Hello ! Weather is getting warmer in Toronto. I start to ride so happy.
Problem I have now is no matter how I adjust idle mix screw bike always backfiring except with half open choke on.

With half open choke - no backfire
with choke close - always backfire
Both condition is anytime condition when bike is in idle and riding after engine warmed up. So far I replaced spark plug, cleaned carburetor, replaced battery.  

How can I fix this issue? Thank you for help in advance.

Everything is stock condition.
Mine is 2008 with only 14880 km or 9246 mile.
Thanks

Thank you advice.
If I ride with half open choke all the time what is going to happen?
Is it going to damage engine?

Riding with the choke on isn't a great idea ,  If your bike is backfiring when you close the throttle all the way, leaving the choke pulled out all the time will mean your bike is running rich any time your not at idle. This will allow your combustion chamber and sparkplug to be covered increasingly with soot., fouling the plug (can be cleaned) but could form glowing hot spot in the chamber which may cause detonation (very bad) or the unburned fuel could pass your rings washing oil from them and the cylinder walls causing premature wear. not to mention killing your mileage. The TEV valve on the right side of the carb ,under the small plate with three screws , there is a diaphragm and small spring under that cover . when the throttle is closed the vacuum downstream from it goes high and travels up a passage through a small hole in the "finger" off the diaphragm , defeats the spring pressure opening the valve ,sending extra fuel from the bowl ,down stream of the throttle plate to make the fuel mix richer and prevent misfire, backfire, or stalling . If the diaphragm has holes / cracks (other than the one in the finger) or the passage behind that normal hole is plugged , the valve won't open . You 'll need to clean the passage by spraying carb cleaner or inserting a fine small copper wire ,or both , to make sure it's open .You need to address the problem , not run with the choke on.

Thank you . I just went to test drive with turning idle mix screw quarter by quarter from fast idle to slow idle, slow to fast and tried very gentle throttle too. Result is same every time throttle closed then pop pop or pop pop pop backfire and also when I down shift, slow down then backfire. I will try Mr. batman's suggestion. Does backfire will damage engine?

backfire occurs in the exhaust ,and should cause no damage ,except maybe hearing loss.  

kylejang wrote on 05/01/18 at 14:51:45:

You can check for exhaust leaks on a running engine with a lit candle.
wave it around the joints, if it flickers, you have a leak.
use wd40 on the intake, if the rpm picks up, you gotta leak.

I wouldn't do the white spacer mod until you have the jets right.

kylejang wrote on 05/02/18 at 17:57:22:




It's the rubber boot/tube where the carb is connected to the engine.