Not running well after warm up

[Craig]

Hi everyone. This is my first post here. I have been working on motorcycles for many years, and am pretty good at bringing old dead motorcycles back to life.
My son has an 82 400 Maxim. It runs great for about 30 minutes, then looses about half its power between 2K and 6K rpm. It idles fine. At full throttle, it gradually makes it way up to 6k rpm and kicks back in. Happens in all gears. Carbs have been rebuilt. Fuel is flowing. It is running on both cylinders, both ignition coils ohm out correctly. Because this is only happening after the bike is running awhile, I'm thinking it is either the pick up coils or TCI breaking down. Can someone confirm the pick up coil resistance of 120 ohm is correct, as stated in the 82 xs400 service manual. I am reading 30 ohm on one pickup and 50 ohm on the other.
Thanks.

 

[NewHavenMike]

Running conditions that change with engine temps seems like it's always carburetor related.

Original mufflers? Stock air filters and intake tube?

 

[16VGTIDave]

30 minutes running time is about how long it takes to get the engine up to full operating temp (oil above 80C /175F). It is possible that the ignition could be the issue, if the symptoms occur regardless of throttle position. The wiring harness for the pickups runs close to the countershaft sprocket and could have been damaged. If your pickup coils are metering that far out of spec, you may want to change them even if they aren't the problem. The TCI modules are known to go bad and are getting more difficult to find. When mine went bad, it was initially at higher RPMs, then it would cut out randomly.

That said, 2k to 6k RPM with lots of throttle would be roughly the operating range of the needles in the carbs. Are the diaphragms intact? Slides moving freely? Are the needles or the needle jets worn? Are the clips in the middle groove on the needles? Have you checked the float levels according to the manual (with the carbs mounted on the bike) as it sounds like the fuel level may be too high.

 

[Craig]

I found pulser coils on E-Bay for a 86-90 Yamaha radian. They physically looked the same and when I received them, they ohmed out at the correct 120 ohms. Installed them Friday night and went for a 2 hour test ride Saturday and another 2 hour ride today. The bike had no running issues at all, so I'm assuming problem solved.
Next up, is to fix an oil leak. I'll start a new topic.