Friendly Dissagreement about Mixture Screws

LTGTR

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Im having a friendly dissagreement/discussion about the way the mixture screw works on our BS38 Carbs.
My friend says the early BS38's (he means up to TX) have a mixture screw that richens the mixture when you turn it out, and on later models (my 77D) it works the opposite i.e. it leans the mixture if you turn the screw out.
From my learning on this forum I beleive the mixture screw works the same on all BS38's i.e. you richen the mixture if you turn the screw out. I remember 5twins/Jim or Gary describing the mixture screw as a "real mixture screw" because what arrives, and is metered by the screw, is really a mixture of air and petrol and turning the screw out allows more mixture in.
My understanding is because the mixture screw is on the engine side of the throttle valve then its a mixture (or fuel) screw - on the air cleaner side of the throttle valve its an air screw. Did the design of the BS38's change over the models.
My friend did explain his opinion to me - he says its because of the different way the air mixes with the petrol in the bowl - early design the air mixes via the holes in the pilot jet - later models the air mixes with the petrol as it exits the top of the pilot jet.
Can somebody clarify this please,
Thanks Ray.
 
You are right. All years of BS38 mixture screw work (pretty much) the same way. They control emulsified fuel not the air flow, out is richer.
The slight change was the location of where the air/fuel gets mixed. Was after, then changed to inside, the pilot. But always mixed BEFORE the idle mix screw.
Very early carbs have all the idle mix through the mix screw and out a single hole into the bore, all later carbs have 3 holes with the screw controlling only one hole.
bs38 float bowl passages pilots.jpg

Or something like that....
 
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My friend did explain his opinion to me - he says its because of the different way the air mixes with the petrol in the bowl - early design the air mixes via the holes in the pilot jet - later models the air mixes with the petrol as it exits the top of the pilot jet.
Can somebody clarify this please,

Your friend explained himself out of his own argument. In both his examples he's drawing a fuel/air mix out of the float bowls. He's correct in that there's two different float bowls on the -38's and yes, two different types of pilots... and two different way of mixing the air. But at the end of the day we're still talking about fuel (and air) drawn from the pilot and fed to the mix screw. Opening up the mix screw allows more of the fuel/air mix into the intake side.

So yes, two ways of doing it... but no, both cases regulate the amount of fuel.
 
I've never seen a very early set of carbs so I can't comment on them, but all the later sets actually have 4 holes from the pilot jet into the carb's main bore. Three are clustered together at the top of the bore about where the butterfly plate closes. These flow the fuel/air mix directly from the pilot jet and there is no adjustment. The 4th hole is off to the side and flows past the mix screw so it is adjustable .....

Pilot Outlets BS38.jpg


This gives you control over roughly 25% of the amount of fuel/air mix being delivered, and is why the mix screws only make fine adjustments. Now, this changes the amount of mix coming through but not it's strength. To do that, you must change the pilot jet size.
 
Thanks everybody - I didnt know the early carbs only had one hole versus the later carbs having 3 (not controlled) and 1 controlled (by the mixture screw).
All I have to do now is "politely" suggest to him that screwing out means richer in all BS38's.
Thanks - Ive learnt something as well.
Ray
 
Went down, looked and took some pics of early carbs, there's two direct from pilot holes, not one.
Early should perhaps be amended to VERY early, we are talking about the first 2500 bikes of the line in 1969/70
1691939313713.jpeg



Early BS38 ID SB.jpg
 
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