throttle bogging down until engine is warm

howdy. i've got an 81 xs400 that up until now has been running great. weather has started to turn a bit and now when i start the bike, until she is pretty well warmed up, she will bog down when i pull the throttle at anywhere below 3 revs. above 3 revs the throttle response is fine, and once she's warmed up same story.

any ideas?

sucked up a bit of crud making it run a little lean. open up the pilot mix a turn and if it improves, that's what it is!

He's right.

Below 3,000 rpms primarily the pilot/idle circuit is providing fuel.
As the engine warms up the air entering it warms up, is less dense, and contributes to creating a richer mixture.

I always like to know why so I can figure out other problems myself in the future

Makes sense. Had to go from zero and start all over again but got it runnin ok. I couldnt tell when the pilot screws were all the way screwed in and didnt want to force it. Alot easier on my old Honda.

Thanks for the help.

If the carbs are off the bike you can see a tiny pin hole inside below the idle screw. When the screws are closed all the way they just poke through this tiny hole. If the carbs are still attached the process is a little sketchy unless you remove the screws completely and know for certain the threads are not bunged up and providing resistance. Having cleaned mine a lot I can screw them in by hand against the spring pressure. When I meet resistance and the screws no longer turn smoothly I have closed them all the way.

So, did adjusting the idle mix help? Did you clean the pilot jets?

bump

sooooo pretty common problem then aye?
My bike just stared doing this. Thought it might be timing or contact breakers( pitted so I filed with nail file stuck to a feeler gauge,cleaned and re-set)
My pilot screws are still capped and I wasnt sure if I wanted to mess with them
If this dosn't improve the situation then suggestions recomendations?

My bike boggs or hesitates at initial throttle but then takes off.
I was bummed thinking 'fat guy-little bike' but feel better knowing its a common problem
sheeesh

You're simply too lean or the engine is still not at max temperature. I'd uncap the idle screws and turn them out as a first course of action.

This may be illegal in Ca though!

If it is, you can try raising the main needle.