ADVrider

Go Back   ADVrider > Bikes > Thumpers
User Name
Password
Register Inmates Photos Site Rules Search Today's Posts Mark Forums Read

Reply
 
Thread Tools Search this Thread Display Modes
Old 05-23-2011, 12:09 PM   #766
motolab
Beastly Adventurer
 
motolab's Avatar
 
Joined: Feb 2006
Oddometer: 2,301
Quote:
Originally Posted by Sparrowhawk View Post
I'm not being critical here but isn't the pilot screw controllng air and not fuel? As I understand it adjusting the screw out increases the amount of air and leaning the idle mixture, not making it richer.
On a BST40, the pilot screw is a fuel screw. In fact, I don't know of any CV carb that uses an air screw. If you are unsure whether a pilot screw is a fuel screw or an air screw, the rules of thumb are that fuel screws usually use an o-ring, whereas air screws don't, and that fuel screws are usually located downstream of the throttle valve, whereas air screws are located upstream.

Regards,

Derek
motolab is offline   Reply With Quote
Old 05-23-2011, 12:30 PM   #767
makazica
Studly Adventurer
 
makazica's Avatar
 
Joined: May 2008
Location: Zagreb, Croatia
Oddometer: 738
Quote:
Originally Posted by laramie LC4 View Post
bummer, that's a nice looking bike. if you decide to part it out, let me know and i will take the tank. been looking for a 12L tank for some time now. right now my poor engine is sitting on the garage floor waiting to be shipped off to Cut7 for a total overhaul. crank lightened/balanced, counter balancer balanced, ported and polished head, over-sized valves, custom cut cam, and my FCR is going from 39mm to 43mm. cant wait to get her back and running....

laramie
You Laramie are a bad, bad boy!



M.
makazica is offline   Reply With Quote
Old 05-23-2011, 12:41 PM   #768
laramie LC4 OP
crash test dummy!
 
laramie LC4's Avatar
 
Joined: Sep 2004
Location: Tucson, Az
Oddometer: 2,351
Quote:
Originally Posted by makazica View Post
You Laramie are a bad, bad boy!



M.

sometimes i just cant help myself.

i'm just trying to not think about how much it is gonna cost.....

as for the fuel screw....

The pilot circuit has two adjustable parts, the pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulates fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

laters,

laramie
__________________
DON'T TRUST CUT 7! HE IS A CROOK! ASK ME HOW I KNOW.

'12 LC8 990R, '02 LC4 640, '05 WR 450f (part-out), '98 XR400R, '76 KE100, '05 525 (Step-Child)
laramie LC4 is offline   Reply With Quote
Old 05-23-2011, 02:00 PM   #769
Rumlover
Ed
 
Joined: Sep 2010
Location: Utah
Oddometer: 707
Quote:
Originally Posted by laramie LC4 View Post
sometimes i just cant help myself.

i'm just trying to not think about how much it is gonna cost.....

as for the fuel screw....

The pilot circuit has two adjustable parts, the pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulates fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

laters,

laramie
I am confused (nothing new), the manual for the BST40 on my DR650 shows a pilot jet (fuel), and a pilot air jet. They combine in a passageway and the MIXTURE is then discharged through two seperate ports: one is a fixed opening and the other is controlled/metered by the pilot screw.

Rumlover screwed with this post 05-23-2011 at 02:09 PM
Rumlover is offline   Reply With Quote
Old 05-23-2011, 02:14 PM   #770
laramie LC4 OP
crash test dummy!
 
laramie LC4's Avatar
 
Joined: Sep 2004
Location: Tucson, Az
Oddometer: 2,351
Quote:
Originally Posted by Rumlover View Post
I am confused (nothing new), the manual for the BST40 on my DR650 shows a pilot jet (fuel), and a pilot air jet. They combine in a passageway and the mixture is then discharged through two separate ports: one is a fixed opening and the other is controlled/metered by the pilot screw.

the pilot jet controls the fuel flow while the pilot air jet controls the air. what most people do when working on the bst-40 is increase the idle jet, this richens the system by providing more fuel. the other option that some take is to install a smaller pilot air jet, this basically has the same affect of richening the mixture by reducing the amount of air flow being mixed in. most go for the idle jet though because the want more fuel, not just richer.

laramie
__________________
DON'T TRUST CUT 7! HE IS A CROOK! ASK ME HOW I KNOW.

'12 LC8 990R, '02 LC4 640, '05 WR 450f (part-out), '98 XR400R, '76 KE100, '05 525 (Step-Child)
laramie LC4 is offline   Reply With Quote
Old 05-23-2011, 02:23 PM   #771
Rumlover
Ed
 
Joined: Sep 2010
Location: Utah
Oddometer: 707
Quote:
Originally Posted by laramie LC4 View Post
the pilot jet controls the fuel flow while the pilot air jet controls the air. what most people do when working on the bst-40 is increase the idle jet, this richens the system by providing more fuel. the other option that some take is to install a smaller pilot air jet, this basically has the same affect of richening the mixture by reducing the amount of air flow being mixed in. most go for the idle jet though because the want more fuel, not just richer.

laramie
That was my understanding. I was just confused by some of the questions and replies and wanted to make sure we were all on the same page so to speak.

