A bad centre seal normally makes it much harder to start, and sometimes difficult to idle. The revs will also often "hang" if you blip the throttle, or when riding and slowing down to a stop. You'll usually see a significant decrease in bottom-end performance too.

It's very difficult to diagnose without seeing the bike, but if the crank was rebuilt by reputable agent, it sounds more like it could be an air leak. Maybe the inlet manifold, or the crank seal behind the flywheel. You can spray a little carb cleaner in both areas and see if the revs rise, which would indicate a leak, but a proper leak down test is the only real way to be sure. That will also [usually] show up a failing centre seal.

However, you want to check the simple things first: carb balance and air screw settings.

To potentially reduce the number of more generic replies, or maybe prompt replies from members with specific experience, t would help if you put the model in your profile, or stated it in the opening post.
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Andy.
NSR-WORLD.COM

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