Problems starting/staying started

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Sassafrass89

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I have 81 xs400, first bike build, having problems getting it started. I have cleaned the carbs about three times now, beginning to feel like a pro at doing that now. I have checked the spark plugs both are firing. The longest I have gotten it to run is about 20 seconds, and when I do get it started I can get maybe 2-3 good tugs on the throttle before it will cut out. Engine has compression, is there any safety relays or anything that would cause this? I know im probably being pretty vague but any suggestions id appreciate
 
Quick question: do you have a fuel filter inline between the tank and the carb?

It's pretty easy to have a clean carb clog back up if the tank is a mess. I wouldn't suspect the intake boots. While they may cause the bike to run lean, they usually won't stop the bike from running...
 
No I don't, the tank is a little rough on the inside but I thought the screen on the petcock would at least keep enough gunk out in the mean time to get it at least started and idling. But if that's the case the fuel filter inline will help?
 
I would flush the tank good and put new fuel in it. Then add a fuel filter after it. Replace the intake manifolds and gaskets. Take the carbs completely apart and make sure all the small passages are clear. Using compressed air (125-150psi) will do the job. Make sure all the rubber o-rings and plugs are good and soft and fit well. Make sure the bike has air filters and mufflers also.
 
You might also want to take a look at the ignition timing. I bought our 80 XS400 from a guy who had cleaned the carbs a few times but couldn't get any throttle response at all (bike would idle but engine simply would not rev up with the throttle). Setting the ignition timing correctly cleared up the problem.
 
Gas mixture and spark are the usual suspects , if carbs are clean and plugs are firing - check fuel supply. Turn petcock to "prime" and see how fast the fuel is flowing, if it is flowing - try starting in "prime" . Perhaps try fresh gas.

I would also look at spark plugs after engine dies, are they wet or dry? if they are wet - turn off choke earlier and try that way. Try new spark plugs, could also help.

just ideas....
 
Have you confirmed that your choke/enricher is intact and functioning properly?

I assume your bike is a sohc and not a dohc.
But if it's a dohc like mine it will be fussy about cold starting without lots of enricher initially.
And if I'd cut the choke too soon, it would die and then be REALLY hard to start up again. If I did it right though, it would start on the first crank and smooth out nicely as long as I reduced the enricher at the correct rate - or started riding with partial enricher after 30 seconds of warm-up.
 

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