I have a 1983 harley sporster ironhead that pops out of 2nd gear when down shifting. What needs to be replaced??

SOURCE: valve adjustment 1983 Harley Davidson XLH 1000 Sportster

To adjust the valves on your Ironhead Sportster, first collapse all the pushrod tubes, remove the spark plugs, and get the rear wheel off the ground. Put the transmission in fourth gear. Now, use the rear wheel to turn the engine.

To adjust the valves, bring the front cylinder to Top Dead Center. Use a common plastic drinking straw down in the spark plug hole to make sure the piston is at Top Center. Make sure both tappets for the front cylinder are all the way down. Now, loosen the lock nut slightly and turn the adjuster to make the tappet longer. You want to make it just long enough so that you can't turn it with your fingers. Gradually back down on the tappet until you can turn the pushrod with your fingers. Lock the locknut down when done. Now do the other tappet.

Once you've done the front cylinder, turn the engine using the rear wheel until the rear piston comes to Top Center. Always turn the engine in the normal direction of rotation. Adjust the two tappet the same way as you did the front cylinder tappets.

When finished, put the transmission in neutral, lower the rear wheel, put the plugs back in, and close up the pushrod tubes.

These tappets set to Zero Backlash, none. Therefore this must be done on a completely cold engine. When the engine starts up and as it warms up, the cylinders and head "grow" due to the heat expansion properties of the metal. As they grow, you pick up valve lash and the valves will clatter a bit. This is where the old saying came from, "You can tell it's a Harley from a mile away by the way the engine sounds. You can tell it's an Ironhead from a half mile away from the valve clatter". If you get the valves too tight, when the weather cools down in the winter, you could wind up with a valve standing open just enough to make the bike impossible to start due to low starting compression. It's better to have the valves a bit loose and have them clatter than have them too tight making the engine difficult to start.

Good Luck,
Steve

SOURCE: i have a 1983 harley sporster 1000cc and the gear

Yes, it is. The gear is on the clutch shell