Surging engine, hanging RPMs, black spark plugs?

[avemachina]

Howdy again! The old boy's developed fun new issue after a cold snap kept me off two wheels for a week. Last change I made was checking my plugs and noticing they had a healthy dry black soot deposit on them, so I wire-brushed them off and adjusted my idle mix screws 1/4 turn out to help lean it down. However, here's where the issue starts.

Symptoms:
- Takes a long time to warm up. Ambient temperatures are normally 60-80F (15-26C). I have to leave him on full choke for about a minute, then half-choke for two or three, then he's still stumbly until I ride for five or so.
- Idle RPMs are good and stable. In neutral, he'll stay at 1200-1500 RPM all day long.
- Once I'm in 1st and give him even a little throttle, he'll fly up to 5000-6000 RPM. Closing the throttle will ~eventually~ bring the idle back to normal. Same with pulling the clutch lever in--at a point the RPM will go back down, but the amount of time it takes is worrying.
- RPMs surge and hang.

Troubleshooting:
- Adjusted idle speed. No change.
- Made sure the throttle cable wasn't binding. Cable moves freely with no impediments at both ends.

Suspected culprit:
- Probably the carburetors. Maybe a busted seal?
- Going to clean the carbs and make sure the boots aren't letting extra air in.
- Poor jetting choice? I'm pretty sure I'm all stock, but I'm at 3200ft above sea level, and the temperature change with the seasons (about 20F or 10C range) may have changed some things.

I'm wondering if this is symptomatic of an air leak at the carbs. Does that sound correct? I don't think 1/4 turn on the idle screws would create such an extreme change to engine speed. Just wanted a second opinion to make sure I'm on the right track with troubleshooting--I'm going to replace the butterfly shaft seals and clean the carbs anyway, but if there's some other thing that could cause this, I'm all ears! 1981 XS400 SH, all stock.

 

[CaptChrome]

One conflicting thing that doesn't make sense to me is:

checking my plugs and noticing they had a healthy dry black soot deposit on them, so I wire-brushed them off and adjusted my idle mix screws 1/4 turn out to help lean it down.

Black plugs do indicate a rich condition, so you would turn the mix screws in, not out, to lean things out. Maybe a typo.

The symptoms you describe do sound like a lean condition at this point. What color are the plugs now? Have they become whitish?

Black plugs can also be because of an oil control issue in the cylinder (valve seals leaking in the head or the oil control rings leaking on the piston), so you may have two problems (fuel mixture and oil control). That said, tackle one at a time and the best place to start is always the carbs. Your plan sounds like a solid one.

 

[avemachina]

Not a typo. I turned them out. Is that wrong? I though out=more air.

Given that, would a 1/4 turn out really cause that dramatic a difference? If so, that could be my problem! Just to clarify, the correct idle screw adjustment is
out=richer
in=leaner.

Correct?

 

[tstidham]

Correct. Most idle mixture screws adjust air quantity, but our mikuni carbs actually adjust fuel quantity. So in is lean and out is rich. So I'd try turning them back in a half turn, maybe even 3/4 and see what you get.

 

[tstidham]

In addition, depending on how they were adjusted, it possible they were already far to lean and the idle speed screw was used to obtain an idle. This can have the advers effect of idling off the main jet. So a reset maybe a better option. Lightly seat each mixture screw, then turn them out 2.5 turns each. Start the bike and adjust the idle speed screw to idle as low as possible. Then start turning the mix screws out 1/2 turn each to obtain the highest idle. If the idle starts dropping down on a turn out step, turn it back in. Note that if during this, the idle starts getting too high (1500 or higher), lower the idle speed screw back down to 1000-1200, then continue with the mixture screws.

 

[avemachina]

Thanks for the tips! I'm going to clean the carbs anyway since it's never a bad idea. Let me just make sure I've got the order of operations right:

- Clean, reassemble, sync, set mix screws at 2.5 turns out.
- Start bike, warm up, set idle screw to lowest possible idle (1000ish RPM)
- From the 2.5-turn starting point, turn mix screws OUT to get highest possible idle. Adjust idle screw to keep the engine around 1000ish RPM. Do this in 1/2-turn increments.
- When the idle drops instead of rising, stop turning OUT. Turn mix screws 1/2 IN.

 

[tstidham]

You got it!

 

[avemachina]

I've gotten myself turned around a little bit; I'm cleaning and polishing the parts and reassembling while I wait for the shaft seals to arrive.

On the needles, does the nylon spring seat go above or below the circlip? i.e., does the order go:
1. spring seat->clip->needle point, or
2. clip-> spring seat->needle point?

Option 2 makes the most sense to me because otherwise the upper clip positions would be unusable with the spring seat not having any room to grab on, but I wanted to double-check.

 

[avemachina]

^ answered my own question--I checked one of the manuals since the original had the wrong carb diagram; the nylon donut goes under the clip.

 

[avemachina]

OK! Fingers crossed, everything seems to synced, idly, and responsive without any weird hanging. Took something like 5 turns out to get the idle to stop rising. We'll see how he goes next weekend when the rain stops again

One tip, it was a lot easier to keep track of the turns when I colored half of the top of the idle mix screws black with a sharpie. Don't know if anyone else does this, but it's a great visual indicator of how far you've turned the screw.