I bought my 73 Honda 500-4 new in 1973 and still own it today. I always had to warm the bike up initially to keep it running. I would start the bike and then tweak the idle adjust knob a little bit to keep the bike running. If I didn´t do this it would not stay running. However, once I rode it a short distance I would have to reset the idle knob back to where it was so it would run and idle normally. If after a normal warm up as I described above, it won´t stay running you may have other problems.