Only running on 3 cylinders

[Monkymann2]

Bought the bike about 6 months ago knowing nothing about bikes and wanting to start working on them.was running rough so I cleaned out the carbs found different videos on how to correctly do it and benched synced them and messed with the screws and had it running great rode it around for about two weeks and realized it was running lean tried to mess with it some more bought a rebuild kit new intake manifold gaskets and got it where I wanted it. Ran fine for a while then one day I went to check on the spark plugs looked fine plugged everything back in and boom one of the fuses blew and started running horrible(seems like the fuse was for the lights because the neutral light stopped working and all the gauges also didn’t work). I went back to work and couldn’t get the carbs right which I thought was the problem ended up buying some Murray’s carbs and after I plugged those in I would have to crank the throttle to even get it to start. Some times I would get it to idle and other times I would have to hold the throttle open. I felt the exhausts and 3,4 were scolding hot. 2 was hot and 1 was stone cold Steve Austin. Switched the Murray’s around to test if it was the carb or the bike and still had the same situation. Tried testing the coils couldn’t get a good reading i snipped the spark wire about 1/4 and that still didn’t work. Called Murray and he said that it could be the TCI module or the CUD unit but I don’t know where to start and need help.

[DiverRay]

Swap plug wires 1 and 4. Start the bike and see if 4 is now cold. My guess is it is cold. If you still have the original coils and wires, replace them. I need to know what year and model bike so you can get the correct coils, or just look in the mod section for coil mods as it lists everything you will need to know.

[oldyam80sg]

Also are you running the original fuse box? You said one went boom. I never had any blade fuse make any noise when they popped. The glass fuses are unreliable and blade fuses so much better.

[Radioguylogs]

The spark for #1 and #4 are wired in series.

The spark current must pass through #1 if it is passing thought #4.

The only way #1 is not sparking is if the #1 high voltage is shorted to ground:
(a) #1 plug is fouled,
(b) #1 high tension wire is arcing to ground

I would pull the #1 plug and the #4 plug, connect the threaded potion of both plugs together using a car battery jumper cable. Crank the engine and look at both plugs to see if there is a spark.

If both will spark, then look to see if there is evidence of arcing on the #1 plug wire. If there is no evidence of arcing, maybe it's not the Ignition system.

[DiverRay]

The ignition system is a "wasted spark" system. BOTH 1 & 4 fire together. If #1 is not firing moving the leads between the two will tell you if it's the plug or the wire.

[LAB3]

To sum things up if cylinder #4 is getting spark then your problem isn't any further away than the coil that serves cylinder 1&4. Replacing CDI is wasted money! The problem is occurring with either the #1 spark plug, #1 spark plug wire or the coil itself.

[DEEBS11]

A decent $15.00 fuse box replacement part:

https://www.ebay.com/itm/143445206467

[Monkymann2]

So I swapped 1 and 4 plugs and the issue moved to number 4. #1 was hot and #4 was cold. 2 seemed like it was still moderately warm but 3 was still scolding. Not sure if I should if I should replace Both coils or just the one but I’m definitely going to buy that fuse box to make it a little easier because trying to find those glass fuses is impossible.also my bike is sneezing not sure if that has anything to do with it.

[LAB3]

So I swapped 1 and 4 plugs and the issue moved to number 4. #1 was hot and #4 was cold. 2 seemed like it was still moderately warm but 3 was still scolding. Not sure if I should if I should replace Both coils or just the one but I’m definitely going to buy that fuse box to make it a little easier because trying to find those glass fuses is impossible.also my bike is sneezing not sure if that has anything to do with it.

Have you checked the resistance in the spark plug caps? If they measure significantly more or less than 5k ohm that would certainly explain the issues you're dealing with.

[DEEBS11]

You could also have a cracked spark plug wire that is arcing to the motor.

[Radioguylogs]

You could also have a cracked spark plug wire that is arcing to the motor.

I would conclude that the #1 plug wire is arcing to the engine if the plugs were not swapped when the #1 and #4 wires were reversed for the test.

[DiverRay]

I would conclude that the #1 plug wire is arcing to the engine if the plugs were not swapped when the #1 and #4 wires were reversed for the test.

And at 40+ years old, I would replace the coils/wires and caps with aftermarket. Put in resistor plugs, and solid copper wires with non resistor caps and you should be good to go.

[Monkymann2]

Hey guys thanks for all the advice I’m going to buy some new coils and spark plug wires and caps. I’ll keep you posted when they get here.

[oldyam80sg]

You could take off the spark plug boots,snip 1/4" off the wire and screw in boot again. Sometimes the ends get corroded. This happens all the time.Did you look at the condition of the spark plug? If it is dry you could put a small amount of fuel in well,reinstall spark plug and see what happens when you hit the start button.

[DiverRay]

It's probably the "resistor" that is inside the cap. They are a carbon block, and do go bad. You can unscrew the bit that snaps over the plug, remove the resistor, and put in a short bit of #12 or #14 copper wire. You would need to do this to all 4 caps to keep the bike running correctly.