1976 BS38 IDLE MIXTURE SCREW QUESTIONS

[Norton7d]

After reading many threads, there is differences of opinion if they are an air, or a fuel screw,nevertheless my questions are in reference to the capability of my screws.
According to my carbs, my screws are plastic and only have the capability to turn 3/4 turn due to a plastic nob on the plastic screw, and a nob on the body of the left carb, which doesn't allow the screw to turn more than 3/4 turn.
Do I have this right, are these the air / fuel screws?

 

[member28833]

Yes, fuel mixture screws. Seeing your original plastic caps still on there I’d call a bonus
As a rule of thumb on carburetors. I consider mixture screws in front of the venturi, fuel mixture screws.
And behind the venturi, air mixture screws.
Anybody chime in if this is not correct but I’ve used this as a general rule ever since Briggs & Stratton mini bike days
-R

 

[Norton7d]

Yes, fuel mixture screws. Seeing your original plastic caps still on there I’d call a bonus
As a rule of thumb on carburetors. I consider mixture screws in front of the venturi, fuel mixture screws.
And behind the venturi, air mixture screws.
Anybody chime in if this is not correct but I’ve used this as a general rule ever since Briggs & Stratton mini bike days
-R

Yes, that is the rule of thumb, prior to fuel = air, after fuel = fuel.
so, I have some sort of "original screws", I assume they are limited to keep the average shadetree mechanic out of trouble?

 

[member28833]

Don’t know the reasoning but guessing it was an attempt on your 76 era bike to control emissions ?

 

[Norton7d]

Don’t know the reasoning but guessing it was an attempt on your 76 era bike to control emissions ?

You may have something there..would this mean every year after has the same?

 

[member28833]

I’ve personally only found them remaining on one 78.
I suspect your statement is correct.

 

[5twins]

They were installed on the '76 through '79 BS38's to, as you've guessed, keep you from "tinkering". Pull them off and you'll be left with a somewhat normal screw head that you can adjust to your heart's content, lol. They just pop off with a couple flat screwdriver blades prying behind them in 2 spots, or you can grab them with pliers. Here's what the mix screw will look like once you remove the plastic cap. It's on the far right .....

 

[5twins]

These are true mixture screws on these carbs. They don't meter or deliver just air, but they don't do straight fuel either. They flow some of the fuel/air mix coming from the pilot jet. Notice I said some of the mix. There are 4 small holes exiting into the main bore of the carb coming from the pilot jet. One is set off to the side and comes from the mix screw. The other 3 come directly from the pilot jet. You have no control on how much those 3 holes flow, only on the amount that comes out the mix screw hole .....

That means you can adjust roughly 25% of the flow.

 

[650Skull]

Technically they are part of the pilot circuit and on parts manuals they are refereed as Pilot Screws.........