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

[Squishy]

Hi,

I found this mentioned in the manual.
I'm gonna put my 400 on a dyno soon and wonder how I would go about changing the mixture at idle?
Apart from the fact that that the manual mentions this.. if you look at the picture from the manual:



you can see the pilot screw actually only meters the airflow that already has both fuel and air.. and therefore doesn't really change the mixture just allows more/less air/fuel mixture.

How would one change the mixture ?

Thanks

[greg737]

The "Do not disturb the pilot screw" statement is something that the manufacturer has to include in the motorcycle's documentation to please the governmental "powers that be".  Are you familiar with the term "Boiler Plate"?  The motorcycle had to pass emissions tests and the idle engine speed test is apparently a very important part of this testing. 

My other bike, a Kawasaki EX-250, has the exact same statement in its service manual.  The EX-250 has Keihin carbs and the Keihin guys even put plastic plugs behind the pilot screws to prevent the owner from changing/adjusting them away from the factory setting.  It is, however, a very common practice to drill out these plastic plugs to re-set the idle screws to a less environmentally-friendly setting to get a good idle (especially on EX-250s that have been modified with aftermarket exhausts and/or airbox).

As far as changing the mixture at idle, the screw indicated by the number 5 on the diagram is the way to do it.

[Squishy]

The "Do not disturb the pilot screw" statement is something that the manufacturer has to include in the motorcycle's documentation to please the governmental "powers that be".  Are you familiar with the term "Boiler Plate"?  The motorcycle had to pass emissions tests and the idle engine speed test is apparently a very important part of this testing. 

My other bike, a Kawasaki EX-250, has the exact same statement in its service manual.  The EX-250 has Keihin carbs and the Keihin guys even put plastic plugs behind the pilot screws to prevent the owner from changing/adjusting them away from the factory setting.  It is, however, a very common practice to drill out these plastic plugs to re-set the idle screws to a less environmentally-friendly setting to get a good idle (especially on EX-250s that have been modified with aftermarket exhausts and/or airbox).

I guess that makes sense. I didn't think they were so strict on it in 1990... Still I think it's weird why they would add the line: "this component is PRE-SET at the factory by very specialized equipment". I think they wouldn't need to make an excuse?

As far as changing the mixture at idle, the screw indicated by the number 5 on the diagram is the way to do it.

But if you look at the diagram, opening or closing will only add more or less air AND fuel (grey arrow). It also says so in the text (I edited the picture above, now with text).
How does this make the mixture leaner or richer? If the screw would be before the part where the fuel enters it would make sense.
Also it's not like it receives only air from another path where adding air/fuel would make it more rich.

[greg737]

well, if you wanted to change the air/fuel mixture ratio that your Bandit's "Slow System" delivers to the Bypass Ports (number 6 on the diagram) and to the Pilot Outlet (number 7 on the diagram) you would need to make a change to the Pilot Jet (number 3 on the diagram).  You would have to either install a larger Pilot Jet or modify the existing Pilot Jet in some way (bore it out to a larger size with a drill bit).

but why would you need to do this? 

[Squishy]

well, if you wanted to change the air/fuel mixture ratio that your Bandit's "Slow System" delivers to the Bypass Ports (number 6 on the diagram) and to the Pilot Outlet (number 7 on the diagram) you would need to make a change to the Pilot Jet (number 3 on the diagram).  You would have to either install a larger Pilot Jet or modify the existing Pilot Jet in some way (bore it out to a larger size with a drill bit).

but why would you need to do this?

Well, how else does one make the idle mixture leaner or richer on the bandit 400?
As far as I know, on most carbs this is what a pilot screw does. It either allows more air or more fuel at idle or slight throttle. However from the picture here it seems it does both on the 400.
If the pilot screw doesn't make it leaner or richer then what does it do?

[greg737]

I would guess that the process of dialing in the B400's idle should go like this: 

If the bike has the original, factory-installed air filter and exhaust header and exhaust canister, all you need to do is dial the idle knob that is provided to achieve the correct idle RPMs (you know the knob I'm talking about, the one on the flexible stalk that extends downward from the middle of the carb bank, the one that when adjusted changes, ever-so-slightly, the angle that the carb's throttle-plates come to rest at when the handle-bar throttle is released).  If the intake and exhaust are stock then the factory pilot jet size and the factory pilot screw setting should deliver the correct air/fuel ratio at idle.

However, if modifications have been made to the B400's intake and/or exhaust that improve the volumetric efficiency of the engine (in other words, if the engine is flowing more air as the result of performance mods) then it will need to be re-tuned so that it gets more fuel at every single point throughout its performance envelope, which includes idle speed.  Without this the engine might end up running air/fuel mixtures that are too lean which could hurt performance, or mess up the engine's throttle-response or even possibly cause burned piston crowns.  In this case something would need to be done to introduce more fuel into the idle mixture.  I would imagine that this could be achieved by simply opening up the idle screw a little bit (making it richer than the factory-spec setting) and if this action caused the engine's idle speed to go above factory-spec then you would ever-so-slightly close the throttle plates with the plastic knob mentioned earlier.