I have always assumed the BST40 for the KTM is basically the same carb for the DR650 (except for some minor differences like jetting)

Thanks for all the great info!

Edit: Just to clarify the point I was trying to make is that the pilot screw is actually metering a "mixture" of fuel and air. And as you indicated that can be made leaner or richer by changing either the pilot jet or pilot air jet.

Rumlover screwed with this post 05-23-2011 at 02:59 PM
Rumlover is offline   Reply With Quote
Old 05-23-2011, 04:40 PM   #772
Sparrowhawk
Beastly Adventurer
 
Sparrowhawk's Avatar
 
Joined: Sep 2005
Location: Eastern Washington, USA
Oddometer: 1,979
Quote:
Originally Posted by motolab View Post
On a BST40, the pilot screw is a fuel screw.
Regards,

Derek
Thanks for the clarification. I was thinking it was the other way around, an air screw. My day is complete because I've learned something new. I can start drinking now.


When making final adjustments on the BST40 pilot screw is it best to:
  1. Set for the highest idle speed.
  2. Set for the smoothest first or second gear riding with the throttle opened slightly above the idle stop. (No surging)
  3. Determined by how the engine returns to idle after lightly blipping the throttle (lean > stays at higher RPM a little before returing to idle, correct > return to idle RPM, or rich > drops below idle RPM and then back up). or
  4. Set for how well the engine responds when the throttle is blipped. (quick, slow, stumble, etc.)
Sparrowhawk is offline   Reply With Quote
Old 05-23-2011, 04:57 PM   #773
Droptarotter
Beastly Adventurer
 
Joined: Jul 2007
Location: Cloverdale
Oddometer: 1,187
Quote:
Originally Posted by Sparrowhawk View Post
Thanks for the clarification. I was thinking it was the other way around, an air screw. My day is complete because I've learned something new. I can start drinking now.


When making final adjustments on the BST40 pilot screw is it best to:
  1. Set for the highest idle speed.
  2. Set for the smoothest first or second gear riding with the throttle opened slightly above the idle stop. (No surging)
  3. Determined by how the engine returns to idle after lightly blipping the throttle (lean > stays at higher RPM a little before returing to idle, correct > return to idle RPM, or rich > drops below idle RPM and then back up). or
  4. Set for how well the engine responds when the throttle is blipped. (quick, slow, stumble, etc.)

#2 & #4

Cheers
__________________
HP2 Sport, R1200S, R100S, KTM 640 Adventure, KTM 520 EXC/Motard, Yamaha RZ350, Hodaka 125 Wombat
Droptarotter is offline   Reply With Quote
Old 05-23-2011, 05:07 PM   #774
Rumlover
Ed
 
Joined: Sep 2010
Location: Utah
Oddometer: 707
Quote:
Originally Posted by Sparrowhawk View Post
Thanks for the clarification. I was thinking it was the other way around, an air screw.

It is neither the fuel screw or the air screw. It is the pilot screw and it meters a mixture of fuel and air. The air fuel ratio of that mixture is pre-determined by the pilot jet and pilot air jet. Sorry for beating a dead horse.

Rumlover screwed with this post 05-23-2011 at 05:29 PM
Rumlover is offline   Reply With Quote
Old 05-23-2011, 05:27 PM   #775
motolab
Beastly Adventurer
 
motolab's Avatar
 
Joined: Feb 2006
Oddometer: 2,301
Quote:
Originally Posted by Sparrowhawk View Post
Thanks for the clarification. I was thinking it was the other way around, an air screw. My day is complete because I've learned something new. I can start drinking now.


When making final adjustments on the BST40 pilot screw is it best to:
  1. Set for the highest idle speed.
  2. Set for the smoothest first or second gear riding with the throttle opened slightly above the idle stop. (No surging)
  3. Determined by how the engine returns to idle after lightly blipping the throttle (lean > stays at higher RPM a little before returing to idle, correct > return to idle RPM, or rich > drops below idle RPM and then back up). or
  4. Set for how well the engine responds when the throttle is blipped. (quick, slow, stumble, etc.)
With the engine all the way warmed up, set the CO to between 2-4 percent. If you don't have a way to measure CO, lower the idle speed a little bit, and then adjust the idle mixture for strongest idle. Then adjust the idle speed back up to normal. Once this is done, the engine will return to idle rapidly. If it then does not run well during 1/16 to 1/8 throttle opening operation, it needs a different sized pilot jet installed (at which point the idle mixture will have to be readjusted via the above method). If it does not run well at low rpm (up to ~3K rpm) regardless of throttle opening, then the float height is incorrect, the emulsion tube is worn out/the wrong size, or the needle base diameter is incorrect.

