I've had this bike for about 2 months now - owner claimed it had this same problem. 964 miles, so I've put about 30 on it in that time. One thing I'll admit to (and may very well be contributing) is a mistake I made when I did the head. When I took it off I noticed the head bolts were stripped and whoever did it before me managed to just catch the next thread, gave it 1/4 turn and that was it. So I ordered some new studs and bought a really cheap $20 click type torque wrench from discount. Bad move. I used it to torque my plugs in my car, to torque the plug in this batavus (with the head off), and to do my lugnuts. When it came time to put the new studs back in, I set it to 11 ft lbs, and started going to town in the cross pattern. It started to get a tiny bit difficult, then it snapped at the head end. I didn't realize it until I broke 3 studs. One of them in the crankcase. Lousy wrench had stopped clicking. So 3 studs later, a broken screw extractor, and a week of yelling at it, got the stud out, but unfortunately I noticed a hairline crack.
Its in the bottom right stud as you look at the sparkplug hole. It extends diagonally from top left to bottom right for 3-4 mm towards the screw hole, and also extends 2mm past the other side of the screw. I cannot find where this crack leads, and I'm assuming it just cracked the casting there. It doesn't seem to enter the crankcase or to the outside. I attempted to press jb weld (liquid kind) into the crack (it is VERY tiny and shallow) let it set up overnight, and hoped for the best. I ran a 6MM 1.0 pitch tap through all the holes with no resistance, and no metal, so I believe the threads are OK in the crankcase. Had all new bolts lock washers and flat washers, and put the studs in AOK. I torqued in an odd way since I don't have a wrnech. I tightened until the 10" ratchet could support a gallon of water (8.3lbs), approximating 6-7 ft lbs, then gave 1 more 1/4 turn, then an additional 1/8 turn. The cracked hole gave no indication of any problem, the stud went in fine and it torqued AOK.
Just some background. Does anyone think the crack could be contributing? I don't believe so. The bike ran just like this when I first got it, (i.e. hard to start, would stall, but once running, ran for 10 minutes.) Going over the ignition, replacing the condensor and timing, new plug, and cleaning the carb got it so before the stud incident it always started on the first kick, and ran for 10 minutes. Now although it did start on the 2nd kick after the head was back on, and ran like a dream for 10 minutes, it is now difficult to start. Sorry this is so long I'm just going over everything that was done, so I have an idea in my head.
I think your right on the fuel delivery. I did replace the fuel hose with black thick "fuel line" hose from Lowes. and I do have a clear fuel filter inline. There is fuel in it. I cannot find the brass screen in the banjo nut for the life of me, I had it apart twice now, so I think I may not have it.
I do have the white plastic hollow float, and Chris at MWH recommends that I replace that with the black solid float, due to a recall. It sounds good, and I'll be doing that in the near future. I think I'm also going to be getting a new top end of the float (top cover I think), and gasket and banjo and screen. I am intermittently dripping gas on the pavement and I think that might actually be flooding the engine, causing the problem. I think that might actually fix this. However, the dripping gas doesn't coincide with the stalling.
My GF watched it happen and with her limited engine knowledge said that she thought it was just overheating....