I have a 1963 Allstate with a 150 cc upgrade. The complete fuel system was replaced with new parts (tank, fuel tap, line, carburetor). It starts easy and runs very well.
Today I went for a short ride and about at the 3 mile mark while cruising at 30 mph, the bike suddenly acted like the brakes were applied. I quickly pulled in the clutch and it then started to roll freely. The engine was off. When I came to almost a stop I shifted into 2nd gear and let the clutch out. The engine started and I was able to ride back to home. This has happened before so I put on a new carb SI20/17 which has a 98 main jet. I had previously drilled a 1/16 inch hole in the gas cap to eliminate the vapor lock problem. What is the likely cause of this sudden stoppage of the engine while going down the road?
I also notice a slight sluggishness when I turn the throttle from idle to just off idle. It happens more when the engine is cold. I have tweaked the air mix screw for best idle. BTW, when I removed the rubber plug on the back of the air box to adjust the air idle screw, some fuel came out and ran down the engine. This does not seem right. Below is a photo of my spark plug after returning home from the ride where it suddenly quit.
Any help on the sudden engine stoppage while riding and the sluggishness from just off idle will be most appreciated. Needless to say, I do not want to venture too far from home with a ride that is not dependable.