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The bike is a 2008 Hyosung GV250. The ONLY time I have problems with the bike running is on mornings when the temp is below 65F. After letting it sit running (anywhere between 2 minutes to 15 minutes), I start out ok and then either the bike dies at a stop and won't restart unless the bike is in neutral, kick stand up and clutch in, or it starts loosing power as I ride down the road. If it looses it when I am moving, it will sometimes go from 4-5k RPM down to zero, then back up and down causing the bike to buck. This will usually go on for the first 3-4 miles of the trip, and then will work fine. I have had it happen the whole 7 miles into work though.

For the initial start and warm-up, I have used the choke to start and then turned the choke off and let it idle (per the dealer's instructions), I have left the choke on so the RPM's stay between 2-3k (done for various amounts of time up to 10 minutes). I have tried riding with the choke partially on hoping to prevent it from dying at stops.

Now for the kicker. I can let the bike sit from Friday after work to Sunday afternoon, and if it is warm out, it starts right up and is ready to ride with no more than a minute or two of warming up.

I suspect I just need to take it to the dealer and leave it with him overnight, but I wanted to get opinions from others on the site before I do so.

What might be causing the problems, and any ideas on solutions? I've had carberated cars many times before, and some were a little temperamental on cold mornings, but nothing like this!

I don't have the GV250, but have a similar issue around 50 degrees with my Qlink Legend. If I warm it up I have found that I have to actually with the choke partially engaged for a few miles and then things smooth out. Mine does not drop RPM as drastically, but throttle response is lacking in 5th at highway speed. If I choke it a bit while running it smooths out and runs great Then I can ease off the choke after a few miles and it runs just fine. I am going to check with the dealer tomorrow to see if there is any adjustment that needs to be made, but my feeling is that it takes a long time to warm up to operating temp in cooler weather.

Try the choking the bike a bit and see if you regain power.

no doubt this bike is running very lean at idle. Thats the way the EPA wants it. Im a harley mechanic and i used to work in a US dealer. all carb bikes at set up time had the carbs "readjusted". We drilled out the EPA plug and turned the idle air screw out and drilled out the pilot jet too. The bikes worked much better. we were at sea level too so the bikes were very lean from the factory. I can imagine that after the new legislation that came into force they are even leaner and require a full warm up to run properly. Warmed up means you put your hand on the motors head and its hot enought to have to jerk it away. this could take more than a couple minutes. and yes our "readjustment" was illegal. i advice you to do the same.

I'd be really careful about drilling out any pilot jets. Unless you have a complete drill index, and know exactly what your doing, you will frack up the carb so it never works again.

Thanks for the replies everyone. I have done some more fiddling and think I have figured out at least what is causing it, but not necessarily why, or how to fix it.

Turns out the kickstand down switch has been a problem on these bikes. Bike thinks the kickstand is down even when it is up causing it to die when you put it in gear.

More importantly, yesterday morning when I was only about a mile from home, the bike died again. I was low on fuel so I switched to reserve. Bike started up and ran fine to the gas station. I filled up and left the petcock on reserve. Bike ran fine the rest of the way in to work. This morning, I left it on reserve again and the bike ran fine all the way in.

Appears when the petcock is set to ON there is not sufficient fuel getting to the carbs, but switching to reserve ensures a good flow. How should I go about trying to fix this? Or should I just take it to the dealer since it is only two weeks old and the kickstand switch needs to be replaced anyway?

U can bypass the switch by using a jumper. If fuel is not flowing into the tank, it may be dirty from the factory and already clogged up the filter or the vacuum part of the petcock may not be working and the carbs are pulling all the fuel out of the float bowls at highway speeds faster then it goes in.
to drill a carb, im sorry you realy have to know what you are doing. we use .0018' drill bits (eighteen thou) which was roughtly a #46-47 idle jet. drilling it out this way was cheaper then buying them new. the previous post was correct. dont try that at home.