I've read several thoughts about this screw and seen it called several things. So what does it do?
How important is its setting?
How important is its setting?
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That mix adjustment screw.
- Thread starter weekendrider
- Start date
I don't think it affects much except idle. When the throttle plate opens a little the holes behind in come into play.
Bob Kelly III
Ranch Kid from way back,.... that got Old !
What does it do ? it's very important as it sets the mixture of air to fuel ratio for the engine at Idle....
if its set too rich the engine will load up and puff a bunch of black smoke on throttle up
set too lean it will sogg or bog down, then pick up , upon throttle up so the setting is very important for correct running and throttle response
From what I gather the BS38's adjustment isn't all that critical like 1-1/4 turns out is usually good enough
However the BS34's are very critical indeed.... being capped off at the factory so you can't adjust them does not change the fact they should be adjusted for the proper running engine.... the factory actually detuned the engine to get by epa B.S. this is why the recommended idle is 1200rpm instead of 800 rpm like it should be , as well as the charging circuit not putting out enough till 1200rpm .
that's my take on it anyway f.w.i.w.
You will be surprised ( I was) as how crisp the engine responds after you do a dead cylinder adjustment of the air screws on either carb.
it's just one of those areas where close is Ok, and it works .... but perfect is allot better.
......
Bob.........
if its set too rich the engine will load up and puff a bunch of black smoke on throttle up
set too lean it will sogg or bog down, then pick up , upon throttle up so the setting is very important for correct running and throttle response
From what I gather the BS38's adjustment isn't all that critical like 1-1/4 turns out is usually good enough
However the BS34's are very critical indeed.... being capped off at the factory so you can't adjust them does not change the fact they should be adjusted for the proper running engine.... the factory actually detuned the engine to get by epa B.S. this is why the recommended idle is 1200rpm instead of 800 rpm like it should be , as well as the charging circuit not putting out enough till 1200rpm .
that's my take on it anyway f.w.i.w.
You will be surprised ( I was) as how crisp the engine responds after you do a dead cylinder adjustment of the air screws on either carb.
it's just one of those areas where close is Ok, and it works .... but perfect is allot better.
......
Bob.........
OK xjwmx but it doesn't close so shirley it affects the hole range?
Don't call me Shirley. With the throttle plate closed it's close to 100% but with the throttle wide open I think the effect of it's setting would be small. Pilot jet size would have more of an effect because it effects all the little holes. I've seen people equating a couple extra turns out with an increase in jet size but I don't think that's quite right except at idle. No carb expert at all here, just saying what seems obvious to me.
Bob Kelly III
Ranch Kid from way back,.... that got Old !
I remember reading a thing on the carbs that had a chart on it where it showed the effects of the jetting and RPM
and the idle setting does effect the engine into the entire range ( as you surmised the thing doesn't shut off)
but it's effect after about 4000 RPM is minimal where it is then buried by the throttle valve cutaway and jet needle .....( mid-range)
followed by the main jet above that ...
but yes it does stay on, so it's setting is important as it is mixed with all the other jets in the carb at various RPM's
....
Bob........
and the idle setting does effect the engine into the entire range ( as you surmised the thing doesn't shut off)
but it's effect after about 4000 RPM is minimal where it is then buried by the throttle valve cutaway and jet needle .....( mid-range)
followed by the main jet above that ...
but yes it does stay on, so it's setting is important as it is mixed with all the other jets in the carb at various RPM's
....
Bob........
If you carefully adjust the mixture screw (pilot screw), you can find the "sweet spot". Once the "sweet spot" is found, the engine will run smooth, with its best power and with its best fuel consumption. On my 1978 SE, I use stock air boxes and filters. I also use the stock exhaust. I was using stock sized pilot and main jets on my BS38 carbs. I had quite a few air leaks, which took a while to find but eventually solved that problem.
I used the dead cylinder method to find the "sweet spot", and always ended up with the mixture screw around 3 to 3.25 turns. Yet, after much adjusting, I still would have a wandering and hanging idle. I removed the stock #27.5 pilot jet and installed a #30 pilot jet, one size up. Using the dead cylinder method again, I then found that 1.75 turns on the mixture screw was perfect for both carbs. No wandering or hanging idle. That was 5 years ago, and the mixture screws are still at the 1.75 turns. My engine has run perfectly over that time.
