I am feeling a bit ridiculous going over this problem again!!  I got a new fuel valve and installed it yesterday and also cleaned the plug.  It started right up with the valve on the regular position and I took it out for a run, and in a short distance 200 yards it backfired and came to a stop so I switched over to prime and it ran well.  I tried switching it over to normal position a few times with the same result, it would cut out but would ride on prime (without vacuum line being pinched off).  I filled up with gas and got home (running on prime), and I decided after having it sit for awhile, to start it and run it on regular position.  It ran and at one point revved up and then cut out and I put it away.  This morning I went to go for a ride and it wouldn't start in regular position, in prime or reserve.  I thought that maybe there was an air block so I undid the bottom of the carb and drained some fuel out with still the same result.  I am getting a  frustrated with finding out the problem, I know it is something concerning the fuel valve but don't know what to do next.  
DocNeedles

It sounds like a vacuum leak to me...or a bad diaphram.  Now what are the chances the new diaphram is bad?  Anybody got any lottery tickets?

BTW, have you done anything to your bike that would cause problems with the vacuum?

Also, check your fuel flow and report back with results.  How long does it take to fill an 8 ounce cup on PRI?

barry68v10 wrote on 06/14/08 at 11:54:55:

I did disconnect the fuel line and turned valve to prime and it took 30 seconds to fill a cup.  Today I took off the tank, (tried starting it a number of times with no go) and took out the spark plug which was black with coating, (not oil),  I then cleaned and set my spark plug (plus others I have had around since I have had this as an ongoing problem) and put back the original fuel valve, and put new fuel lines and vacuum lines in.  The bike started, not easily and exhausted some unburnt gas.  I am again running it on Pri with the vacuum line disconnected and plugged.  I took the bike out for a burn and it seems to be running like it was before with just a bit of unburnt gas exhausting when it has sat for a length of time and is cold.
I understand that there is some concern on the island that the gas tanks are contaminated with fresh water and sea water.  That is where I purchased gas the last time when I was trying out the new fuel valve, prior to that I purchased gas at a station where the tanks are high and dry from the ocean.
So, I think that with the brand new fuel valve there was a possibility that the engine was flooding on both the regular setting and the Pri setting.  Any feedback, I would like to understand what is happening with this.  I am almost considering rebuilding the fuel valve as specified in the tech section and just having an on and off valve with no diaphragm.
DocNeedles

Have you checked the needle valve and seat?  I had a run in with a bad needle valve, combined with a finicky petcock diaphram.  
After changing the needle valve and seat, I run now on prime and just keep track of my mileage.