I know that you just got this Bandit, and, of course, it's 22 years old so just about anything could have happen to it during that time. 

Along those lines, this thought just occurred to me: Does your Bandit have its plastic idle knob?  As I mentioned it is the knob that is attached to a flexible stalk that is in turn attached to the middle of the carb bank.  When not needed this flexible stalk is meant to be "hung up" out of the way on a little hook provided on the bottom-left-hand side of the carb bank, pointing toward the left-hand side of the bike.

[Squishy]

Yes I understand the above. But I'm not trying to adjust the idle, I know how to do that. I'm asking about the idle mixture (rich/lean).
I will be going to the dyno soon to get a print out of the air/fuel ratio. This is because last year I was way up on a mountain (2800m/9000ft) and I couldn't even drive off from a stand still without clutching to 6000rpm+.

Anyway.. I expect it to be too rich (since there's less dense air up high). I think it might be because of worn needles, but also because my pilot screws are fiddled with (they are not set equally among all carbs).

Now I know how to change the mixture for mid-throttle (needles) and for full throttle (main jet) but I can't figure out how to change the mixture for idle.
Normally, this is done with pilot screw..but when I looked in the manual I noticed this only allows more or less air/fuel... so by judging from the manual the pilot screw will not make the mixture leaner or richer (just like the idle knob, which yes I have).

That's why I'm wondering how to change the idle mixture on the BST33SS carb.
I only have an aftermarket muffler. Stock airfilter/carbs.
Btw I didn't just get this bandit.. I've had it for almost 5 years ;).

[ventYl]

Despite the fact that diagram shows, pilot screw can add air/fuel aerosol, it in fact *DOES* change idle mixture. Amount of air/fuel aerosol is added to air, which passes throught throttle plates. Thus more air/fuel aerosol -> more rich mixture on idle/slow RPM. less air/fuel aerosol (tighter pilot screw) -> leaner mixture.

[tomacGTi]

First off: read Factory Pro's page on how CV carbs work.

The screws are supposedly preset from the factory and then plugged. While this may be the case, they're probably all set differently so they're most likely wrong. It's amazing that they're still in there.

You have BST33s with adjustable needles. If everything else is still stock, dial all of the screws in and back them out two turns. This is the standard for these and the 32s we got gipped with in the states. Slip-on add another 1/2 turn. Your mains from the factory may be less than what we got which is a 102.5. With the factory needles, this is enough for most slip-ons though maybe a bit fat.

Do not mess with the idle other than to get it to sit at spec when HOT. It should come up and settle down. If it hangs, it's lean, if it drops below and comes back, it's rich.

The basics are:

-2 to 2 1/2 turns out

-14.5mm float level (very important)

-In your case: middle clip position

-Stock main (102.5 for the US) rest of world IIRC is 97.5

-Sync your carbs. This does much more than you think

This is working with what you have. If you install a jet kit, header, air filter: everything changes. Refer to the jetting database that's stickied at the top for REFERENCE, not all is gospel as every setup is different.

If you've had this bike for some time and plan to work on it yourself, the first step is knowing how these carbs work. They are the single biggest chestpain of these bikes and I personally have wasted a lifetime's worth of Sundays on mine. Know what all of the circuits effect and use the database as a guide for your setup.

[Squishy]

The basics are:
The screws are supposedly preset from the factory and then plugged. While this may be the case, they're probably all set differently so they're most likely wrong. It's amazing that they're still in there.

-2 to 2 1/2 turns out

-14.5mm float level (very important)

-In your case: middle clip position

-Stock main (102.5 for the US) rest of world IIRC is 97.5

-Sync your carbs. This does much more than you think

Thanks for your reply.

What do you mean it's amazing that they're still in there? I don't have any plugs - I can see the screw directly from the bottom.
I always thought all 4 should be set the same turns? I noticed that the left 2 carbs are set differently than the right 2 carbs. I thought they should be set similarly because all 4 carbs are basically the same?

Anyway,
- According to my manual the stock setting is 1½ turns out.. not 2 turns. (picture)
- Stock for BST33 in my country is 100 main which I have.
- I set the needles clip to +1 richer 2 years ago because I bought a slip-on. Rest of bike is stock. I did this because I figured it would be leaner with slip-on? And I thought it would be lean from factory. Should I put it back?
- Unfortunately I can't turn the screws while the carbs are mounted.. I can't even see the screw and can't reach with a screwdriver.. tips?
- Carbs are synced 1 year ago after I did the valves.
- I checked float height and they are exactly as described in the manual (14.6mm +-1mm) (picture)

I still don't see how adding more of the same mixture makes the idle mixture richer...but OK.

