Cy Welch

If it's a sharp popping sound it likely lean, if it's more muffled it's likely rich. First off look at your plugs, if their white it's definetely lean, if really dark that would tend towards rich but on an older bike that is not quite certain. Especially with higher rpm backfire it's more likely lean than rich though, and while you don't need the guages to adjust the mixture (they won't help with that at all) you do need some sort of gauge to syncronize the carbs. Do a search as their is an article on a homemade syncronizer using clear plastic tubing that is supposed to actually work better than the gauges and you can put one together for just a few $$. That said, if they carbs are in sync now, don't seperate them when you work on them, and as long they use a single throttle cable and you don't touch the screw on the shaft between them they should stay in sync although the manuals for my bike at least recommend checking/adjusting the carb sync every few 1000 miles. That said, if you don't want to make the homemade one, I have seen decent sets for around $35 on ebay. As for the plug advice, my experience has been that you use hotter plugs if you do a lot of low speed short trip stuff, and a lot of long trips or high speed/rpm driving usually requires a lower temp plug. The standard plug for your bike is generally the right temp for average mixed driving.

The headlight relay is a simple latching relay (has an internal diode that locks it into the on position until voltage is removed once it's been energized by a voltage pulse from the diode in the wiring harness off the stator. You can test if it's the problem by unplugging the relay and and putting a jumper across the connection (at the bottom I believe, but it might be the top two, just try one then the other, if you do them that way with the engine off there will be no problem), if it's the relay then the headlight will come on when you put the jumper on. I ran mine with a jumper for a couple of years because I didn't have the relay. You can open the relay and clean the contacts if that is the problem, as it's a mechanical unit. However, there seem to be plenty of them on ebay for a reasonable price if the relay is the problem. Also check the headlight fuse connections as those are notorious being poor (corrosion and such). I replaced my fuse box with an aftermarket unit made for off-road vehicles that uses the modern blade type fuses.

While how tight the chain is will not affect the final drive ratio, make sure that you check the chain deflection with the proper suspension preloading (on my XS400G is says to make sure that the point where the chain has the least deflection in the suspension travel) as otherwise the chain could actually be too tight as the deflection spec is based on where the chain is tightest. According to the manual this requires two persons to adjust the chain, although I would guess with some creative use of heavy weights it may be doable by a single person.

I don't know about in the UK, but here in the US, Bikebandit.com carries many of the rubber parts, although some of the key parts don't see to be available anywhere. As for the handlebar switches and such, ebay seems to have a really good selection of parts, often in very good if not almost new condition. Some parts are hard to find, but overall it's not too hard to build one from the ground up (I have even seen complete frames and wiring harnesses).

I notice that someone posted a wiring diagram. If you need something better I have both a PDF of the maint manual and a hardcopy of the shop manual supplement which I could scan the relevant sections. As I am just finishing up a daily rider restoration of my XS400 I may be able to provide some guidance. It may help if you could give the exact model number? PS, I noticed that the diagram is for an XS400D which I am not sure was available in 1980 or wired the same as the 1980's. My electronic copy is a bit clearer as well.