Just bought a 400ex for pretty cheap. Doesnt run of course lol. Guy i bought it from said he was riding it when it just killed on him. But anyway, heres what ive got...
I have fire.
I have good fuel flow.
I dont have a compression gauge to check it but it the starter is turning and the engine is turning over.
What im leaning towards from other threads is the decompression mechanism.
Maybe it broke or something? any help??

 

Usually the only time you have a problem with the decompression mechanism is when you have the motor apart, put the stock cam back in and forget to put the little pice back in.
I would do a compression check, check out the timing, then start looking at the electrical system.

 

Yea now that you say it that is when everyone had problems. I didn't replace the cam or anything on it. I'm just trying to get it to start lol. And the guy I got it from did do some rewiring. He said the start button went out so he just took right off from the start solenoid to a push button switch making and breaking power. Whick works. Same as a screwdriver across the two posts. But what gets me is how he said that he was riding and it just killed.

 

If he did that I would go over the electrical system and clean the carb, who knows how long the ex was sitting.

 

He claims he rode it pretty often. But I did soak to carb and blew it out really well. I'm starting to think more like you now and I believe it may be in the electrical. Which I'm not at all experienced with. I just want to try and fix it on my own and not have to bring it to a shop

 

He may not be telling you the truth about how it quit.

 

How can it be electrical if you said it's got spark?

 

I think theres something maybe shorted maybe. I was thinking kill switch but that didnt make since with good spark. Oh yeah..forgot to mention at the beginning, the guy said the start button had went out so he just ran a couple wires off the start solenoid to a push button to make and break power to make it start. which would work like jumping the solenoid with a screwdriver. But thats all ive found thats been fooled with in the wiring. I ordered a new start/kill/lights switch today so i could have a normal electric start. Maybe theres something messed up in there or the wiring harness? I dont know. Im lost and lookin for help lol

 

Oh haha nevermind i said that already about the rewiring

 

If you have good spark it's not electrical. Dosen't matter if your lights, horn, or anything else don't work; if your getting good spark your good to go with that.