I've read in old threads where a number of people had nearly identical problems as mine but never found posts of anyone's solutions. I've got an '02 Sportsman 400. It will start fine and idle smoothly but, when I try to bring it up off of idle, even in neutral, it will bog down and the engine won't acclerate. It does this even with air filter removed. However, if I remove the air box and the tube so that nothing is connected to the carb inlet, it runs like a scalded cat. With the air box on but no filter, I can deflect air slightly with my hand as it goes into the tube and improve it so that it will accelerate. I've been through the carb, soaked and cleaned it. All looked good, float height is normal, etc. Diaphram is good and slide pulls all the way up. Has new plug and a hot spark but plug gets black quickly when fooling with the thing. Checked timing - OK. In process of pulling all the stuff off to be able to inspect the cam and valve lash. Don't believe it is that because of the way it runs without the air box & tube installed.

This thing sits most of the time and only has 140 hours on it. Symptoms sort of appeared some time ago.

I'm sure it's something simple - I just haven't figured it out yet. Please help!

 

fuel pump working ok and the vacuum lines all hooked?

 

Check the air tube for restrictions.If nothing restrictingg you need to check engine vacuum

 

sounds like pilot jet is plugged, maybe try raising the needle in the carb a notch .

 

Well, I believe the pump is working okay as it flows plenty of fuel. W/O air box installed, bike did 50+ mph on a test run. With the box... blaaahhh.

The only vacuum line I'm aware of is the pump impulse line that connects to the right side fitting on the carb. Other lines are two vent lines that go to air box and don't make a difference whether or not they are hooked up.

Don't believe tube between carb and engine is cracked because I've sprayed stuff around it while idling and had no change. I've not removed it for inspection but plan to do that.