engine runs at high rpm but coughs and spits at idle.any ideas why.

 

The pilot jet in the carburetor is probably clogged. Just clean out the carb.

 

What kind of bike is it?
The Prairies are notorious for this when its cold and mine has done it since it was new as has every one of the other 5 or 6 that I have been around.

After it warms up though you should loose the couch for the most part.

 

Backfiring through the carb at idle is a lean condition. My old big bear did that until I got the carb good and clean.

 

May try to adjust the pilot screw mixture[idle mixture screw] try adjusting bout 3 full turns out to start with an see if it helps..If not then do like suggested clean carb very good.

 

Yep, that is the typical diagnostics on the coughing, but I've never seen a Prairie V Twin that didn't do it even if it was brand new. I have seen three that have done it from the day they rolled off the assembly line, and know of several others in the family that also do it although they were not purchased new.
After they warm up, they run fine, but when cold and without the choke, they cough badly.
Like I said, Its just this bike that I've seen it do it on... That's why I asked what bike he had. Some people don't realize that they are prone to it so badly.

 

I wonder if richening the pilot screw 1/4-1/2 turn on each carb would help? Might be worth a try. I know a lot of ATVs are tuned a little lean from the factory to help with emissions I guess.

 

Fixed the thing last week....pulled thr carb off threw in corner of shop....bought a new Mikuni manual side type carb.....all I can say is SWEET...runs great.