inconsistent spark

[peterman]

Hey there,
I have a 1983 Yamaha xs400k. I got the bike in October and have been working on it off and on ever since. When I got the bike it had been sitting for 9 years under a tarp outside. I can only imagine some of the issues that still may be lurking. I have made huge asthetic improvements and small mechanical ones along the way. I was able to find a carb at a junk yard and pretty much replace all the jets and even the diaphram, floats and float needles. Its as clean as its going to get and seems to be working well. They may need to be syncronized but that is the most pressing problem at the moment.

I haven't been able to get consistent spark. Two days ago I had it running, albeit very rich on the right side. Today I go to start it and I have no spark what so ever on either side. This has happened in the past but if I wait a couple days I'll go out and things will work fine.

My problem is I don't know everything that controls the spark on this bike and would love some help in this matter. The wires, although old don't seem to be cracked or arching. There are two boxes on the left side under the seat and above the airbox, one says "ignitor" and the other unlabled. I assume that the one that says "ignitor" controls the spark. Am I wrong in this and are there other factors I should look to troubleshoot as well?

Any help would be greatly appreciated. If I can't fix this problem I may just scrap the bike all together.

Thanks for any ideas.

Peter

[Skier]

Check your shop manual for the section on ignition. You probably have ignition coils that have seen better days, and your manual will tell you how to test them. Basically you unhook the two wires from the wiring harness to the coil, then test the resistance on the two contacts, then test the resistance on the ends of the plug wires (put the multimeter's test probes in where the spark plugs go). Your manual will tell you what's acceptable for resistance and what's not.

[kar_the_terrible]

the ignitor box under the seat is the TCI (or CDI) ignition system. I cant remember which one is on there. Those should be good for the life of the bike, and are probably not the problem. as skier says, check the coils. this should be one or two coils attached to the spine of the bike under the gas tank (so you reomve the tank). check their resistances etc. its possible that if they're degraded you wont get a good spark.

[peterman]

Thanks, it appears as though the coils are in the spine. They look a little tricky to get to. I have seen replacements on-line but they are $90 per. And if the deteriorate then I am not sure a junk yard would be a good choice. The last time I pulled the plugs to check for spark there was no visible spark to speak of.

Thanks for the ideas, I appreciate the help.

Peter

[kar_the_terrible]

well invest about $10 for a multimeter from radio shack, so you can measure voltage, resistance and current (the current option isnt useful for bikes, coz the meter only measures in milliamps). Now you can verify if your coils are alright.
A junkyard might still have a bike with good coils. Chekck yours first, it might not be just that. I might also be something as simple as a bad spark plug wire (cheap) or end cap resistor, etc etc.

You need a manual to check what the correct vaules are for these things, I'll look it up, but it might be different for the 78-82 bikes, since things changed radically in the 83 xs400.

[peterman]

well invest about $10 for a multimeter from radio shack, so you can measure voltage, resistance and current (the current option isnt useful for bikes, coz the meter only measures in milliamps). Now you can verify if your coils are alright.
A junkyard might still have a bike with good coils. Chekck yours first, it might not be just that. I might also be something as simple as a bad spark plug wire (cheap) or end cap resistor, etc etc.

You need a manual to check what the correct vaules are for these things, I'll look it up, but it might be different for the 78-82 bikes, since things changed radically in the 83 xs400.

Hey Kar, I hope you'll forgive the ingnorance here but is an "end cap resistor" the same as a plug cap assembly, or the cap that goes over the spark plug and connects to the plug wire?

Thanks

[MotoF150]

check and unplug all the wiring connectors, inspect for corriosion and damage, coils and cdi boxes don't go bad even on old bikes, its a broken wire or a bad elecrical connector, and after you find the bad electricial connnector, after you clean and repair it and corrects your problems apply di electric grease to the inside of the connector and plug it in so it never happens again

[peterman]

Thanks MotoF150, I'll start digging around to see if I can find anything.

Have a great day!