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Ok, heres a good one.

This is an 82 Maxim 400 (yamaha) that is my project. It does not currently run.

Last night (after getting the service manual fianlly) I was checking resistance and reseating wires and such, hooked up a battery and tried to fire the bike. After just a couple of tries (5-6 seconds at the most) I noticed, as we say in the computer world, the magic blue smoke. I looked down where the smoke was comming from to see melting electrical tape comming from the wiring harness.

Disconnect the battery, carefully cut open the tape and I find a clear(ish) rubber/plastic tube surrounding what apears to be a blown diode. This is between the power (not sure if rectifier or ignitor or what) and the starter relay. This diode is most fully toast.

Pictures of the part:

http://www.adamandnicole.com/gallery2/m ... temId=4831
http://www.adamandnicole.com/gallery2/m ... temId=4835
http://www.adamandnicole.com/gallery2/m ... temId=4841
http://www.adamandnicole.com/gallery2/m ... temId=4844
http://www.adamandnicole.com/gallery2/m ... temId=4847

Now, my issues are these:

1. The Service manual shows WHERE the diode is, but not any information about the diode, and theres no diode listed in the parts lists.

2. Would a blowing diode cause starting issues (bearing in mind the starter has been turing the engine just fine) and why would the diode have fried?

3. Could the frying diode caused other problems I'm not thinking of


On top of that all, on the advice of Dan's Motorcycle Repair Course, I was doing some more troubleshooting on the bike that won't run and was able to get a spark on the plugs across a fully bent out electrode on an old plug, so at this point I've got spark, fuel, compression, but still no starting


PS... I can't seem to get images to work.. can anyone help?

quick reply, I finally found the diode on the parts list and will be ordering it, the rest of the questions still stand if anyone has any ideas.

Hi,Mr shake i am no expert,but i think the diode is to keep the battery from feeding back thru the altenator when the key is off,thus not draining the battery.
hope this helps.
Thurm

I do believe that the diode will keep the starter from backfeeding current in the wrong direction when the engine kicks over and the starter disengages. Diodes can be like capacitors when they get old. They like to explode or catch on fire...

You may want to order a few. If a new one fries, you could have two much voltage (or amperage) coming back from the starter to the diode. Like all electonic components, they will have a rating of how much they can really handle before something bad will happen to them. If it's rated for 12v and it's getting 20v, it could fry. If I remember correctly, diodes are rated by amps, so it's possible there is a short somewhere or something not hooked up right. Most likely, it was just old and tired.

A short somewhere causing the diode to blow will cause the bike not to start... However, since you have spark, I still stand by it was probably just an old diode and not a short.

Have you cleaned out the carbs or did a rebuild of them yet? If you have a good spark the next thing is probably fuel. If the fuel is flowing from the tank fine, the carbs could not be getting enough out.

Thanks for the replys guys!! I had suspected "old Diode" myself (it being probably 20+ years old).

In terms of the bike starting...

I've cleaned / rebuilt the carbs and they appear to be working fine.

The new plugs come out afetr trying to start clean and a tiny bit damp smelling of gas

I've yet to get a real compression tester (can't find the right adaptor around here) but I did shoot an old spark plug across the room (it was only screwed in maybe a quarter turn) and Im not able to hold anything over the plug hole, so there is at least SOME compression.

Other things I've tried:
Carb Cleaner into the carbs while trying to start (thought I heard a pop once but it was faint)
Carb Cleaner directly into the cylinder
Played With Choke
Played with Throttle
Plugs will spark on an old plug with electrode bent straight out

Nothing results in any firing at all

Checked the timing? Maybe it's so far out that it's not even sparking on the compression stroke...

I wish I could adjust the timing. The Maxims have TCI inginition system that is electronicly timed.

You are not making this easy on me.

There isn't much left to check other than the ignition module is shot. If the module is shot and it's not working right, it is possible it could be sparking, but not at the right times. It's also possible that the coils could be flaky and not working right all the time.

I guess a way to check that the timing in the module is ok, is to use your home made spark checker and put a finger by the plug hole.. If you feel pressure coming up and then a spark, it should be close enough. I would think that if you see a consistant spark, the coils would be ok.

Just a silly thought... Does this have a safety switch that will not allow the bike to be started without the clutch pulled in or the side/centerstand up?? I know that some of the late model XS650s had neutral switches where you could not start the bike without the clutch pulled in, even if in neutral... Some of them even had them on the side stand where the side stand had to be up for it to start... Check both of those and the centerstand, just in case.

I haven't tried the clutch in thing.. I'll have to after I replace the diode.

if your plugs are damp then try removing the airbox/ filter you are runnign too rich or you are flooding the engine. wet plugs do not work very well