This is either a vacuum leak, or lack of fuel

• Spray carburetor cleaner around the the throttle body gasket and mounting gasket. If the idle changes then you found your leak.

Other places for vacuum leaks:

• Intake manifold
• Vacuum hoses
• Carburetor mount
• Throttle body to float bowl gasket.

Replace or clean your fuel filter: Some of these carburetors have a filter inside the fuel inlet.

Adjust the idle mixture: Start by turning it out slowly. If the RPM gets better then you may have found the problem. Make sure the engine is at operating temperature and the idle is at specification. Turn the idle mixture screw out 1/4 turn at a time, wait a second for the engine to catch up, then turn out again. Do this until the RPM doesn't increase. Start turning it in 1/4 turn at a time until the RPM starts to drop. Turn back out 1/4 turn and leave it.

Test the fuel pump: Should be about 4 lbs. If too low it will starve for fuel.

Check needle and seat: The needle may be sticking in the seat and not allow enough fuel to enter.

Blocked Passage: Dirty passages will cause fuel starvation. Check each passage carefully by blowing air through each passage.Take a look at the illustration. You can see where the fuel flows from the float bowl to the idle mixture screw. If this circuit is dirty, then you won't get enough fuel to idle.

Check float level: Set to specifications. Fudging the float level will probably introduce another problem.

Get a manual on this carburetor

It is full of carburetor theory which will help you understand how things work.