How important is the setting of the mixture screw..........................its very important to find that "sweet spot". However, be aware that the mixture screw is just a fine trim. You must have the correct size pilot jet, in order to get you into a range for the mixture screw to be able to do the final precise trim.
Once your mixture screw is open 3 or more turns, it no longer adjusts the mixture. As soon as I went up one size on the pilot jet, and adjusted the mixture screw to 1.75 turns, I was able to easily find the "sweet spot".
I used the dead cylinder method to find the "sweet spot", and always ended up with the mixture screw around 3 to 3.25 turns. Yet, after much adjusting, I still would have a wandering and hanging idle. I removed the stock #27.5 pilot jet and installed a #30 pilot jet, one size up. Using the dead cylinder method again, I then found that 1.75 turns on the mixture screw was perfect for both carbs. No wandering or hanging idle. That was 5 years ago, and the mixture screws are still at the 1.75 turns. My engine has run perfectly over that time.
How important is the setting of the mixture screw..........................its very important to find that "sweet spot". However, be aware that the mixture screw is just a fine trim. You must have the correct size pilot jet, in order to get you into a range for the mixture screw to be able to do the final precise trim.
Once your mixture screw is open 3 or more turns, it no longer adjusts the mixture. As soon as I went up one size on the pilot jet, and adjusted the mixture screw to 1.75 turns, I was able to easily find the "sweet spot".
Bob Kelly III
Ranch Kid from way back,.... that got Old !
Ahhh Very interesting RG ! thank you...
I assume that the pilot jets are on the small size on my 83'XS650HS because they are Stock other than the adjustment screw
and Yes it was hard to find the "Sweet spot" because the pilot jet was so lean....... Now that I know why I will try to find out what size they are and then buy one step larger Pilot jets and then re adjust ! ... that has got to be better on fuel economy with more air coming in at idle ! LOL
My air screws..... are about 3.5 turns out if I remember correctly or close to it.... I didn't run the screws back in for fear of loosing what I had LOL
but they are in that area i think.
I always thought that 3 to 4 turns out was ODD.... and that explains it... and with the proper pilot jets in the carbs I'm sure they would adjust easier too !
thank you for mentioning that !!!!
....
Bob.........
I assume that the pilot jets are on the small size on my 83'XS650HS because they are Stock other than the adjustment screw
and Yes it was hard to find the "Sweet spot" because the pilot jet was so lean....... Now that I know why I will try to find out what size they are and then buy one step larger Pilot jets and then re adjust ! ... that has got to be better on fuel economy with more air coming in at idle ! LOL
My air screws..... are about 3.5 turns out if I remember correctly or close to it.... I didn't run the screws back in for fear of loosing what I had LOL
but they are in that area i think.
I always thought that 3 to 4 turns out was ODD.... and that explains it... and with the proper pilot jets in the carbs I'm sure they would adjust easier too !
thank you for mentioning that !!!!
....
Bob.........
Bob Kelly III
Ranch Kid from way back,.... that got Old !
it's interesting about the aforementioned adjustment screw ... I call it the idle /air adjustment screw but it does not adjust the air at all
that screw adjusts the amount of fuel and the pilot Jet at the throat of the carb is the air controler....
I call that screw just about everything in the book some times , but it's purpose is to adjust the Fuel air ratio the engine is getting at idle
if the burn at idle is correct the throttle response will be quicker and the engine will actually run cooler than if it was all leaned out as it was stock on the show room floor.... although you never idle an air cooled engine long without a good airflow across the engine
the temperature difference in a 2 minute idle, can be a big difference... between properly adjusted and stock EPA LEAN ....
that I noticed real fast... when I first got the bike going with the stock air screw settings it heated up to too hot to touch on the cylinders before a Minute was up... after the adjustment it takes almost 2 minutes to heat up to that stage now... and that should only improve if I get the right size pilot jets ! ...LOL.... Fun stuff !
.....
Bob.......
that screw adjusts the amount of fuel and the pilot Jet at the throat of the carb is the air controler....