Regards,

Derek
motolab is offline   Reply With Quote
Old 05-23-2011, 05:31 PM   #776
motolab
Beastly Adventurer
 
motolab's Avatar
 
Joined: Feb 2006
Oddometer: 2,301
Quote:
Originally Posted by Rumlover View Post
It is neither the fuel screw or the air screw. It is the pilot screw and it meters a mixture of fuel and air, the ratio of which is pre-determined by the pilot jet and pilot air jet. Sorry for beating a dead horse.
It is a fuel screw because it adds or subtracts fuel in order to change the mixture (that there is some air bled into the fuel prior does not change this). An air screw takes the place of the pilot air bleed jet and changes the mixture by changing the amount of air that is bled in.

Regards,

Derek

motolab screwed with this post 05-23-2011 at 06:20 PM
motolab is offline   Reply With Quote
Old 05-23-2011, 05:43 PM   #777
Rumlover
Ed
 
Joined: Sep 2010
Location: Utah
Oddometer: 707
Quote:
Originally Posted by motolab View Post
It is a fuel screw because it adds or subtracts fuel in order to change the mixture (that there is some air bled into the fuel prior does not change this). An air screw takes the place of the pilot air bleed jet and changes the mixture by changing the amount of air that is bled in.

Regards,

Derek

I don't disagree it adds or subtracts fuel. But the air that is bleeding in is relavent. Both jets are fixed in size and establish an A/F ratio for that circuit. The (DR) manual refers to it as a pilot screw that meters a mixture of fuel and air. If you want to call it a fuel screw go ahead, I will refer to it as the manual states it and explains it.

Edit: I have never seen it refered to as a pilot air "bleed" jet, only a pilot air jet.

Rumlover screwed with this post 05-23-2011 at 06:00 PM
Rumlover is offline   Reply With Quote
Old 05-23-2011, 06:05 PM   #778
motolab
Beastly Adventurer
 
motolab's Avatar
 
Joined: Feb 2006
Oddometer: 2,301
Quote:
Originally Posted by Rumlover View Post
I don't disagree it adds or subtracts fuel. But the air that is bleeding in is relavent. Both jets are fixed in size and establish an A/F ratio for that circuit. The (DR) manual refers to it as a pilot screw that meters a mixture of fuel and air. If you want to call it a fuel screw go ahead, I will refer to it as the manual states it and explains it.
Using the term "pilot screw" is not technically incorrect, as it encompasses both fuel and air screws, but using the terms "fuel screw" or "air screw" is more precise, as it allows an understanding of whether screwing the screw out will richen the mixture by adding fuel (in the case of the fuel screw) or lean the mixture by adding air (in the case of the air screw), or whether screwing the screw in will lean the mixture by subtracting fuel (in the case of the fuel screw) or richen the mixture by subtracting air (in the case of the air screw).

Regards,

Derek
motolab is offline   Reply With Quote
Old 05-23-2011, 06:19 PM   #779
Rumlover
Ed
 
Joined: Sep 2010
Location: Utah
Oddometer: 707
Quote:
Originally Posted by motolab View Post
Using the term "pilot screw" is not technically incorrect, as it encompasses both fuel and air screws, but using the terms "fuel screw" or "air screw" is more precise, as it allows an understanding of whether screwing the screw out will richen the mixture by adding fuel (in the case of the fuel screw) or lean the mixture by adding air (in the case of the air screw), or whether screwing the screw in will lean the mixture by subtracting fuel (in the case of the fuel screw) or richen the mixture by subtracting air (in the case of the air screw).

Regards,

Derek
IMHO I think using the terms "fuel screw" and "air screw" is the confusing issue (and less "precise"), as neither exist on this carb. Jets have replaced both those items and continuing to use those terms only adds to confusion.
Will have to continue later -- gotta go
Rumlover is offline   Reply With Quote
Old 05-23-2011, 06:36 PM   #780
laramie LC4 OP
crash test dummy!
 
laramie LC4's Avatar
 
Joined: Sep 2004
Location: Tucson, Az
Oddometer: 2,351
I see Derek is trying to make friends again....

Laramie
__________________
DON'T TRUST CUT 7! HE IS A CROOK! ASK ME HOW I KNOW.

'12 LC8 990R, '02 LC4 640, '05 WR 450f (part-out), '98 XR400R, '76 KE100, '05 525 (Step-Child)
laramie LC4 is offline   Reply With Quote
Reply

Share

Thread Tools Search this Thread
Search this Thread:

.
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump


Times are GMT -7.   It's 06:56 PM.


Powered by vBulletin® Version 3.8.5
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright ADVrider 2011-2014