Diagnostic help: can't figure out what my spark plugs are telling me.

avemachina

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Evening y'all! 1981 XS400 SH giving me trouble again.

I'll start with what's on my bike before I explain the symptoms:

LHside.png RHside.png
Left-hand side, right-hand side. Spark plugs removed.


- UNI PK-92 pod filters direct to the carburetors, due to a massive tear in the H-hose (I know, I know, kinda set myself up for this)
- Carburetors rejetted one size up to 137.5 to compensate for pods.
- Carburetor needles at stock 3rd-groove position.
- Carburetor floats replaced with rubber type, tube-tested to have a static fuel level of 1mm below mating surface of the bowl for each carb. (23mm height)
- UNI UP-103 filter to the breather outlet.
- Gravity-fed fuel petcock; vacuum ports capped with fresh caps.
- Freshly replaced carburetor mounts.
- Carbs synchronized by manometer, freshly cleaned with new butterfly valves, oiled throttle cable with no snags.
- Autolite AP63 spark plugs gapped to 0.028".
- My little gear indicator has been removed; I'm back to a regular old neutral position sensor.
- Valve clearance set to 0.09mm inlet, 0.18mm exhaust.

. The bike runs decently, except I've been haunted by a hanging rev--even in 1st, it'll find its way up to 4000 RPM. After a little riding, the idle speed creeps up to 1500. Additionally, the neutral light seems to be a little dodgy.

As part of my testing, I ran it idle for maybe 10 minutes (3 turns out on 137.5). Nothing out of the ordinary, spark plugs were a golden color by the end, sat comfortably at 1200ish RPM.

I then rode for about 2 mi and noticed the rev issue coming back. This is also when I noticed the neutral light not always coming on. This has been happening occasionally since before the pods, when I had 142.5 main jets (what the bike had when I bought it; I set it to stock 135 for a time) and all-stock air intakes. By my reasoning, this means my idle circuit is okay, but maybe the main is still having issues? So I checked the spark plugs and I'm a little stumped--they aren't giving me anything obvious (no white, no real soot?)

Here's the left plug:
Lplug1.pngLplug2.jpg
There's a little bit of what I can only describe as "caramelization" on the ground electrode and, alarmingly, little metal shavings. I assume (hope) these came from me being reckless screwing it in. The center electrode is starting to turn black.

And the right plug:
Rplug1.jpg Rplug2.jpg
Right plug looks much cleaner, with light blackening on opposite sides of the center electrode and the neck of the ground electrode.

I'm not sure what this all means--is it even a carburetion problem? There's no smoke from the exhaust, and no popping on acceleration, just wild revs that fly up and want to stay up.

Any insight?
 
Hanging revs not a spark plug issue- definitely a carb issue. 1st thing to check is a vacuum leak ie intake boots, diagragm. Extreme misadjustment of carb balance could maybe do something like that. Probably worth double-checking the float valve ie does it leak, or is the float set way off or sinking
 
Cameras seem to do funny things when capturing plug color. My photos never look exactly like what my eyes are seeing. So it is hard to evaluate your plugs other than to say they don't look that bad to me. Here is a chart I use to help me when I look at mine:
Spark Plug Color Chart.jpg


I second the idea of looking for a vacuum leak. A hanging idle is a classic symptom of this. The other is the carbs are providing too lean of a mixture, but your plugs sort of indicate that this might not be so. I would spray some brake cleaner around the carbs and holders to see if you find any leaks. If not I would start tuning out your idle mixture screws in increments and then riding to see if that is the culprit.
 
gpounce, good thinking. I tried WD-40 to check for air leaks, but maybe it's too thick to identify a leak--I'll try the propane torch method and let y'all know what I find.

CaptChrome, that chart is awesome--since there aren't any real matches and they're fresh plugs, I bet some more runtime on them will give a better result.
 
And to add a little extra data to the chart, when it says cold or hot, it's referring to the plug heat value. Most plugs are available in a few heat ranges and the part number usually denotes this. For example, you're using AP63 and the hotter plug might be an AP73 or AP64. It's not the engine temp.
Did you bench sync the butterflies when you rebuilt the carbs?
 
I did, I bench synced, then synced with a manometer (the "clear hose with liquid on a stick" type, not a gauge-type). They're pulling symmetrically, within 1/2-inch of each other.

How can you tell the plug "heat"--what's the naming convention? Higher number fires hotter? I'm not sure how the naming relates to things like threading, length, diameter, etc, so I'm curious how you can tell what the different values are.
 
I just Google it for autolite.
Autolite indicates the heat range with the last digit of the part number. For example, 3923 has a heat range of 3. The higher the number, the hotter the plug. The lower the number, the colder the plug.
 
Did you check if the engine is running with only one spark plugged ?

