Total Motorcycle Community Forums

78 xs400, and i just started it for the first time (i actually started it a couple days ago, but it didnt run cuz it blew one of the sparkplugs rite out, cuz the threads are almost done, so JB weld to the rescue)

SO, the sparkplug is holding well, but, heres a few issues i wouldnt mind a little advice on...

the idle is a little wierd, after its warmed up, i keep having to fiddle with the idle screw to get it to idle rite, i cant get it perfect, if it keeps dying, i bump up the idle, OH idle gets too high, so i bump it down, and the process continues itself...... and the other thing thats related to this problem...

on decelerration (when i roll off the throttle), i can hear distinct backfires (about 3 a time)

The carbs have to be responsible (i think its overfueling, cuz of the strong smell), could the backfires im hearing be just unburned fuel combusting in the exhaust?

What sort of things can i do to the carbs (short of a total rebuild), to try and stabilze the idle/get it running better?

anyways, thanks for sticking with me guys, from one rider to the next, i really appriciate it.

p.s. bike has plenty of power on the road, if i giv'er on the throttle, she responds

Check your carb manifold is tight and the gaskets are good. If it’s sucking air it can cause the problems you are describing.

If it has the same petcock setup as my 80' it also can suck air through the vacuum line to the petcock, make sure it's connected tight on the manifold and petcock.

Mine also occasionally backfires when slowing down, especially after a long period at speed with a heavy throttle and is running a bit rich. I think it's the unburnt fuel in the exhaust.

Make sure your chokes are going all the way in. There is an adjustment screw that could be loose on the choke linkages. If it got lose your bike may be choked even if you pushed the button all the way in.

Don't know if this will help in any way but just some things to check.