Hi, Anonymous you need to make sure you have no intake leaks by spraying some Brake Kleen, Contact Cleaner, Starting Fluid, etc. around the intake manifold and seals, if the RPM's go up or down significantly then you have a leak that needs to be fixed before you can continue with the carburetor adjustment.
The engine should be at operating temperature with the choke off to set the idle between 950-1050 rpm. Turn the idle skrew clockwise to increase the idle speed and counterclockwise to decrease the idle speed. If you have access to your air-fuel mixture skrew you may turn it as well, if not there is a video below on how to gain access, make sure you start with clean or new spark plugs. Gently turn the air-fuel mixture skrew clockwise until it starts to bottom out "STOP" do not over tighten as it will damage the taper on the end, now back the skrew out 1-1/2 full turns to establish a starting point start the engine and check the idle rpm's and turn the idle skrew accordingly for 950-1050 rpm if necessary, turn the air-fuel mixture skrew 1/4 turn at a time and let the engine settle for about 10 seconds, turn the skrew clockwise for a leaner mixture and counterclockwise for a richer mixture, you are seeking the highest rpm your air-fuel mixture skrew can acquire without going past 3 full turns from the bottom then resetting the idle skrew back down to 950-1050 rpm If you can not detect any rpm change and can not stall the engine by turning the air-fuel mixture skrew all the way in then your pilot/low-speed jet is too big and needs to be replaced with the next size smaller. Final adjustment should be made with a clean air filter mounted to the carburetor.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day. http://www.kymcoforum.com/index.php?topic=9388.0 https://www.youtube.com/watch?v=KNaIEwy2Tkk KYMCO VENOX 250 Service Manual Kymco Scooter Parts ATVs Scooter Parts Performance Scooter Parts... https://www.kymcousa.com/admin/dealeronly/ownerman/onroad/Venox250.pdf
If it idles and revs fine, then make sure there is no binding in the throttle cables. Turn your handlebars, right lock to left lock while twisting the throttle open to closed. Everything should operate smoothly.
Hi, Mohommed if your bike has been sitting idle for months or years and you did not do any pre-storage maintenance I feel your pain it will probably have a dead battery and not want to start or if it starts it will not idle unless the choke is full-on and run poorly then stall, here are the following steps necessary to complete in order to get your bike back to an acceptable running condition and in the future pour in a bottle of fuel stabilizer and injector cleaner for you FI folks at least 2 times a year and before storage.
1. If your battery was 2-3 years old when you last had the bike running you should replace it.
2. If you believe your battery might still be serviceable remove it from the bike and put it on a 1 or 2 amp trickle charger for 24 hours. If it is the old lead acid type with visible cells and acid levels fill each cell to the top line with distilled water and replace the caps, run the vent tube into a plastic or styrofoam cup, any cells that are cloudy/milky replace the battery.
3. After charging the battery remove the leads and let it sit for a couple of hours then check the battery voltage with a voltmeter, you should have 12.5 volts or more, any readings in the 11 volt range you need to do a proper "LOAD" test on the battery and replace as necessary, you may have 12.5 volts or better but little or zero amps, any readings in the 10 volt range you have a dead cell and the battery needs to be replaced.
4. Drain and flush fuel tank if it rusty there is a cheap and easy fix. https://www.youtube.com/watch?v=fUYr_7SwGms
5. Remove and inspect your air cleaner paper elements that are not oil soaked can be cleaned with a soft brush and low pressure compressed air, oil-soaked elements must be replaced. Gause mesh and foam elements can be cleaned by soaking them in a container big enough to completely cover them with a solution of 1 gallon of water and 1 oz. of Dawn dishwashing liquid for small and medium size elements, for monster size double the formula and let soak for at least one hour then rinse with warm water shake off excess and let air dry, "WARNING" do not use compressed air as this will embed micro-sized dirt and road grime and destroy the mesh pattern and stretch foam elements out of shape just squeeze it like a sponge and let air dry, use a fan if you're in a hurry. When completely dry spray a very fine mist of air filter oil evenly around the whole element.
6. Remove the carburetors, disassemble and decontaminate with a "CARB DIP" or if you have EFI remove injectors and clean with carb spray and compressed air
7. Check intake manifold and seals for leaks and craks.
8. Remove fuel valve and filter disassemble and clean as necessary, remove, clean, and inspect fuel and vacuum lines and replace as necessary.
9. Replace spark plugs with new ones and check for spark.
For more information about your issue and valuable "FREE" downloads that you will need for viewing or printing please click on the blue links below. Good luck and have a wonderful day. https://www.google.com/search?q=Fuel+overflow+2003+Kymco+Venox+250&tbm=isch&source=univ&sa=X&ved=2ahUKEwivg7_trbnjAhUDCM0KHZ5FCRsQsAR6BAgFEAE&biw=1440&bih=757 https://www.allstate.com/blog/common-fixes-when-your-motorcycle-has-a-fuel-leak-video KYMCO VENOX 250 Service Manual Kymco Scooter Parts ATVs Scooter Parts Performance Scooter Parts... https://www.kymcousa.com/admin/dealeronly/ownerman/onroad/Venox250.pdf
Possible failure of the ignition unit. you need to trace the voltage to the coil, the trigger voltage if absent then the ignition system is defective. Check the input and also the 12 volt to the coil , if coil is heating it can be faulty.
Leaving the gas in for 2 months won't hurt anything, my bike has been sitting all winter and is ready to go, except for a dead battery. Are you storing it somewhere that requires this? Just drop the fuel line and let it run into a container. You will always have fumes, and the carbs, if it has them, will still be holding gas.
You can download a User Manual for your motorcycle from here This bike most probably has a vacuum operated fuel petcock that turns off the fuel automatically when the engine is not running. On page 40 of the manual I spotted a "fuel hose connector" that looks suspiciously like a petcock. Worth checking out. Cheers