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heres the prob.... my z400 idles fine but when i hit the throttle hard it starts to misfire while im riding or going up steep hills never in first gear though..!! i changed spark plug it carbon fouled.. which indicates fuel to rich... cleaned carb and air filter..still misfiring... so how do i adjust the air fuel mix?????????
 

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heres the prob.... my z400 idles fine but when i hit the throttle hard it starts to misfire while im riding or going up steep hills never in first gear though..!! i changed spark plug it carbon fouled.. which indicates fuel to rich... cleaned carb and air filter..still misfiring... so how do i adjust the air fuel mix?????????
You may need to pull the carb, and take out the jets and clean them with carb cleaner. Could possibly be fuel screw. What mods do you have?

No need to post twice.
 

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I have the same exact problem. I have a full exhaust, uni filter no lid, hi-rev box, jet kit. Not sure what jet I have in there, but my carb needed some work done to fix a few minor things. While having it apart we changed out jets and adjusted all that we could to get it perfect. While doing so, my buddy suggested we replace the long spring in the center of the carb since I had a new one(I ordered a bunch of replacement parts, wasnt sure which I needed so ordered a bunch J.I.C.), as soon as we did we got the same problem listed above. Starts perfect, idols perfect, low throttle is great, medium throttle is sputtering, backfiring and bogged down. High throttle is fine, and I noticed it really only seems to do this in 1st and 2nd gears, 3rd and up are fine. Also I noticed when the fan is on, if I shut it off, the head light comes on and the turning the key takes it a few seconds before it kills the engine. Kind of makes me think an electrical issue. Any thoughts?

Cant figure out how to embed it but here is the video of the switch/light issue:
 

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heres the prob.... my z400 idles fine but when i hit the throttle hard it starts to misfire while im riding or going up steep hills never in first gear though..!! i changed spark plug it carbon fouled.. which indicates fuel to rich... cleaned carb and air filter..still misfiring... so how do i adjust the air fuel mix?????????
Take choke off the carb and see if it's stuck. Mine was doing the same thing and when I took plug out it was always fouled. When I took choke off and turned it on and off it wouldn't function so I bought pull choke and problem gone. Just a thought if you havent checked it yet.
 
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