Thanks.

[tomacGTi]

-Most mixture plugs have been removed by now. If yours were intact, then that's rare.

-Lower the needle back to the middle setting. You have changed the midrange fueling unless your bike runs well there.

-The mixture screw affects fueling across the entire range, moreso in the low. In theory, the inners should have more turns than the outers (hotter, needing more fuel to cool the cylinders) without a dyno and serious time, most just turn them all the same.

-You can access the screws with the carbs mounted. Be creative. I use a long screwdriver bit and a mirror.

-Stock screw setting is known to be lean. Adjusting to 2 or 2.5 is common knowledge for accommodating a slip-on

-You are adding in more fuel when you turn the mixture screws out: see the third bullet point.

-READ the FP CV tuning guide as well as the jetting database.

-I have found that even turning the idle screw will affect the synch of the carbs. Yes, they are that sensitive. Do you have to synch every time: no, but it helps the smoothness of the bike. Caveat emptor.

-At 3000m, your carb is going to be PIG RICH. There is no jetting that can accommodate that swing from sea level to that altitude. You jet for one or the other. If you are trying to do this, you are the perfect candidate for fuel injection. If you find yourself up there for an extended period of time, you can dial out the mixture screw 1/2 turn at a time until it's OK. When you return to sea level, return back to your previous settings because you will find you are now extremely LEAN.

-If this is the reason for you mucking about with your jetting in the first place and the bike ran fine at every other location: don't touch a thing. It's already set-up and fine. You don't need to muck about further.

[Squishy]

-At 3000m, your carb is going to be PIG RICH. There is no jetting that can accommodate that swing from sea level to that altitude. You jet for one or the other. If you are trying to do this, you are the perfect candidate for fuel injection. If you find yourself up there for an extended period of time, you can dial out the mixture screw 1/2 turn at a time until it's OK. When you return to sea level, return back to your previous settings because you will find you are now extremely LEAN.

-If this is the reason for you mucking about with your jetting in the first place and the bike ran fine at every other location: don't touch a thing. It's already set-up and fine. You don't need to muck about further.

Yes, this is the reason I want to change the needles and screws for a slightly leaner mixture. This year i'm gonna do a trip with lots of mountain passes (2000-2800m).

I know you can never have a perfect mixture for all altitudes, but mine was so bad it was unrideable.
I could hardly get up the mountain, because at every U-turn I had to clutch completely, then get it to 8000rpm to slip the clutch.
At the top of the mountain, I was in 1st gear standing still, and I could crank the throttle fully open while sliping the clutch and nothing would happen.
The way I see it, the mixture must be on the rich side for this to happen so badly.

A friend who was with us, driving his SV650 with carbs (old model) only suffered some minor power loss.

Normally, the bike runs great (altitude: 0 meters). But I noticed the screws were not set the same for all 4 carbs and I had +1 on the needle.
I figured I could set them back to the middle clip and set the 2 screws that were turned open more to the same position of the other 2 carbs.

Surely I could just screw them back to the current position if it doesn't run well?

What do you think?

Thanks.

[tomacGTi]

You're comparing a torque-rich twin to a small bore four cylinder. Your friend will have less of an issue than you almost always at take-off.

I would set all the screws the same and in worst case scenario, bring the items to set the screws with you to adjust at altitude. CVs can compensate but you have to remember that there is no replacement for displacement and at altitude, it makes even more of a difference.

Your needle setting may have contributed as well, it will still pull some from the orifice since the needle is never "fully" seated. All of the circuits have overlap, I would give yourself a baseline and adjust accordingly. If that means your needles where they are now or one clip up. I personally would use the screws but you're 6000kms away and your bike setup is drastically different than mine.

[tomacGTi]

Something else to consider: At 1500m, there is 10% less oxygen at sea level. Once you start adding altitude, the ratio is 7% less for each 1000m.

Ideally for optimum fuel consumption, the ratio is 14.5:1. For performance, it is always set richer at around 12.5-13:1. In heavy accelleration, it can dip down to almost 10, depending on how it is set up.

These bikes do not like to be lean, they like to be set a little fatter for best performance so think of this when jetting the bike.

If spending time at higher altitudes, you can possibly replicate the lack of oxygen by taping off the airbox hole and trying to run the bike. You can then try to jet around the lack and take notes. Not ideal, but it should give you an idea. Mess with the screws first, then drop the needle.

[Squishy]

Thanks, very useful.
I'll give it a try.
I think I'll just set the screws to the 2 screws that were leanest and see how it runs. If it runs OK at 0 altitude, surely it can only be better in the mountains compared to before.

Same goes for the needle. And surely running the bike on stock needle settings can't be that off?
Something I'm considering is that due to wear the needles are letting through more fuel thus being even richer than they would on 1 setting richer.

I could also try removing the silencer from the slip-on once in the mountain...