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Discussion Starter #1
let my friend barrow it to run down to the store he said it just shut off on him and wouldn't start back up went and got it..had it running but would only go about 10mph full throttle got it home put a new plug in it same problem u could only get about 1/10 of the throttle and it would miss extremely bad parked it and now it wont even attempt to start back up any ideas what could be wrong
 

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first thing i would do is check to see if you have spark. also probably get a load test done on the battery to check for how many cold cranking amps it has
 

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headlights still working? that would be a sign of possibly the voltage regulator when if the bulbs are blown. failed ECU could cause this, although not common, same with the voltage regulator, not common. possibly the diaphram in the fuel pump wore and got a hole in it. how is the fuel flow? could also be a issue in the carb, and it might need to be pulled apart and looked at inside
 

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Discussion Starter #7
headlights both work 1 is brighter then other tho,i took carb apart and cleaned it everything seemed fine,i dont think it has a fuel pump and it seems to get plenty of gas,and cca on the battery is 240 i think
 

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The next thing I would do is check compression. The 400 is known for piston and ring failure, that's why I asked.
 

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Discussion Starter #9 (Edited)
any way to do that without a gauge?if u place ur finger over the hole it blows ur finger away from it...i know a good bit about atvs and have tried everything i know and still cant get it goin thats why ive came for help
 

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I do know that thee 400 is notorious for breaking the piston skirt and then it will fall into the crank and will roll it around back to the top of the crank case and bust it. This will cause an air leak and it will never pull vacuum right. Look at your cases right under the carb, right where the left and right cases meet is usually where it will bust out or crack. Also check the cylinder base gasket to see if it has pushed or blown out. Have you tried a bit of starting fluid in the carb to see what happens?
 

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Discussion Starter #11
no ive put gas on the plug and didnt get anything not even a sputter.i will check the case and gasket tomorrow thanks
 

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Discussion Starter #13
checked the case didnt see any cracks or breaks in it didnt have time to take head off it began snowing again so ill have to wait till weather breaks again to check gasket on cylinder
 

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Discussion Starter #15
its not to bad feels smooth and looks fine i will take it and have someone look at it when i order new rings havent had a chance yet to do
 

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Discussion Starter #16 (Edited)
i took off the flywheel yesterday and one of my magnets was exploded and shredded into pieces so i believe this was the problem all along cant find the cord to upload it or i would post a picture of it for everyone
 
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