Stubborn Idle, but not hanging or sagging.

[MisterCinders]

As I dial in the pilot jetting for my bike, I have worked out most of the mixture issues but continue to have a strange idle issue.

As many will recall, the bike has pods (don't hate) and a 4-1 pipe. The current jetting is:

Pilot - 17.5
Fuel Screw - 1.25 turns out
Air Screw - 2 turns adjusted with ColorTune
Needle - 4th slot
Mains - 117.5
Fresh intake boots and o-rings.
Carbs are fully cleaned, with new o-rings, etc.
Valve clearances are healthy.
Carbs vacuum synched.

The bike starts and runs OK. It doesn't like to idle below about 1,200, which I think is a tad high. Otherwise it idles and pulls from a stop through the gears, etc. Pilot circuit plug chops show decent, perhaps a bit lean, results, except for the No. 1 cylinder which tends to be slightly rich. More on that mystery in a separate thread, I think.

Here's where it gets weird. After a few minutes of riding, the idle likes to settle in a bit higher, about 2,000 rpm. It doesn't hang there, as a throttle twist will rev the bike fine and then snap back to the 2,000 RPM mark. If I blip the throttle very slightly, however, the idle will return to 1,200 RPM.

Oddly, neither idle levels (the high 2,000 or the lower 1,200) seem to respond to turns of the main idle knob. When I have tried to adjust that screw, I detect no changes until the next stop light, where the idle settles in much lower or higher, and then responds to the idle screw adjustment.

This sure seems like a mechanical issue on the throttle or slides, such that the slides aren't returning to the proper closed position. I have repeatedly checked the slides for snags or sticking. They move nice and easily in the carbs. The cables also seem to be free of snags, and I have tried to fine-tune those adjustments to see if that's what causes the idle to misbehave.

The issue does not happen in the garage BTW. Only after riding around a short while. Perhaps the engine heat somehow affects the cables running under the tank and causes them to bind slightly?

Any other ideas?

[hampshirehog]

If the plug chops are showing lean on the pilot circuit it might be worth winding the airscrews in a tad.

Also what condition are the slides in?

[MisterCinders]

If the plug chops are showing lean on the pilot circuit it might be worth winding the airscrews in a tad.

Also what condition are the slides in?

The slides are in good shape, and I keep looking for grime or other things that might cause them to stick. In the past, I had a sticking slide cause the idle to "hang" badly, but when I pulled the slides that time, the sticking slide was easy to notice when moving them by hand and the gummed bit was very obvious.

Here the slides are clean (I just cleaned them, twice) and the slides move like buttered glass by hand.

[koolaid_kid]

If the plug chops are showing lean on the pilot circuit it might be worth winding the airscrews in a tad.

Also what condition are the slides in?

Actually, you have it backwards. In for leaner, out for richer. You are too lean, take it out 1/2 turn at a time until the idle settles down, then go back 1/4 turn to see if it still settles down.

[hampshirehog]

Actually, you have it backwards. In for leaner, out for richer. You are too lean, take it out 1/2 turn at a time until the idle settles down, then go back 1/4 turn to see if it still settles down.

He's got VMs on there so it's in for richer out for leaner on the airscrews, the other way round to the fuel screws.

[koolaid_kid]

Oops, my bad. I got in a hurry again. I actually posted once, saw he had VMs and deleted my post. Dammm. What a day.
My apologies.

[MisterCinders]

OK - I have spent the last few nights meticulously tuning the pilot. Highest idle never works well in my experience, since I never hear idle changes except when I all but bottom out the air screws. Then, of course the idle starts to bog and/or lose that cylinder that is about to starve for air.

With the Colortune though, I have been able to dial in the pilot screws to pretty good results. Last plug chop showed good tan colors across all four, and the Colortune shows nice blue flames, for the most part.

This idle weirdness, however, persists. I think I can isolate the symptom now, so maybe my GSR pals can help figure it out with some more info.

