640 Adv won't drop below 3k, intermittent...
 

Hi,
I've a problem with the running of my 04 640 Adventure. If I come off the gas whilst riding the bike won't drop below 3k. If I pull the clutch in it won't drop, but if I leave it in gear and brake to slow the engine below 3k, it's then fine. The problem is intermittent and the idle is set to approx 1800.

Thanks in advance,

Rick

Occam's Razor
 

Check the throttle cable - could simply be sticking.

Other possibilities - an air leak in one of the carburetor's boots. Lean conditions can make engines "race", then settle, eventually.

idle at 1800rpm is way to high in the first place.. it should be around 1500 to 1560rpm for a start... try adjusting the mix.

How many miles do you have on the bike? Any significant use with bad fuel? It could be the carburator jet needle and needle jet that are worn out. New parts are about $30 and will help to keep gas mileage at a good level.

The other place to ask for help would be advrider, maybe check the LC4 thread there.

Sounds like a leak on the vacuum side of the carb as Genghis said.
Have a look at the carb rubber for cracks, test it whilst running by painting some oil onto the rubber with a brush.
Do the obvious carb strip down and clean.
Sticky slide perhaps?
Good luck with it.
Dave.

Also check the vacuum line that runs from the fuel pump to the right side of the intake manifold on the head....Look to where the hose bends before it slips over the manifold spigot as it will sometimes develop cracks and allow air to leak in...