"Do not disturb the pilot screw" ? Idle mixture question.

[tomacGTi]

Unless you inspect the needle for wear, I would ASSume that things are fine. The factory needle, needle jet setup is known to wear less than aftermarket due to the materials used.

Always assume the manufacturer is in CYA mode, not maximum performance. If you are running the needles one clip up from the middle (factory setting) then they are already set richer from factory.

[Squishy]

Always assume the manufacturer is in CYA mode, not maximum performance. If you are running the needles one clip up from the middle (factory setting) then they are already set richer from factory.

What are you saying? Clip in the middle = factory setting right? I indeed thought they would not be set for maximum performance.. so together with the slip-on I decided to put the needles 1 step richer.
However this means the clip is 1 down from the middle, this way the needle sits higher in the carb thus allowing more fuel.

[greg737]

Here's a thought that crossed my mind when you mentioned that your friend's SV 650 (with carbs) handled the high mountain passes way better than your Bandit 400:

It may be that the Bandit 400 simply isn't the bike for that sort of riding (i.e. rides that include massive altitude changes). 

For instance: the Bandit uses Mikuni BST33 carbs on 100cc cylinders which is a pretty monstrous carb-to-cylinder ratio.  Your buddy's SV uses Mikuni BDSR39 carbs on 324cc cylinders, which is obviously a much smaller ratio.

So any change to the atmosphere that the Bandit is breathing will have a much greater overall effect than the same change would have on the SV.

[Squishy]

Here's a thought that crossed my mind when you mentioned that your friend's SV 650 (with carbs) handled the high mountain passes way better than your Bandit 400:

It may be that the Bandit 400 simply isn't the bike for that sort of riding (i.e. rides that include massive altitude changes). 

For instance: the Bandit uses Mikuni BST33 carbs on 100cc cylinders which is a pretty monstrous carb-to-cylinder ratio.  Your buddy's SV uses Mikuni BDSR39 carbs on 324cc cylinders, which is obviously a much smaller ratio.

So any change to the atmosphere that the Bandit is breathing will have a much greater overall effect than the same change would have on the SV.

You might be right.. although I don't think it's normal for any bike, regardless of CC/carb ratio, to be able to give full throttle in 1st gear and just keep standing still. I mean it's not the himalaya.. it was 'only' 2500-2800m.

[greg737]

I fly for a living and during my Air Force days we had to re-qualify in an altitude chamber every 3 or 5 years, depending on type of aircraft.

One of the bits of data that the USAF Aerospace Physiologists repeated over and over was, "the majority of the earth's atmosphere is from sea level to 5,000 feet."  At 5,000 feet there's 14% less oxygen available than at sea level and at 10,000 feet there's 30% less than at sea level.  So 8,000 to 9,000 feet is pretty high.  You gotta respect that.

And the percent of oxygen content in the atmosphere is just half the problem. 

The other, often overlooked factor is the significantly reduced atmospheric pressure at altitude.  It greatly reduces the engine's ability to fill it's cylinders.  This is the factor that is magnified in the Bandit vs. the SV.  The Bandit's huge carb-bore to cylinder size ratio means that its intake vacuum is very "peaky" rather than smooth or uniform in nature (i.e. if you were to see a graphical printout of the Bandit's manifold absolute pressure it would all sharp peaks and valleys, which really means "speed-up then slow-down" over and over.  Very hard to do at low atmospheric pressures).  This is a large part of why the Bandit's engine ends up on its knees at 8,000 feet.

[andrewsw]

Wow, super interesting conversation folks. I just want to second a couple of things and talk about needle jet wear.

First, these carbs are *really* touchy. I find I have to adjust my idle mixture screws for seasonal changes. As a year around commuter, I notice the difference as the seasons change. The bike runs significantly differently in different weather conditions. And humidity has a *huge* impact. So I find myself adjusting the idle mixture several times a year independently of other work I might be doing on the bike. I also second the impact of a good carb synchronization. Time spent getting it as close to perfect as you can stand is well worth it.

Now, needle jet wear is a real thing. After 22 years, the slide carriers wear causing the slides to fit more loosely and most importantly, ride slightly closer to the engine than they should. This induces wear on that side of the needle jet (or emulsion tube) and if you can *see* the wear, it's  having a real impact on performance. I haven't really come up with a good solution for how to deal with the slide carrier wear -- I'll probably shim it by securing some thin brass plate on the surface. Even if you can't see the wear on the needle jet, it's probably there. It's softer than the needle (in my experience, but I don't have stock needles) so you won't see wear on the needle itself, making one think it's all good.

that's my .02

A

[Squishy]

I've never changed the idle mixture in 5 years...  and it runs pretty fine in all condition. From 5c to 30c. Winter to summer.
In that regard I don't have any complains.
Then again I ran the bike on 34hp restriction and it had the clips fully riched, and then stock exhaust clip in the middle, and now clip +1 rich.. and it ran fine in all.

Tomorrow I'm going to remove the carbs (gotta check my throttle cable anyway) and check how the screws are set right now.

[tomacGTi]

I've noticed that you really can only pick up jetting inadequacies at either take-off or full boil.

The fact that you're touring moreso than going full out and really spending the predominance of your time at 8k and below means you're primarily on the needles. If you spent more of it riding aggressively, the main would be more of an issue. Loaded take-off at altitude will show this as well as you've experienced. You are at the maximum though, as there are few places over 3000m that most people would ride. Hell, hop off the bike and sprint 100m at that altitude and see how you do: that's how your bike feels.

I've found as well that needle wear is more apparent in stop-and-go versus more full throttle situations. Reason being, the slide is constantly up and down versus fully up or fully down sawing away at the needle jet.

If your set-up works, then by all means continue on. The rest of us are really just trying to diagnose a problem from a million miles away. My observations are from my bike which is set-up more on the aggressive end: more track oriented which means lots of WFO, good carburation from 1/2 to full throttle for corner exit. Not 3000m loaded touring.

Ironically, the factory settings I've found are much more friendly in most cases to putting around town and easier riding. In the states, they were a touch lean at idle and most folks found that running at 2-2 1/2 turns out on the screws helped the cold-bloodedness as well with the addition of a hotter thermostat to make jetting more consistant. A shim under the needle helped with midrange being lean and the larger main was more than enough at high revs. This is assuming the addition of a slip-on and very few if no airbox mods which affect jetting MORE than if you added a slip-on.

Like I keep saying: you need to know what the circuits do, where the overlap is and how what you change affects everything to YOUR style of riding. Blindly twiddling screws to someone's internuts suggestions may open more of a can of worms than it would to fix a problem.

[Squishy]

Update:
Checked the carbs today. The settings on the pilot screws were:
1: 1¾ out
2: 2¼ out
3: 2 out
4: 2 out

I set them all to 2 out. Set the needles back to middle position.
It runs fine.. I don't really notice any difference to be honest.