Why when i twist throttle does the engine bog down then run fine?

SOURCE: hd carb idles fine will

You didn't say what year or model bike it is but I'm going to assume that it has the stock Keihin CV carburetor on it. You need to check two things, the accelerator pump and the slide vacuum diaphragm.

To check these, first take the air filter off but leave the backing plate on the carb. With the engine OFF, crank the throttle wide open while looking into the mouth of the carb. You'll see a brass nozzle just inside the mouth of the carb. When you crank the throttle open, you should see a squirt of fuel come out of this nozzle. If not, you need to replace the diaphragm in the accelerator pump that is on the bottom of the carb float bowl.

If that is working correctly, start the engine and gently rev the engine while watching the slide inside the carb. DO NOT GET YOUR FACE TOO CLOSE TO THE CARB IN THE EVENT THE ENGINE SPITS BACK THROUGH THE CARB. The slide should be moving up and down. If not, the vacuum diaphragm that operates the slide may have a hole in it. You must take the carb off the engine and take the top plate off to get the slide and diaphragm out.

One other thing. If the bike has been sitting for an extended period of time, the jets may be clogged inside of the carb. This will require a complete disassembly of the carb to clean out the jets and the small fuel and air passageways.


Good Luck
Steve