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Cold blooded idle

  • Thread starter Thread starter MisterCinders
  • Start date Start date
M

MisterCinders

Guest
Soooo close.

After shimming the needle to work through a flat spot at mid throttle, the bike seems to run nicely now, save one thing at idle.

Starts up fine and idles well. After riding a bit, the idle tends to sag and sometimes wants to whither out. Blipping the throttle brings it back with no hang, but at longer stops, it will sag after about ten seconds.

I suspect it is a touch rich and thus bogs when warm. Will pull plugs and try to dial in the screws. Right now, pilot settings are:

Fuel 7/8 turn.
Air about 2 turns after highest idle.
Pilots are 15

Valves adjusted, carbs clean, synched, timing adjusted, battery good, etc.

Gotta say that these last few issues are pretty maddening, since they only manifest on the road instead if in the garage.
 
I have heard it said that, if you blip the throttle and it hangs a bit, then comes down, the idle circuit is a bit lean.
If you blip the throttle and it comes right down, but dips below the set point and comes back up, it's a bit rich.

Since yours seems to be hanging a bit, try turning the fuel screws out about 1/8 turn.
Might have to turn the air screws out just a bit less than 1/4 turn to compensate.

.
 
Thanks Steve. This is more like a sag than a hang. It idles at about 1,100 but then will wander down. Blipping to about 2,000-2,500 will drop right back to normal idle with no hang. Few seconds later it will stray down to 500 or so. Sometimes it wanders back to idle, but sometimes it withers to stall.

Since it emerges after riding a bit, I figure it is a cold-blooded and rich issue. We shall see.
 
Btw - having snapped a fuel screw once before, I am very wary of going too far when bottoming them out. Any tips (see what I did there?) for getting to true bottom on these? When setting them, I suspect that I back them out from less than seated. The springs and oring sometimes prentice enough resistance to make seating them hard to find.
 
Bump with an update and request for further advice. I am definitely chasing my tail here, and am close to empty on ideas.

I changed the pilots to 17.5s because my old stock pilots looked to be clogged, even after soaking in some dip. Once back together, the bike was really hard to start at first, and then ran rich as hell at idle.

So I picked up some brand new Mikuni 15 pilot jets and put those in. I also changed to some new pilot fuel screws from Z1 that I really like, as they are much easier to adjust. I'll do a quick product review on those in another thread.

Checked the floats again and fine-tuned them to 24mm.

Buttoned her back up and set the pilot fuel screws to 1 turn out and pilot air screws to 2 turns out. Charged the battery to make sure I had good juice after all the starting and testing.

After struggling a bit for the first start-up, I got the bike running and idling at about 1,000. Did my best to adjust the air screws to highest idle, but the idle still wanders/sags. It does not hang after a throttle blip, but when it idles for more than 15-30 seconds, the RPMs will randomly dip and sometimes peter out to zero and die. This happens regardless of where the idle screw is set, and a throttle blip will rev fine and return crisply to a decent idle, or a high idle when I've overcompensated on the idle screw. After a short while, the idle would falter, often after a pod cough or backfire. It also seemed to rattle in the front.

Concerned about that rattling noise, I pulled the valve cover and checked out my clearances. I adjusted them last Fall and re-checked them a few weeks ago, finding all the clearances good and healthy. On this check, the clearances were still excellent, all well above 0.06 mm and most in the 0.08 mm - 0.09 mm range. Cams looked good, showing no wear or rust, etc. Plugs were a bit sooty.

I reset the fuel screws to about 7/8 and checked the floats yet again. Following some of the tips in other threads here, I also checked my choke forks and operation to make sure they were not hanging up anywhere. All good.

Here are all the specs for this last reassembly:
Mains 122.5
Needle - 4th slot but with two of the small rubber spacers on top and the large spacer below to shim them up a bit.
Pilot - brand new Mikuni 15
Fuel screw - 7/8 turn out
Air screw - about 2 turns out
Throttle cables are good, and snap back at all bar positions
No leaky fuel drips from the bowls, pods or overflow tubes.
Valves are in spec.
Carbs fully cleaned, stripped, new o-rings, etc.
Timing confirmed/adjusted with static light per DynaS instructions.
Plugs are new and clean (I have two sets rotating for these tests, one set soaks in dip for scrubbing and spraying before changing out).
Battery is healthy and fully charged.
Choke assemblies are functioning properly.
Petcock is working properly.
​
Despite all of this, the idle still wanders south randomly, and it still backfires now and then. I took the bike for a couple of laps around the block to test it under load. It pulls away decently and doesn't seem to hesitate or "hunt for power." It also feels pretty smooth through the lower half of the throttle range, with no flat patch between pilot and needle. On decel, however, it pops quite a bit through the exhaust. At a stop, the ADD idle resumes, requiring throttle blips to keep the engine running.

All I can think of next is to check the synch. I vacuum synched the carbs when I changed the needle the other day, but the rattling and erratic idle might indicate something still out of synch. I don't know how the synch might have slipped, since I haven't disrupted the slides or changed any valve gaps.

Other than re-synching the carbs, what other issue should I investigate?
 
Ugh. Found the culprit. Intake boot on No. 1 has a nice split at the seam.

Awesome.
 
If #1 is bad, imagine the evil cracks lurking in the other 3

I guess you were lean.

Now you get to retune your carbs again
 
Pulled the other boots, but they check out. Definitely throws off my synch.

Still cannot figure out how air leak lead to black plugs, including No. 1.
 
Ordered new boots, but while I wait for them to come in, I JB welded the crack so I could do a little riding. It's a ghetto fix, I know, but the new parts won't get here until Tuesday, the weather is sweet, etc.

The idle still meanders, and I suspect that the other boots may have some leaks as well, though I found nothing when I checked them out. When the idle tries to hang, just a gentle lift on the carbs brings it back down. The No.1 tear was at the top, probably from the strain of cantilevering the rack with no airbox (more ammo for the pod-haters). Probably have similar cracks starting on the other boots as well, since the light lift affects the idle.

Replacing the whole set in a couple days to be sure. I also tied a loop of wire to help support the pods at the back. Might look into a pipe strap or other means of support for the back of the rack/pods.

Other than the wonky idle, the bike performs pretty nicely. Pulls through idle and mid throttle well. Haven't gotten to open her up fully (not much opportunity in the city). Will wait until I have proper boots to get things really dialed in.
 
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