Confused about sudden mixture/misfiring issue

So, the bike has been recently synced after a carb rebuilt and was running without a hiccup, if a bit lean, with the stock mixture settings. I just wanted to dial it in as best as I could by ear since I don't have a colortune yet so yesterday I set about tweaking. It went really well at first. I was able to clearly hear the changes in RPM and was shooting for highest RPM's on each cylinder plus a smidge richer. All the screws ended up at around 3 turns out, which seemed pretty reasonable to me.

Then I decided it was time to swap in some new plugs to start getting an idea of readings. Gapped these new plugs which were the same type as were in there already, and popped them in. The bike proceeded to run like absolute shit. Wouldn't hold an idle and at least one cylinder was misfiring a lot.

Thinking I should just start over and try again I swapped the old plugs back in and set the mixtures back to stock, exactly as it had been when it was running like a top. It idled better with the old plugs, but I still needed to bump the mixture up slightly on one carb to get smooth it out. I did get it to idle very smoothly with no misfiring but as soon as I tried to take it for a ride the acceleration across the whole range was very inconsistent and lurching as if one cylinder was dropping in and out. Weirdly, it goes back to being completely smooth on idle, and in neutral it revs smoothly. Can a 4 cylinder sound completely normal on idle with only 3 cylinders firing? Pulling the plugs, none showed unburned fuel or soot, but I couldn't read for leanness since they were the old plugs.

To cut to the chase, I don't know where I went wrong since setting everything back to the way it was still doesn't fix it. All I can think is that messing with the spark plug wires caused a problem there or one of the mixture screws randomly stopped working while using it. I'm doubting sync could be far off enough to cause such crap running since I did sync it before touching the mixtures which are back to stock anyway.

I feel like I must be missing something really simple but important here.

 

Are the valves in spec?

Float levels accurately wet-set?

Yes, the bike can "sound fine" while only running on 3 cylinders at idle.

IN ORDER:

-Get the valve clearances in spec if they're not.

-Wet-verify the float levels if you haven't already.

-Put the mixture screws at 3 turns out.

-Do another RUNNING VACUUM SYNC.

Then and ONLY then, start fiddling with mixtures.

It sounds like you may be trying to do this in reverse order, or you're skipping steps.

Also understand that "tweaking" the mixtures substantially will throw off your vac sync which usually requires a "touch up sync."

 

Valves in spec.

Float levels wet set.

Running vac sync performed a few weeks back, but will try again now that the mixtures could be a little different. Prior to my first sync it ran much better than it does currently.

Mixtures at 2.75 turns out.

Not aware of any steps I skipped. Plan was to go back for another sync after dialing in the mixtures. Never got that far because of above issues.

Like I said, the bike was running flawlessly in the days leading up to this mixture tweak. I just hadn't gotten around to it yet.

 

I agree with Fitz on everything he said above....

But I have one additional thought... are you sure that you are still running on 4 cylanders? messing with the plugs you could have dropped one

 

I'm pretty sure I'm not running on 4 cylinders, but I have no idea why. Mixture being too lean doesn't seem to be possible unless one of the pilot passages spontaneously clogged while tuning the bike. Plugs were working so I don't think one of them spontaneously stopped sparking right when I was tuning. The only thing I can think is that jostling the wires around so much caused a break. I'll be verifying that I get a spark on all 4 tonight or tomorrow.

 

You could check exhaust header temps when idling, this shold confirm which cylinder is not firing correctly.

infra red thermometers are good but cheaper substitute is spit or hold your hand close, but do not touch..! it Fooking hurts.. trust me.

 

Sounds like you're on the right track. I'd check the plug caps, be sure you don't have a loose resistor core; plus unscrew each one, lop about 1/4" off the wire, and screw the cap back on for a fresh "bite" into the wire.

 

If everything was OK before you swapped the plugs, I would think that one of your plug wires didn't like the way you handled it and quit working!

 

Well, we were on the right track. Everything is back in order now! In retrospect it was a very simple issue.

Took all the spark plug caps off and started testing the resistance. Sure enough, one didn't have continuity (I think the resistor core was loose). After taking it apart and cleaning all the bits up it was showing in-spec resistance. Also clipped back the wires while I had the caps off.

Bike is purring along as it was before. Now back to tuning and syncing.

 

Yep, that's the fun with old bikes. Go to fix one thing, and ya break something else sometimes :/

 

Glad to know that you sorted this out.