Is the "Air Screw" the same as the "Idle Mixture Screw" on Yamaha 225DX?

[mattb348]

The manual for the 225DX only mentions 2 adjustable screws on the carb (the idle speed screw and the "Idle Mixture Screw".

Isn't there also an "Air Screw" or something? Or is that the same thing as the "Idle Mixture Screw" that the manual refers to?

Thanks.

[yaegerb]

air screw is used on 2 stroke carbs, but it acts exactly like your mixture screw. You only have 2 adjustments on your carb. Idle speed and mixture...aka air screw.

[mattb348]

You only have 2 adjustments on your carb. Idle speed and mixture...aka air screw.

I'm guessing the idle mixture screw also affects how the engine runs when the throttle is VERY slightly pressed?

[yaegerb]

No, idle mixture screw sets how much air and fuel the carb delivers to the engine at idle. Once you press the thumb throttle you are moving the slide and needle.

[El Camexican]

The idles speed screw is the large one that raises and lowers the slide using a Phillips screw driver, or your fingers.

The idle mix screw is a fancy name for the little flat headed slot screw that controls fuel flow on that carb and affects engine response from 0 to 1/4 throttle

[mattb348]

The idle mix screw is a fancy name for the little flat headed slot screw that controls fuel flow on that carb and affects engine response from 0 to 1/4 throttle

I wonder why they don't call it the "low speed mixture screw" or "pilot jet wannabee screw" or something then :)

And I thought it affected 0 - 1/8th throttle position. Maybe we are BOTH wrong???? :)

[El Camexican]

I wonder why they don't call it the "low speed mixture screw" or "pilot jet wannabee screw" or something then :)

And I thought it affected 0 - 1/8th throttle position. Maybe we are BOTH wrong???? :)

I don’t know why Yamaha calls them what they do, but the 0 to 1/8 or 0 to 1/4 thing is debatable depending on who you ask. The charts you may see in some manuals that depict a fuel circuit abruptly signing off at a specific throttle position are misleading. If they did work like that it would feel like you were shifting gears as you opened the throttle.

As far as I’m concerned every fuel circuit interacts with the others in some way even if it’s unnoticable, but the one that has the greatest noticeable range of effect is the needle.

Anyway, once you get your choke fixed your fuel screw setting will become relevant from idle speed up to the point that your pilot jet has the most noticeable effect on how your engine runs. The reason your fuel screw is accessible from the outside of the carb and adjustable is so you can easily tune for conditions.

For example, if you were driving up a mountain that was 6,000 feet higher than where yo started off, you wouldn’t want to be disassembling your carb every 2,000 ft to change pilot jets. Instead you would be turning in or closing your fuel screw and turning up your idle speed every 2,000 ft or so when your engine started to blubber and stall. By the time you got to 6,000 ft your fuel screw might be only 1/8 of a turn from closed to compensate for the now oversized pilot jet, but if you had changed the pilot jet to a smaller one halfway up your fuel screw might only need to be 1 turn out. I hope this makes sense.

[El Camexican]

It's not debatable. Every fuel mix screw setting on every carb in the world affects off idle throttle response.

From 0 to 1/8 or 0 to 1/4? Please be specific.

[El Camexican]

I was specific. I just didnt adhere to your spec, because it doesnt accurately depict the function of the fuel circuits and the fact that they all overlap. The idle mix screw, air or fuel, some have both, definitely affects the off-idle throttle response on every carb, that's the only statement i am making.

My "0 to 1/8 or 0 to 1/4 thing" being debatable referred to only between the 1/8" and 1/4" setting, not the "0" part, but if my zero is less accurate than your zero I sincerely apologize.

[mattb348]

When I said 0 - 1/8th throttle position I meant it has an affect on idle, and it also affects how the engine runs when you actually HOLD the throttle open a little bit (IE: 1/8th open or LESS).

I wasn't just talking about stabbing the throttle quickly off of idle (which can lead to the ultra-classic slit-second bog on MANY engines, in which case the pilot jet seems to be the most helpful when it comes to correcting this.)

I'm sure the idle-fuel mixture / air screw affect this too.

But does it also affect how it runs when you actually keep the throttle held at 1/8th (or less) open?