I just bought a used 400, It is very hard to start. I cleaned the carb, and noticed that the low speed jet was only 3/4 turn open. I downloaded a manual and saw that the needle should be 2 1/8 turns open. which I did. Also when I checked the choke, there was a lot of free play before it would start to move the plunger, I guess the cable is stretched and there is no way to adjust it. So I added a spacer on one end to get it to open the port on the top of the carb. It seems to have helped. Still is hard starting. Plan on adjusting the valves next. Very touchy on idle speed. The previous owner said that he always used the primer pump on the carb to start it. And it will not charge the battery unless the lights are on. That seems strange to me. Anyone else had this happen? I need to check the front drive shaft actuator also as I don't believe it is working, hoping it is just a fuse. as the actuator costs $190.

 

Is the petcock on the arctic cat vacuum actuated? I know the Suzuki Eiger is, and the AC 400 uses the same engine/tranny, but im not sure if they use the vacuum petcock like suzuki uses. Checking the valves is a good idea for sure. See if the airbox/air filter is dirty. If it is/was then all that ingested dirt will cause your intake valves to wear and tighten up on the adjusters, which can cause hard starting. Make sure the carb is really really clean. It takes only a very slight amount of crud in a jet to cause issues. Use a piece of wire to probe through the jets, then some carb cleaner and compressed air through all the passages and jets. You may also try about 3 turns on the pilot screw and see if that helps. These things are usually a little lean from the factory.

As for the charging issue, you need to put a volt meter on the battery terminals at the battery to confirm what is going on. Check voltage at idle, then at idle with the lights on, then do the same at half throttle with no lights, and half throttle with lights and report back with what you find. It sounds to me like you could have a voltage regulator issue though. When the voltage regulator on my old 250 suzuki died, it was overcharging and the engine was running like it had a rev limiter. If i turned the headlights on, it would immediately start running right again because it would use the extra current that it was being fed, and the CDI would allow full RPM again. Could be basically the same case with yours, except in a slightly different way.

Im assuming yours is electric 4wd? Im not real familiar with the issues the AC electric actuated 4wd systems can have, but i would definately check fuses first. Also, make sure all your grounds are good.