I call that screw just about everything in the book some times , but it's purpose is to adjust the Fuel air ratio the engine is getting at idle
if the burn at idle is correct the throttle response will be quicker and the engine will actually run cooler than if it was all leaned out as it was stock on the show room floor.... although you never idle an air cooled engine long without a good airflow across the engine
the temperature difference in a 2 minute idle, can be a big difference... between properly adjusted and stock EPA LEAN ....
that I noticed real fast... when I first got the bike going with the stock air screw settings it heated up to too hot to touch on the cylinders before a Minute was up... after the adjustment it takes almost 2 minutes to heat up to that stage now... and that should only improve if I get the right size pilot jets ! ...LOL.... Fun stuff !
.....
Bob.......
Hitndahedfred
Do I look like a people person to you?
================================================================
MY experience with it is this.
On a VM34
I was experiencing mid throttle "skipping" it wasn't ignition.
So I added another 1/2 turn out on this adjustment,,,, half throttle skip disappeared.
I'm NOT exactly sure what this adjustment does except it sets air/fuel mix.
Not sure if it does affect other circuits but in my case,,, it did correct an issue I was having.
I think they are pretty useful to find a nice idle on BS38's, they don't seem to have much affect on BS34's But i'm both, lazy and impatient. After a couple weeks of use, Period piece (fully rebuilt 81 BS34's) I thought the smooth idle had coarsened up just a touch. Mostly from listening as Allison leaves for work. Then I test rode it after adding a small windshield, about two miles out I had to switch to reserve. I guess I was hearing the difference that a low fuel level made. The gas tank is (as found) inside not a rusty mess but not quite pristine either. Think I will at least drain the float bowls into a container to see what I see.
Bob Kelly III
Ranch Kid from way back,.... that got Old !
LOL isn't that just like a wife to drive something till it's completely out'a gas !?!?!?! LOL
Gott'a train her better Gary ! it ain't a Horse it don't eat grass, it'a Motorcycle and it eats Gas ! ( you can tell her I said that!)
LOL.....
Bob........
Gott'a train her better Gary ! it ain't a Horse it don't eat grass, it'a Motorcycle and it eats Gas ! ( you can tell her I said that!)
LOL.....
Bob........
Gary's Quote;
" I think they are pretty useful to find a nice idle on BS38's, they don't seem to have much affect on BS34's"
Yes I agree. 5twins has often said that the BS34 mixture screws are best just left at the 3 to 3.5 turns.
My friend had a 1980 with the BS34's. I had to clean his pilot jets one time, and I tried to adjust the mixture screws. I tried to find the best spot for them, but it just seemed rather vague...........................no real sweet spot, so I just left then where I found them. Maybe a step larger pilot jet would be better but I have never had a chance to experiment with BS34's.
" I think they are pretty useful to find a nice idle on BS38's, they don't seem to have much affect on BS34's"
Yes I agree. 5twins has often said that the BS34 mixture screws are best just left at the 3 to 3.5 turns.
My friend had a 1980 with the BS34's. I had to clean his pilot jets one time, and I tried to adjust the mixture screws. I tried to find the best spot for them, but it just seemed rather vague...........................no real sweet spot, so I just left then where I found them. Maybe a step larger pilot jet would be better but I have never had a chance to experiment with BS34's.
I did try going up one on the pilot, and went back to stock.............. I did move the mains up two then left them back at one up , and still have the shim under the needle. It's when I put the stock air boxes back that it went to running as good as any XS I've ridden. Early stock headers and "long commando type" permanent baffle mufflers
So when you adjust the screw you changing the fuel mixture?
Well, this is a very interesting and timely discussion!
After installing the PAMCO and E-Advancer ignition system, plus adjusting the valves, I have been experiencing much louder popping on deceleration - fairly heavy backfiring. I'm not suggesting that the ignition change has caused the backfiring - but SOMETHING sure has changed. Lucille (a 1976 Standard with BS38 carbs and stock air boxes and exhaust) always had a mild pop when I backed off the throttle and I just put it down to an elderly gal, who was aging gracefully. Frankly, once I got her going after many years in a barn, she started well, idled OK and ran up to supra-legal speeds without much difficulty. As a result, I had simply never done anything to the carbs except clean the "cottage cheese" out of the float bowls and blow the "coffee grounds" out of the jets.
