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Mysterious dead cylinder

  • Thread starter Thread starter RyanBiggs
  • Start date Start date
Yeah, its crazy. You see it all over the forums "check your connections!" I can't emphasize enough how true this is! I had major drops through so many different connectors and switches, it took me a week to correct it all! The relay mod is is simple and highly effective, well worth the time.
 
RyanBiggs said:
Thanks for the ideas. I have a Dyna ignition, so I've figured that's probably not the problem since the plugs did spark during the plugs-out cranking test after I replaced the coil. The coil comes with the plug wires, so those got replaced at the same time. Could be resistance in the ignitor connection to the coils though, I suppose. I was suspecting that maybe I have spark during the no-compression check but not under load, as p_s suggests. Sounds like the next step is new plugs and cleaning up the low voltage (ignitor) terminals.

If the thing runs good for a week after that and then quits again, maybe it's running so rich it fouls. I was thinking the #2 and 3 sooty plugs might just be a result of poor spark; we'll see. I replaced the plugs last summer and synced the carbs; the old plugs looked OK then. I'll probably just rebuild the carbs anyway for good measure soon, but right now I'm working at the university, and parking sucks so much that the bike saves me 30 minutes a day or more in commute time. So I've been reluctant to tear into it just yet. That and I'm about to move, bad timing. p_s, where did you ride up from?
Click to expand...


Swap out the coils, keeping the same wires on them (change the routing 2&3 and 1&4 of course) . If the problem switches to 1 & 4, you've isolated the problem. You can then start re-swapping things one at a time until you find the culprit.
If no change, must be a carb problem.

Another issue - it isn't enough to see you have spark. The plugs fire on every upward stroke. You need to verify firing on compression.
 
Good idea swapping the coils, should have tried that first. But since the coil for 2&3, the associated plug wires, and the boots have now been replaced since the problem began, I suppose I can rule all that stuff out. Thanks for the observation that plug-out spark doesn't mean a whole lot.

Current plan is new plugs to clear out old fouling, and dinking with idle mixtures. Also, even if idle fouling turns out to be the main problem, I'm going to be cleaning primary side connectors and checking voltages! If fouling isn't the problem, seems like that sort of thing almost has to be. Nothing like 30 year old electrics! Fortunately a lot has been replaced with more modern components on this bike, but not the switchgear/harnesses.
 
flyingace said:
Swap out the coils, keeping the same wires on them (change the routing 2&3 and 1&4 of course) . If the problem switches to 1 & 4, you've isolated the problem. You can then start re-swapping things one at a time until you find the culprit.
If no change, must be a carb problem.
Click to expand...
Not true. That just means it's not a coil problem.
 
Voltage at both coils is about 10.8V - low enough that I see the benefit of the relay mod, but I suspect probably not enough to cause major problems.

My current theory is that the #3 plug is fouling up and then losing good spark. I'm thinking perhaps something is clogged in the #3 pilot circuit, as the idle air adjustment on that carb doesn't seem to have a lot of effect on the idle speed. Probably been this way for a while, but I didn't notice before since I wasn't previously spending much time in the pilot circuit. Clean plug has it going again, but I have a feeling it's going to be fouling again soon. Looks like it's time for a good ole carb rebuild...

Er, looking at some of the carb info on the site - anyone know if these carbs have a pilot air screw or pilot fuel screw? I don't have the Haynes manual in front of me. Maybe backing out the screw is actually making it richer...
 
Nevermind - answered my own question: both, but the ones I've been dinking with are indeed air screws.
 
I am very interested in what you come up with. I have the same problem, well pretty much, on my number 2 carb. I even rebuilt my carbs, and she isn't firing on that cylinder. Mine isn't getting gas, and I can't figure out why. I have tore down carb 2, 3 times now, and can not find anything wrong with it. I wonder if I am missing something somewhere. Somebody said something about the choke cavity being plugged. I will have to check that again, but I am pretty sure I cleaned it.
 
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