First, the Colortune maybe adds another clue. At about 1,100 RPM and no U-Boat mode (see below), the idle sputters and misses, but not regularly. The bike may just have needed a bit more warm up, but when the sputter occurs, the blue flame streaks a bit yellow just for that instant. Not sure if this is just ordinary idle hiccup/colortune read out or not.

When I re-did the Colortune testing after a short ride and pilot plug chop, the sputtering diminished significantly.

Second, the next clue is either another warm-up issue or signs of things to come at the needle chops. For the first 15 or 20 minutes of a ride, the bike feels a tiny bit flat just off the pilot - between 1/4 and 1/3 throttle. Thereafter, it seems to pull through that throttle range fine. Not sure if this just means the engine is still warming up or the needle (currently in the 4th slot) needs some shimming. Keeping an eye on this to make sure it's not a slightly lean pilot causing transition problems.

Third, the idle itself occasionally tries to settle below its regular level. I am calling this U-Boat mode. Even in the garage now, a rev will return the idle to a low RPM. It snaps back and forth to that level, and does not tend to slowly return to normal (e.g., like a sagging idle) or drop lower and bog out (like richness fouling plugs etc). I thought it might just be some richness, but plugs and Colortune seem to rule that out.

When it does this U-Boat dive, a light touch of throttle pops it back to normal, but check this out. In U-Boat mode, the idle screw does nothing. So when the idle settles to 800 RPM instead of 1,200 RPM, turning the idle screw either way does not budge it from 800 RPM. After a light throttle touch, the idle pops back and captures the changed idle screw results. In other words, after idling at 1,200, it returns from a rev to settle low at 800, so I turn in the screw to raise the idle. Nothing happens. A tiny blip on the throttle causes the idle to surface, but now at 1,500 or 2,000 - because of my idle screw turns.

Conversely, if it dips into U-Boat mode and I just do the light blip to bring it back to the surface, the idle screw will move the RPMs per normal.

What would cause a steady low idle that won't react to the idle screw but isn't just a rich mixture and doesn't seem to bog out?

[MisterCinders]

More testing and plug chops. Plugs are still a bit lean, but getting closer. I am hesitant to go further out on fuel screws, since they are in the 1.25 - 1.5 turn range now. But the air screws are <2 turns out and plugs continue to show slightly lean. If I can't balance out that mix, I'd have to jump up another size in pilot (to a 22.5), which just seems wrong (stock is 15, currently have 17.5).

The idle now favors U-Boat mode and almost forces me to hold (but not really turn) the throttle to keep it in the screw adjustment range. I understand that makes no sense, but I don't really get it either. My throttle cable play is minimal, below 2mm for sure, so I don't think it's a snag there.

Before I dig into the slides, I want to try and sort the pilot mix and know more about the needle setting, to avoid dealing with re-synching and/or recreating a slide problem should I need to go back in to deal with the needle clip.

What the hell could submerge the idle like this? Electrical maybe?

[hampshirehog]

What the hell could submerge the idle like this? Electrical maybe?

That's what I'm thinking. The Colortune won't lie. You've gone Dyna S though haven't you? That shouldn't give you any problems bar loose connections / dodgy coil or leads.

[MisterCinders]

That's what I'm thinking. The Colortune won't lie. You've gone Dyna S though haven't you? That shouldn't give you any problems bar loose connections / dodgy coil or leads.

Yep, I have the Dyna S, but still have stock coils. I did trim back the plug wires by 3/8" or so to freshen them up.

I think the oddball streaky Colortune results might (in part) result from the lead on that tool. The Colortune tip has a non-removable aluminum cap that looks like the resistor terminals on an NGK Plug (the one's I remove, since I have a resistor cap). It's not a thick as the NGK tip, but it's thicker than the unrestricted plug tip. I wonder if it "splits the baby" for use with both types of plug leads, adding a hair more resistance to the Colortune than a regular plug would have.

Edit- the tip in question is the end of the long thin extension on the left side in this picture.



Or I am wrong and dumb. Close call.