XS400 Backfiring and High Idle Issue

[JowWeL]

Howdy,

I have an 81 XS 400. Picked it up for free from a friend who couldn't get it running.

I stripped and cleaned the carbs, replaced the spark plugs, installed a new battery, and changed the oil. All of these measures together got the bike running and I've been putting some miles on it over the past week. There are some pretty obvious issues with the bike that I would love your insight for.

1) Literally every time I turn the bike off after I ride, the engine back fires very loudly. I suspect that the timing is way off and have a timing light, but don't really know how to use it on this new bike.

2) The idle is super high. It was hanging out between 6000-8000 before the needle broke off (yes, I am serious, it just broke off and the stub of it is now stuck pointing at 8000 rpms).

That's it for now. I'd really appreciate your help. Thank you.

[duaneage]

You have an intake leak somewhere between the carbs and the head. This must be addressed before damage occurs. Spray wd-40 around the boots when it's running and see if the idle speed changes. Chances are, the boots are bad and for around 40 dollars you can replce thm from ebay.

Make sure teh clamps for the carbs in the boots are very tight, and that the airbox and related filter boxes are also tight. The filters might be very degraded, to the point of having no elements. They are pricey but I replaced the foam on mine with lawnmower filter foams.

Last, check the valve clearances since they are probably way off.

[HoughMade]

Go ahead and check for the intake leak as above. Both the symptoms you have indicate a very lean condition. That results from either too much air- hence checking for the intake leaks, or too little fuel. the first thing I would do is set the idle mixture screws to about 3 to 3.5 turns out, adjust the idle to 1200 rpm and see if it hangs when revved. If not and the backfiring stopped, you're good. If not, check the float level and consider cleaning the carbs again...and again....and again. These CV carbs must be very clean to work correctly.