Miss fire that defies being found ... by me

[79XS11F]

My 79 standard has a miss fire that is proving to be difficult to find the cause of. Carbs are clean as a whistle, Plugs are burning as close to perfect as I would want to see and a color tune shows good even color across all 4 cylinders but I can see the miss with the color tunes in place and with a timing light.
This miss is erratic. It is happenstance and appears at any and all throttle positions. It is not enough to kill the engine and the bike actually pulls pretty good but can see the miss in the tack. I already checked the ignition unit by changing it out with a known to be good unit, fuel supply is good and gas is right out of the pump.
This bike ran flawlessly right up until spring of last year and nothing had been changed.
I'm very sure it's not a carb issue given the color tune results and the plug condition. I have not as yet checked the pick ups but they were NOS units that were installed less then 5K ago
I'm close to stumped. WHAT NEXT?
Rob

[79XS11F]

Well, as Gomer Pyle himself might say, SURPRISE, SURPRISE, SURPRISE!!!
I decided to re & re the coils and replace them with the ones from my hop up bike as a last ditch effort to find the problem. I wasn't expecting that to be the case because they were also NOS units less the 5K ago. I didn't even have to to switch them out because I could see that both were TOAST as soon as I got them off the bike. The Leads for both 1 and 3 were hanging 90% to the coil bodies. Turns out that the wires inside both were broken off inside the insulation right at the bodies of the coils.
I don't think I'll bother paying extra dollars to acquire NOS electrical parts anymore.
She's running much better now with a different set of coils installed. Idle is still a little lumpy but the carb sync is still good. Guess I better check out those "NOS" pick up coils as well ...
Rob

[motoman]

Reading down, looks like you've solved the MOST COMMON ignition issue.....NOS or not.

[11Rider]

I know you're convinced it's not the carbs but mine sat for about six months when I had a back operation an the pilot jets plugged up. It took a piece of guitar string to clean them out. The carbs looked perfectly clean too. Just a thought.

[79XS11F]

Reading down, looks like you've solved the MOST COMMON ignition issue.....NOS or not.

I have difficulty accepting that the ignition system is so fragile now that the bikes are so much older. These were NOS coils with nice flexible leads and they appeared to be perfect when I got them. It was the last thing I would have expected and did in fat prove to be the last thing I checked.
I owned a new out of the box 78 XS11 for 5 years and all it ever needed normal maintenance. I put just over 50K on that bike and never had an ignition issue.
Rob

[79XS11F]

I know you're convinced it's not the carbs but mine sat for about six months when I had a back operation an the pilot jets plugged up. It took a piece of guitar string to clean them out. The carbs looked perfectly clean too. Just a thought.

I appreciate the input but it's not the carbies, They have been cleaned in an ultrasonic cleaner, and blown out with compressed air. I check every part closely during the reassembly process. There are no blocked jets and the plug and color tune colors are very good.
Rob

[Rasputin]

So is your problem gone away with the replacement of the coils or is there still a miss?

[TopCatGr58]

Hey Rob,

The key letter here in N.O.S. is the "O"= OLD!!! Rubber doesn't age well, and at 30+ years old, I wouldn't expect it to hold up for very long or very well!

Now the innards of the coils are probably still very good....takes a bit longer for copper to age sealed inside the coil body, so you could possibly recoup some of your investment with the new wires in old coils mod if you're up to doing that much work!

Glad you found the problem fairly soon in your troubleshooting process.

T.C.

[79XS11F]

So is your problem gone away with the replacement of the coils or is there still a miss?

She's still a little "lumpy" on idle so I will likely change out the plug caps tomorrow. Their the original type with the steel jackets on them.
Rob

[79XS11F]

Hey Rob,

The key letter here in N.O.S. is the "O"= OLD!!! Rubber doesn't age well, and at 30+ years old, I wouldn't expect it to hold up for very long or very well!

Now the innards of the coils are probably still very good....takes a bit longer for copper to age sealed inside the coil body, so you could possibly recoup some of your investment with the new wires in old coils mod if you're up to doing that much work!

Glad you found the problem fairly soon in your troubleshooting process.

T.C.

I'll likely do that and set them up for future use. Has any one tried to do this repair in a fashion that has the wires thread onto the coil?
Rob

[79XS11F]

I changed out the plug caps and the bike was still misfiring. It went from a lumpy idle back to just misfiring so I installed the Excel coils off my hop up bike. There was an improvement but the issue remained. I say remained because I finally found the problem and it was likely the last place anyone would look. It turned out to be the gap between the reluctor and the pickup coil. One was at .30mm and the other was at the original setting of .07mm. How the gap changed is unknown and all screws related to the timing plate were still tight. There's no damage on the coil face so I do not think there was any contact between the reluctor and the pick up. In any case, she is back to normal :-)
Rob