04 vinson 500 wont start without choke and backfires when i let off the gas.  Just changed plug! HELP  

Sounds like too much air....carb. tight?  air filter tight ? or carb is dirty, trash or gummed up.  

valves?  could be a burned valve. could be a head gasket as well.

to check the head gasket one easy way to do it is get it running, and spray crab choke cleaner/starting fluid, around the where the head gasket is and see if it idles up or stalls out.

also spray around where the cylinder jug and the crankcase bolt on.
if there's a leak it will either bog down or idle up when sprayed.

if that's not it and it is 4 cycle check the valves.  

I HAVE 1994 POLARIS 300 SPORTSMAN.PLENTY OF SPARK BUT DOSNT SEEM TO BE GETING GAS TO SPARKPLUG.  

could be alot of things,

the first thing i would do is remove the carb. take the bowl off see if it's clogged or dirty.  also remove the top and pull the diaphram out inpsect it for tears or abrasions.
I don't believe the 300 sportsman has reeds. but i could be wrong.  if so check them to see if they are worn or broken.

if you clean the carb out and it still wont start, it's possible that the rings are worn and compression is down.

the engine draws in fuel from the carb using compression, (duh you probably knew that) but if the rings are worn out the compression will be down and there will be more back pressure from the exhaust.
meaning it wont pull fuel into the cylinder.  a quick easy way to check is:
open the throttle up,
put your hand above the air filter inlet.
if it is blowing air out of the air filter inlet when it turns over then your rings are down.
(this works if you don't have reeds)

if you need new rings it's really not hard to replace them.

Remove the carburetor and intake hose;

remove the cylinder head bolts, ( i think there are six on yours) and remove the cylinder head.
then remove the cylinder (four bolts) you may have to remove the gas tank, and any carburetor snorkling the polaris may have. remove the exhaust and flange, remove the engine mount and bracket from the cylinder,
once you have all this done slide the cylinder up and off the piston.

inspect the cylinder for scratches or gouges, and inpsect the piston for the same.  the rings are located on the piston, remove those, reinstall the new rings, and it's best to replace the wrist pin for the piston as well.

clean the old gaskets from the cylinder head and the cylinder and crankcase, reinstall new gaskets, cylinder, cylinder head, carb intake, carb, and exhaust.  retorque all bolts to factory specs.

once you have everything back on give it a start.

I had both a 1995 polaris trail blazer and trail boss, they were really bad about burning up the rings.  

you could also check the cpmpression with a guage.  i think the 300 needs to have between 125 and 140 psi.  

1st of all I want to thank you you solved my problem by ansering his, J took the head off my 400 Xpoyer and it looked like they were running it on starter fluid, , I ordered new piston rings and gaskets, my question is there a trick to getting the pot bacl on over the new piston, I didn't pull the engine so I will be putting pot over piston...
that is kind of opposate from what i'm used too I usully compress rings and slide them down into the block...