I have a 1981 440 Jag that just will not start. I have gas going to the cylinders, have spark, and compression averages 118 pounds per cylinder. I've cleaned the carb, replaced the fuel filter and replaced the fuel pump. Even when I spray ether into the carb throat, the engine won't fire. Any help would be greatly appreciated.

First off, ether, and 2 stroke engines, not the best of friends. If in fact all the other things are good on this sled I suspect maybe a blown seal. You see, in order for the fuel/air mix to be drawn in it gets pulled in under the piston as it rises. Then when the piston comes down the mix is pushed through the transfer ports to the top side of the piston and bang. If you have a bad crank seal the first part of this event is compromised. You need to recheck all your ducks and then if you still have everything in order, you should look at pressure testing the crankcase.

Couple of things..

I assume that the plugs are new and gapped correctly. Make sure that the spark is good, blue and fat. If not the plug could be failing under compression.

After cranking the engine for a while are the plugs wet if not there is either a restriction somewhere between the carb and the combustion chamber (transfer ports etc). or the crank seals are leaking. You can try spraying wd40 on the crank ends to temporarily make a seal or pour some fuel into the cylinders and try starting.

It could also be a bad Stator. I had a '97 ZRT800 that had this problem. When I checked the spark with the plugs out, they had great spark. When I had the plugs in, I couldn't spin the engine fast enough to make a good enough spark for ignition. This problem drove me nuts. Then I discovered that one of the coils on the stator was shot.
Best of luck

It could also be a bad Stator. I had a '97 ZRT800 that had this problem. When I checked the spark with the plugs out, they had great spark. When I had the plugs in, I couldn't spin the engine fast enough to make a good enough spark for ignition. This problem drove me nuts. Then I discovered that one of the coils on the stator was shot.
Best of luck[/b]

Good tip and often overlooked.
The work around for this is to test with another set screwed in the heads.