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Join Date: Nov 2007
Location: Chandler, Arizona
If it starts good, and runs good for awhile, that eliminates quite a few things. The only things left are gas and ignition. Either the ignition is cutting out, or something is stopping the flow of gas. My Vulcan 750 (both of them actually, I've had 2, a '93, and now an '02) have always had the habit of sometimes starting up on one cylinder, then several seconds the other cylinder kicks in. It doesn't do that all the time, but often enough to notice. Even after I completely overhauled the carbs it still does it. I never found the cause. The '93 had an issue right after I bought it new where the rear cylinder would flood out. It was under warranty, but I did not trust the dealer to work on it after an incident with another bike, so I did it myself. I found the float in the rear carb had a piece of flashing sticking out where the seam was molded together, and it would occasionally catch on the carb body. I trimmed it down, and it ran fine.
You said it starts running on one cylinder, then the other one cuts out later. That means something that can affect one cylinder at the time. Possibly the carbs or pickup coils? It may be difficult to find, since it happens intermittently. By the time you can check it, the problem may have disappeared. Even though the petcock feeds both carbs, I would make sure you are getting a good flow of fuel from that. If you are not in the U.S., I'm assuming you don't have to deal with alcohol laced gas. Has the bike had this problem ever since you bought it? Has it gotten any better or worse? Have you noticed anything else odd? How long do you have to ride it before this issue happens?
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike