Do a leak down and see if it's loosing more than 20% through either the valves or piston rings. You can hear air through the exhaust and also the carb if it is plus if leaking past the rings you can hear air through the crank case vent. I'd still measure the exhaust cam lobe to eliminate it as the problem. Just too many of these years models had cam problems. Plus may be stator,pick up coil or even cdi problems.Quick test on the pick up coil white and white/red wires should show around 97 ohms.Plus you can check the distance between the pick up coil face and flywheel magnetic strip. Shouldn't be more than .040 clearance.If none of this helps then this is where a cheap manual can help with the trouble shooting guides and stator resistance specs in the schematic diagrams.
DOWNLOAD 1996-1998 Polaris ATV Repair Manual ALL MODELS - Download ...

 

The intake valves do leak a bit, took the carb/intake elbow off and can see bubbles around valve seats as I turn it by hand. But shouldn't be enough to backfire..

I was wondering if the cdi box goes would cause incorrect timing. I'll test the pickup as well


Another odd thing.. It will pop/backfire when using ether, but won't with gas.?

 

Ether is lighter than gas and the vapors can try to ignite and back fire out the carb.Cdi controls timing and spark advance,but all manuals show to eliminate other problems first or install a "known good" cdi to test if the cdi is the problem. Plus you can pull the black kill wire from the cdi and see if it helps.

 

Ya, after dinner I'll go and check ohms on components..

 

So put my timing light on it today, timing is retarded. I'm thinking the cdi box is faulty.

If I line up the timing mark(the 3 dash lines on the flywheel to the pointer in the pull start housing the mag pickup lines up, but when cranking over the timing light says its way restarted.

 

The only way to properly check the timing is have the timing light,a good rpm meter(suppose you can go by the tach as it should be close) and rev the engine up to 3500 rpms. Timing light should be on the 30 degree mark. Try advancing the stator plate clockwise a little at a time and see if that solves the problem if you can get it to the 30 degree mark.

 

Hard to do as I can't get it to run. Just got the light on it while cranking

 

If you've pulled the black wire from the cdi with no effect,and the pulse coil reads around 97 ohms and is within 0.20-.040 clearance between the face of the coil with the flywheel tab,I'd check out the stator resistance specs first before jumping for a cdi. Here's what the manual shows.
Grn to blk/red 446 ohms, Grn to red 3.2 ohms,white to white red(pulse coil) 97 ohms, Yellow to Yel/ Brn 0.17 ohms, Y/R to Y/Brn 0.17 ohms. Yellow,Red and White no shorts to ground. Coil primary(tab to ground) 0.3 ohms, Secondary(plug wire to tab) 6300 ohms,Plug cap 5 k ohms. Resistance is not always absolutely accurate,but close. If you find something here more or less than 20% of the given readings,could be a problem other than the cdi.

 

Got Dr running.. Simple I put the carb back on filled carb with fuel thru feed hose, fired up.

Idles but bogs/dies when given throttle.. I assuming it's a fuel pressure thing now without the tank on, and the pump feeding the carb.

Thanks tech for all the help. Very informative.. I think I'll be gettin a manul For it real soon.

 

Gotta have fuel or no go.. Pull the fuel line leading to the carb from the fuel pump and crank the engine over. Should be a steady,sometimes pulsing stream of fuel. If not then pump probably needs to be replaced. If it has a good supply of fuel and it bogs(You HAVE to have the air box, filter and lid on or it will bog!) then you may have some stopped up jets,or sticking needle and seat. Plenty of carb kits on ebay if needed. Don't just try to clean a carb,not worth having to go back into one.http://www.ebay.com/itm/1996-POLARIS-SPORTSMAN-500-CARBURETOR-CARB-REBUILD-KIT-Rebuilds-3130665-r516-/331454160009?pt=Motors_ATV_Parts_Accessories&hash=item4d2c351c89&vxp=mtr.