causes of your problem is a weak trigger magnet in the rotor, so unplugging the reg / rect will disable the alternator and the magnetic field on the rotor that interferes with the trigger magnet.

Then, do a forum google search on "rare earth magnet"...

pamcopete said:

diab,

Try unplugging the reg / rect. The plug is under the left side air filter cover. One of the most common causes of your problem is a weak trigger magnet in the rotor, so unplugging the reg / rect will disable the alternator and the magnetic field on the rotor that interferes with the trigger magnet.

Click to expand...

I'll check this tomorrow.

So I disconnected the regulator/rectifier and my bike revved all the way up no problem. Does that mean I need to put a small rare earth magnet somewhere on the rotor? I've looked around on the forum, and I haven't seen any specific write ups about putting a magnet on.

http://www.xs650.com/forum/showthread.php?t=12253

Use the google search to the left, right under the "Home" tab - it will give you more info