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feliks

1983 XJ750 Shuts Off Randomly When Cool, Runs OK When Hot

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Hey Guys!  It's been a while since I've posted, but have always been lurking around.

I picked up an XJ750 a couple of years ago and have done quite a bit of work to it.  However I'm at a complete loss as to what and why I'm having this random bike shutoff issue.  It's getting to be dangerous because you don't know if it's going to shut off while you're leaning into a turn!

Anyway, here are the symptoms:

When the bike is cold it requires a bit of choke to start, then once the idle goes up, I turn the choke down.  Then the bike idles fine.  It will run for about 30 seconds usually, and then just shut off.  Just recently I've had to wait about another 30 seconds before it will start again.  If you try to start it, it will crank, just not start.  Once you wait 30 seconds it'll start back up again no problem and idle beautifully.  Then... depending on her mood, another 30 seconds or minute later she'll shut off again.  Because she's now just being a bitch.  So I have to sweet talk her a little more, wait another 30 seconds or so.... start her back up.  Caress her.  Tell her she's beautiful.  Then maybe she'll stay running for me and give me a good ride.  However, randomly, she still just shuts off.   I can't even say when it happens it's so random, sometimes when letting off the throttle, sometimes when rolling it on.  Sometimes just when I'm sitting at a stoplight.   Once I've been riding for over 30 minutes, it doesn't do it anymore.  She runs perfect.  No issues.  Only that first 30 minutes it's sketchy.

I would love any opinions or feedback on this crazy issue, because I'm out of ideas!

And I don't see any obvious electrical issues, I've checked all of my kill switches (clutch and side stand).  I thoroughly cleaned the carbs at the beginning of the year (didn't start having these issues up until about a month ago maybe).  And I've checked and replaced spark plugs, swapped coils with my old FZ600, and still the same issues. (now those coils are 30 years old too, so I'm no ruling that out).

I'll get a video of it tomorrow, bike is hot now because I've been out there messing with it, so it will probably run fine if I take a video now.  Next stop is the mechanic, and I've never had to bring a bike there other than for a safety certificate, so I don't want to start now! lol  Thanks in advance, you guys are the best!

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Things I have not checked, but probably should:

Carb sync
Valve Clearances
 

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It's got to be fuel or electrical, can't be anything else really, simple things first try leaving the tank filler open when you start it might be a blocked tank vent, try some NEW coils they do break down and can give good service when cold but stutter and fail when warmed up, check your fuel filter under the tank that can get bunged up as well plus the tank tap pipes can get loose and let air in so that's another thing to check, other than that you seem to have sorted most of the other stuff I would suggest. good luck, come back if some or none of that works and we can have another think about it.

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I had an XJ-650 for best part of 30 years.  Basically same block and ancillaries.  Never had the problems you describe so I agree with what Slice said above. Also look for deterioration in rubber pipes and look where dirt (however small) may come from. How's the inside of your fuel tank iwo rust?

I had a separate fuel filter (one of those small plastic jobbies) in the main fuel line between petcock and carbs that I changed regularly.

Always loved those XJ-750's with their YICS whine.

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