I'm having a problem when I start my 1985 van.  The engine turns over for a second and as the engine catches there is a single noise. I'm assuming this noise is due to a misfire of some sort.  Happens about every fifth time I start the van.  Seems to happen more often when the previous ride was long and the engine fully warmed up.  Otherwise the engine runs OK.  

Any idea as to what this could be?  

Thanks in advance,
Ron Maxseiner
1985 GL 'Stinky Pete'