I have never even tried to synchronize them and I really didn't know any of the adjustments except for the idle speed screw, with which I had to fiddle nearly every day.
I have also replaced the exhaust gaskets and tightened the intake manifolds up a little - but nothing really made a difference to that backfire, so today I finally got to the carbs. After reading the famous (and incredibly well-done) Carb Guide today, I tried adjusting the mixture screws. Initially, I turned the screws out 1/4 turn and that made NO perceptible difference at all on a test ride over dinner time. She was still popping pretty heavily on deceleration.
When I got home tonight, I turned them out another 1/4 turn (1/2 turn total) and the idle sped up - A LOT. I guess I had found the "sweet spot" that is outlined in the Carb Guide! One piece of data I don't have is how far out the mixture screws are out now - and thus, do I need to change jets. I'll have a look at that tomorrow. I
I can't wait till I can go for a test ride tomorrow (I've had a bit of wine tonight, so no riding for me).
Thanks so much to Grizld1 and 5Twins for that excellent Carb Guide - what a valuable tool for all of us!
Pete
After installing the PAMCO and E-Advancer ignition system, plus adjusting the valves, I have been experiencing much louder popping on deceleration - fairly heavy backfiring. I'm not suggesting that the ignition change has caused the backfiring - but SOMETHING sure has changed. Lucille (a 1976 Standard with BS38 carbs and stock air boxes and exhaust) always had a mild pop when I backed off the throttle and I just put it down to an elderly gal, who was aging gracefully. Frankly, once I got her going after many years in a barn, she started well, idled OK and ran up to supra-legal speeds without much difficulty. As a result, I had simply never done anything to the carbs except clean the "cottage cheese" out of the float bowls and blow the "coffee grounds" out of the jets.
I have never even tried to synchronize them and I really didn't know any of the adjustments except for the idle speed screw, with which I had to fiddle nearly every day.
I have also replaced the exhaust gaskets and tightened the intake manifolds up a little - but nothing really made a difference to that backfire, so today I finally got to the carbs. After reading the famous (and incredibly well-done) Carb Guide today, I tried adjusting the mixture screws. Initially, I turned the screws out 1/4 turn and that made NO perceptible difference at all on a test ride over dinner time. She was still popping pretty heavily on deceleration.
When I got home tonight, I turned them out another 1/4 turn (1/2 turn total) and the idle sped up - A LOT. I guess I had found the "sweet spot" that is outlined in the Carb Guide! One piece of data I don't have is how far out the mixture screws are out now - and thus, do I need to change jets. I'll have a look at that tomorrow. I
I can't wait till I can go for a test ride tomorrow (I've had a bit of wine tonight, so no riding for me).
Thanks so much to Grizld1 and 5Twins for that excellent Carb Guide - what a valuable tool for all of us!
Pete
Bob Kelly III
Ranch Kid from way back,.... that got Old !
Good man Pete ! THANK YOU ! did you know that 99% of ALL motorcycle fatalities involve the rider having too much to drink ?
you might get away with it in a Car on the rare occasion, but on a bike... the odds are stacked against you in a big way !
..... sounds like you need to do a dead cylinder test and get those mixture screws set just right... and then synchronize the carbs !
it sounds like it is in need !!!!!! don't neglect the girl ! give her what she wants ! LOL
..... i was surprised at how much smoother my bike ran after doing all that.....my hands used to go to sleep in about 10 miles on the bike but now, ..... its more like 50 miles ! the vibration almost disappeared ! ....NO REALLY !!!!!
too bad there isn't a fix for the Square gear sound on the clutch side .... that Wrrrr is loud on this bike !.... maybe I need thicker OIL ! LOL
..... or as my step dad would say ..... turn up the radio if ya don't like what ya hear ! ( 18 wheeler driver for over a Million miles!)
......
Bob...........
Okey dokey - I'm now sitting in the Chuckwagon Restaurant in Kingsville, ON and have ridden about 25 miles from home. When I pulled out this morning, the popping was reduced - but still there. I stopped at the end of my street and opened the mixture screws one more quarter turn (3/4 turns counter-clockwise total now) and WAH-LA - no more popping.
Touch wood!
Pete
Touch wood!
Pete
So where does that put your setting total? 2 1/2 - 3 turns out?
