Question about 1982 Yamaha XJ 400

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1981 yamaha 400 twin was running o.k. now it back fires and hard to start .spark is white. it seems like its not getting gas ,i dont smell gas that strong when having to kick start.when it does run its like it bogging,with to much gas.i working the choke in/out just to get it to run

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You may have issues with;
Fuel flow, check filter for blockage, line for obstructions and floatbowl needle and seat for dirt.
Air/fuel mix. Clean carby with a aerosol carby cleaner and check operation of slide needles (rise/fall in unison)
Sparkplug; Reset gaps or replace if wary.
Top End, is camchain tensioner working, any undue valve train noise, has it been serviced at all.
Electrical. Check wiring connections and plugcaps, coil earthed and pluglead free of other wiring.

Cheers.

Posted on May 30, 2010

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Replace both of your spark plug caps. They are available after market (NGK brand) for about $5 each. Simply unscrew the old ones off of the end of the wires and screw the new ones on. When a spark plug cap burns out the 5000 OHM resister the motorcycle will stop firing on that cylinder.

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Is there gas coming from the gas line? Its sounds like you need a new spark plug maybe even a carburetor cleaning. Thats my guess.

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Check you have power to the plugs take out both plugs and place one on the cylinder head and see if you get a spark .Try rubbing a pencil lead on the contacts ,as this give a highter spark .If no spark on either plugs you have an electrical fault which could be any number of things ,Try W/D 40 on the ignition switch it my be corroded inside and not making contact .or check your wiring for breaks or corroded terminals.or connectors .

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There is a black and white wire near the coil the should be for the kill switch or on / off if you unplug that wire you will bypass the on / off switch.
The CDI unit on the Yamaha often does not provide a very bright spark and it's often very hard to see in daylight. First make sure you replace the plug, second make sure you have it grounded well on a part of the engine. Make sure you have something covering the spark plug hole because if gas comes out of the cylinder and the plug does fire you can ignite the bike on fire. (I know that one from experience). I'd be willing to bet you don't have an ignition problem unless the key is shorted out. Sounds like the bike was running fine until you shut it off and then would not refire, did you try to push start it?
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