power loss
 

Hope you guys can help. I bought a 2007 GZ 250 for my wife to learn to ride. Just trying to make things simple for her and the bike was a good deal. It had been sitting mostly unused for at least three years but only has 4300 kms and seemed to run great.
Unfortunately what is happening is that the idle is erratic straying from 700 to 2000 rpm at any point. When it gets warm it has a bad spark-knock when leaving a stop and it keeps having a momentary power loss at higher speeds. More of a quick jerk than a real loss.
I've run a bunch of Seafoam through it and am now running premium fuel with a detergent. The idle seems to be better but the high speed jerk is still there.
I suspect that it is running lean but am unfamiliar with this carb design and don't really know where to go.
Any help would be great. My wife is spooked and refuses to ride it. I'm afraid it's going to melt down.
Long story. Thanks in advance.
I would also like to know how many teeth the drive sprocket is supposed to have. It was supposed to have been changed.
Thanks

my guess would be that the carb is gummed up. If it has sat that long especially with fuel in it then this is almost a guarantee. a very lean condition will cause all of those symptoms. you can always read the plug as well but i wouldn't suggest riding it till you get it fixed. You should be able to get it professionally cleaned for pretty cheap if you can't do it yourself. The carb is pretty simple the only abnormal part would be the needle diaphram setup in my opinion. if cleaning isnt for you then you can always buy new jets for usually $5 a piece, but you will still need to clean all the channels, float valve etc. Also the stock sprocket up front is 15 and 41 for the rear.

Although kilo 0195 is probably right about the carb needing attention, you could also have an issue with the petcock. Before I rebuilt mine, my idle was all over the place. I had to put it in gear and hold the brake to get it to idle down and then sometimes it would creep back up to high rpms. Having sat that long, it is probably a combination of all of the above. Lacquer has coated the jets and other surfaces and possibly even the petcock and is causing your problems. Seafoam will certainly help alleviate some of the problem but depending on the severity of it, you may have to go in and manually clean it and/or replace jets etc. to get it back to normal. Not expensive if you do it yourself and it would be a good learning process. Lots of info on this site.

Very true... I didn't even think of the petcock

Get a carb rebuild kit off ebay and replace all the o rings. I am pretty positive that your problem is with the idle circuit. The brass plug will need to be drilled out to get to the jet and O ring. That circuit is responsible for about the first 25% or throttle travel as well as idle.

These Videos are great for finding your way around a CV Carb
https://www.youtube.com/watch?v=IBFq9nHVmks

https://www.youtube.com/watch?v=3Fmf...bhrHn5eNh0CE2g

https://www.youtube.com/watch?v=g6y3...bhrHn5eNh0CE2g

See the attached photo showing where the pilot jet lives on the GZ