is it carb or ignition problem?

[wrgreen]

Hello all. This is my first post. I am wrgreen an artist and lover of all transportation. Live in SoCal. Building a 1980 SX650 with my son. We have come upon this problem ... starts great runs good when cool but when it gets towards warmed up it will not rev up. It just comes up flat like a rev limiter on a car. The carbs have been apart and all looks good. I am starting to think it is related to ignition but a bit confused how the heat could alter any of the components. I could sure use some help

 

[Paul Sutton]

Could be magnetic interference from the alternator rotor's field onto the magnet for ignition timing. If you manually rotate the alternator rotor and watch in the gap by the timing marks, near the outer edge you will see a round patch with a square-ish patch of copper around it. This is the magnet for the TCI ignition. Give the surface a little clean with a bit of steel wool or fine wet/dry paper then try running the bike again.

The alternative is to disconnect the rectifier/regulator plug from the wiring harness so the bike is running off the battery. This will cut out power to the rotor's electromagnet so that the ignition's magnetic pickup will not be drowned out by the strong rotor field. The connector is a big 6 way located on the left side of the bike behind the side cover. You will see 3 white wires from alternator going to it.

As the magnet ages it loses strength and eventually its field strength gets overwhelmed by the field of the rotor. Sometimes just cleaning the little magnet's top solves the problem. Sometimes you need to glue a new 0.5mm thick Neodymium magnet over the top.

Hopefully this is the issue.

 

[5twins]

Well, if the coil is going bad, that often only acts up once it gets hot. There are tests for the coil, measurements you can take, but it will most likely test good if done cold. You'd have to test it hot when it's acting up to get a test fail. A good, cheap, very dependable replacement is an MP08 coil from a Honda.

 

[wrgreen]

Could be magnetic interference from the alternator rotor's field onto the magnet for ignition timing. If you manually rotate the alternator rotor and watch in the gap by the timing marks, near the outer edge you will see a round patch with a square-ish patch of copper around it. This is the magnet for the TCI ignition. Give the surface a little clean with a bit of steel wool or fine wet/dry paper then try running the bike again.

The alternative is to disconnect the rectifier/regulator plug from the wiring harness so the bike is running off the battery. This will cut out power to the rotor's electromagnet so that the ignition's magnetic pickup will not be drowned out by the strong rotor field. The connector is a big 6 way located on the left side of the bike behind the side cover. You will see 3 white wires from alternator going to it.

As the magnet ages it loses strength and eventually its field strength gets overwhelmed by the field of the rotor. Sometimes just cleaning the little magnet's top solves the problem. Sometimes you need to glue a new 0.5mm thick Neodymium magnet over the top.

Hopefully this is the issue.

I will give it a try. Thanks so much

 

[wrgreen]

Well, if the coil is going bad, that often only acts up once it gets hot. There are tests for the coil, measurements you can take, but it will most likely test good if done cold. You'd have to test it hot when it's acting up to get a test fail. A good, cheap, very dependable replacement is an MP08 coil from a Honda.

I was considering the coil and this sure helps. Thanks