I'm writing because I'm stumped as to what's wrong with a 1977 Moby (in gorgeous yellow) that I recently bought. This may get a bit lengthy, but I want to fill you all in from the "git go" as to what I've done so as to eliminate the usual quick diagnoses notes coming back and forth and using precious web space and even more precious web user time.
Symptom: Bike starts up fine when left for a period (i.e. overnight) and rides beautifully (plenty of power, no backfire, smooth throughout rpm range etc) for about 2 miles or so (and that's stop and go moving (red lights and such)) and then (whenever the "problem" starts) if I start to decelerate for a red light, there is this point at which I reach in the rpm range where it just won't come back up. It's almost like I ran out of gas, but that is not the problem. The bike then won't start up again until I let it sit for about 30 minutes or so.
Things I've considered/ done: 1) I first thought I was out of gas. Not the case. The petcock flows smoothly in both on and reserve. Off works well too. I changed the filter (external) and fuel line too. The vent on the gas cap was open (No vacuum lock (I've had that happen to me before--embarassing too)). I checked the float, thinking it was stuck. I ended up sanding down the seam on the float a few thousandths just to "know" that it wasn't touching anything. It works smoother but still the same problem.
2) I pulled the carb and cleaned it up. I then switched carbs with a friend but the same problem.
3) I thought my plug was the wrong heat range, so I switched plugs with a friend, and still the same problem.
4) Then I figured that the exhaust was clogged with carbon, so I pulled it and it is spotless. The same for the piston and the exhaust port. (I was really upset by this because I was convinced that that was the problem.)
5) I knew that I was getting spark the whole time, even when it wouldn't start because I checked and because when I tried to start it, if I held the decompressor lever down a long time, I could hear the backfire in the exhaust, so I knew I was getting spark.
6) The only thing I didn't do was check the point gap, and timing (since it ran beautifully when it ran), but I attacked that yesterday. I filed the points, and regapped them. The timing was off a bit, so I re did that to spec. The bike had slightly less power, but it was running smoothly, and I thought (I've got it now. The timing was a little too far advanced which was good for power but maybe I was getting heat from being really close to preignition (I never heard any pinging.) I rode around the hood but when I reached that critical ~2 miles, the problem came back. Argh!!
Does anybody have any ideas?? I've given the bike to a friend, but she is not mecahnically inclined. Also my pride wants to know that the bike is fixed. Please help me!!