What causes Carburetor motorcycle engine to backfire on start-up?

Bike: 2007 Hyosung GT650R (Stock, still original parts except normal service replacements oil and filter and adjustments)



My bike has backfired on me three times now since its last service in Sept. twice in october 2 weeks apart and just last week. I have not taken it back to the mechanic yet as it檚 not happening all the time, and I'd like to give some direction on the problem.



The engine is a 90 degree V-twin, with Carburetors. The mechanic adjusted the fuel mixture since he said the bike was running lean. Don't know if that's relevant or not. Prior to the last service, the bike never backfired. each time, it occurred during stat-up. either just as i press the starter button, or it cranked once and then backfired.



I looked on the internet, and other people have had similar comments, but they say it's normal or not a big deal. Needless to say, when my bike goes bang like a gun, it concerns me greatly. Other people didn't mention having this happen, but the tail light bulb was broken twice from the shock.



The weather was not cold (not below 30 F) but wasn檛 really warm either (not above 50 F). Always start the bike with at least 戮 choke, and in neutral.



So basically, any idea what the cause could be, and recommend how to fix it (beside sell it)?



Also, don檛 know if this is related or not. There is a hose between the rear shock and kickstand that I noticed has a liquid dripping slowly out during idling. Revving does not change the rate of drip. But it smells like gasoline. Don檛 know where the hose originates yet. But if this helps of you know what this hose it, please let me know.



Any help will be greatly appreciated.What causes Carburetor motorcycle engine to backfire on start-up?
Mine will backfire if I give it too much choke. Try a little less choke, increasing as you start if needed.

Could be the timing, also.

The hose dripping liquid that smells like gas most likely is an overflow from the carburetor. Probably a float needle not seating properly. Very well could have happened if the mechanic was in the float bowl of the carb. Would also contribute to an over rich condition, flooding carb, possibly the cause of your backfire.What causes Carburetor motorcycle engine to backfire on start-up?
backfire usually indicate fuel mixture being incorrect. you stated the bike shop adjusted the carbs then = backfire. have them readjust the carbs since you paid for good service not a backfire services..hehehe.



those hoses you see are overflow for carbs. any unused fuel get dump out using those hoses.What causes Carburetor motorcycle engine to backfire on start-up?
Too much advance on timing and excessively rich fuel mixture. Or too tightly adjusted exhaust valve (most likely not on a new bike), causes excess intake reversion and back fires. Rev it quickly and see if you get a black cloud (rich).