#It looks like from the big cover that an electronic ignition is mounted, so it should be the best setup (i heard about broken or oldered soldering points)
from the order, first ignition, then check with brake cleaner if everything is closed (i saw that not the original air filters are mounted), then find the best way or setting that the engine is running with only one spark (in the manual is standing which side first), then syncronize the carburetors (vacuum watches) and at last do the regulation of the lowest RPM

if both sides have not the same settings, one side is drawing the RPM up, it is a zwo cylinder bike

i think if you play with clutche, RPM is going down because the is no real power at stand gas
 
Did you check if the engine is running with only one spark plugged ?

#It looks like from the big cover that an electronic ignition is mounted, so it should be the best setup (i heard about broken or oldered soldering points)
from the order, first ignition, then check with brake cleaner if everything is closed (i saw that not the original air filters are mounted), then find the best way or setting that the engine is running with only one spark (in the manual is standing which side first), then syncronize the carburetors (vacuum watches) and at last do the regulation of the lowest RPM

if both sides have not the same settings, one side is drawing the RPM up, it is a zwo cylinder bike

i think if you play with clutche, RPM is going down because the is no real power at stand gas

:0 I never thought to do the adjustment one cylinder at a time! That's really clever. I've only ever done it the traditional "make the same change on both carburetors" way before, so this advice is great!

I won't be able to test things out until at least Friday, but here's my plan:

1) Test for air leaks with a propane torch. Fix any leaks I find.
2) Running one cylinder at a time, adjust each carburetor's idle mix screw individually.
3) Resync the carburetors with a manometer once the idle mix is correct for each carb.
4) Readjust the throttle screw for idle RPM back to 1250.

RadlagerMike, you're awesome!
 
OK! Step 1 is underway, and the results were not what I expected. Here's where I applied a propane torch (at idle, on sidestand), and result:

- Around throttle shaft seals: no RPM change.
- Directly into breather filter: RPM increase.
- Around carb mount vacuum port caps: no RPM change.
- Around carb mount (to cylinder head): no change, but I noticed they seemed loose. Tried to tighten them and the bolts stripped right out.
- Around carb mount (to carbs): no change.
- Around carb-to-pod junction: no change.
- Directly against left-side pod: RPM increase.
- Directly against right-side pod: RPM decrease.

So it seems the cylinder idle mixes aren't right, based on what happened when I applied propane to the pods themselves.

I don't know if the breather increasing engine speed is expected behavior--potentially, when the bike is at speed, the increased air passing over it could be causing it to have that hanging idle?

The other thing might be those loose carb mounts. I've already fixed one up with a helicoil and it seems nice and solid; the other one is a job for tomorrow. Maybe the vibration or movement of the throttle cable could be making a leak open up.
 
Good on getting to the testing!

My two cents is I would discount the results at the pods. By putting propane directly into the intake really disrupts the mixture inside the carbs in unexpected, and not subtle ways. Really richening the mixture with propane can cause an rpm increase too. The aim of the testing you are doing is trying to displace air at places that are normally supposed to be air-tight, so don't test at places that are supposed to be open to atmosphere.

Good also that you found the stripped carb holder threads. Fixing those will be important, long term.
 
Everything's assembled again, with the carb holder bolts all fixed--turns out they make 90-degree drill adapters, which made the job a total breeze:

drillattachment.jpg


I retested for air leaks and didn't find any after mending the threads.

I'm taking a little break since my testing ran down the battery and it's recharging now, but I got past the step of adjusting each cylinder independently. LH was 4.5 turns out, RH was 2.0 turns out. What a wild difference.

Once the battery's back in shape, I'll get to resyncing the carbs and maybe adjusting the clutch. It's on the center stand currently.

After that, test ride.
 
The furthest I got this weekend was trying to sync the carbs, and no matter how I set it, I found the RH cylinder always pulled harder than LH.

Then things took a bit of a turn--I got strep and there's a pretty hard cold front coming in, so I'm taking this time to try to refurb the stock H-pipe.

I might have already said, but this hanging idle was present before I experimented with pods. Now that I fixed the carb holder fasteners, I wonder if I fixed the original problem.

The Great H-Pipe Fix is basically clean the things off, fill the gaps/cracks with a full, continuous brush-on coat of RTV silicone, then some flex-seal for good measure. I could've gotten a different one off eBay, but the ones I was seeing kinda looked like they were in the same or worse shape than mine 😄

I was actually kind of shocked at how many tears had developed in the hoses as I was cleaning them. I'd love to "recreate" the pipe with some modern silicone hosing, but seems like a lot of really specific measurements to match.
 
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You've done a solid job dialing in the carbs! The hanging revs could point to a vacuum leak, maybe check the carb boots again, even though they're fresh. Also, try adjusting the mixture screws slightly richer to see if it helps. As for the neutral light, could be a loose connection or worn switch. The plug differences are interesting, maybe a slight imbalance in carb tuning? Keep at it, you're close :)
 
Thanks, I appreciate that!

Still a little sick but I put the rebuilt H-pipe and put all the stock air intake parts back on today. I ran out of light before I got to test things out. Still chipping away.
 
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