Can anyone advise yet again!
After fitting a 2nd hand cylinder head to the front cylinder I'm finding that much of the time the engine runs only on the rear cylinder.   I seem to be getting a spark ok at the plug and fuel is getting in ok.   When I start the bike from cold the front cylinder
refuses to run.   I can drive a few miles only on the rear.   There was some wear in the cylinder bore - piston slap noise is quite apparent.   I havent done a compression test but could this be the problem?   If it was I would have thought the problem would be there all the time, but sometimes it runs ok on both cylinders.   Also the front inlet stub was ripped but I repaired it and it seems to airtight now.   The woodruff key came adrift from the front camshaft sprocket with the result that the sprocket turned while the camshaft was stationary - I drove like this for 3 days before I noticed that the front cylinder wasn't getting hot!   I put it all back together and it runs ok apart from the above.  
Anyway any advise would be much appreciated.
Thanks

Hi Pete
Do a compression test.
You might as well have bought a rear cylinderhead, so you are running 2 rear heads and cams??? this will not work as the cams cannot be interchanged as far as I understand.
Last time I have a Virago running on one cylinder, the float was not floating, that way drowning the cylinder.
Just some ideas , Hi from Erik

Hi Erik Thanks for your suggestions.   I'll check the things you mentioned.   Regarding the rear cylinder head being different, The 2nd hand one I bought has a number 2 stamped on the camshaft so I'm pretty sure it's off a front cylinder.   The engine does run on both cylinders eventually.   I also wondered whether I assembled the replacement head with the valve timing 180 degrees out.   I'm not sure if this is possible - would it run at all if this were the case?
I also used the original head gasket on the front cylinder- I wanted to see if the new head cured the original problem before buying new gaskets.   So this may be leaking as well.   Occasionally I find that if I remove the rubber stub between carb and frame air intake there is very little suction compared to the rear carb - low compression again?
Anyway I'll keep investigating.   Thanks again.   Take it easy.

I had the same problem, usually after the bike had been parked for a while.  There are vent tubes coming from the carbs and running to metal tubes which then run along the frame rearward to the airbox.  Test your bike this way:  when the engine is running (on one cylinder) unplug the rubber vent tube of the carb feeding the dead cylinder (always the front on my bike) from the metal tube. You will need to jack up the rear of the tank for access.  On my bike, this results in immediate perfect operation.  The solution is to rod out the metal tube all the way back to the airbox.  I have found that a bicycle brake cable about 18" long works best; it's flexible and the right diameter if you leave on the road bike mushroom end.  Don't be surprised of you discover cobwebs and dead bugs clogging the vent tube.  This worked for me.

Thanks stephen  I'll try your suggestions and see if it helps.   I think I've still got an inlet manifold leak on the front cylinder which wont help either, so I'll attend to that as well.
Thanks again, keep you posted.
Take it easy!

Hi Peter
If you turn the timing 180 degrees the pistontop will hit the intake valve as it then is open on tdc.
A compresion test is a good indicator of problems se if you can borrow one
Low compression - have you tighted the valve ajusting screw to much so the valve is not closing - just an idea.
Take care  - hi from Erik