Air pilot screws/please someone clear up the confusion

[YammyMan]

Ok, I have been working on outboards for about 25 years and now I have some confusion. I purchased a 2007 25 hp 2 stroke yamaha msh and it had the EPA covers on the air pilot screws, which is similar to the mixture screws on my evinrudes and johnsons. But there is something a little different. First let me say, the epa caps and needles are crap. Got rid of them and purchased some from a few years prior to replaced the epa needles. Part # 6L2-1432-00. Just in case anyone else wants to get rid of that epa crap. These are the same needles except for the fact you can adjust these and they have no caps. Ok so my question is. Are these air mixture screws or fuel mixture screws? If they are air mixture screws, bottomed out would be the richest setting and backed all the way out would be the leanest setting and vise versa if they are fuel mixture. If they are indeed air mixture, which I think they are, they would be adjusted bass akwards from a tradional mixture screw. I hope I have expressed this question clearly enough to understand. Any help would be greatly appreciated. Also where would be the starting point to adjust. I dont think it would be the traditional 1 1/4 turns out because of how fine the threads are on the needles. Thanks in advance.

[99yam40]

From what I understand some of the carbs have fixed fuel and air jets inside to regulate how much of each is metered together, the pilot screws just adjusts how much of that mix you allow into the motor

[boscoe99]

Yamaha refers to it as an air adjusting screw. What do you think that it might be con*****ing?

I have heard of an EPA anti-tamper plug but not an EPA adjusting screw. The standard Yamaha screw is adjustable by turning it in or out to alter the air/fuel ratio.

The starting point for any adjustment is the number of turns out that you counted when turning the screw in, as the carburetor came from the factory.

If you did not measure the number of turns before the screw was removed the base setting is 2 plus or minus 3/4 turns out from fully seated.

Now if a non-Yamaha adjustment screw is being used all bets are off. The geometry of the screw may not be the same as the Yamaha screw, so who knows what the mixture will be.

Assumption is that this is a two cylinder 25 HP Yamaha.

[rodbolt17]

I have had no problems with the anti-tamper device.
occasionally they break if your not careful but no worries.

easiest way to tell is LOOK at the parts diagram.
on the 25 2cyl you will notice a PILOT fuel jet and NO PILOT air jet.
guess what the needle controls?

on some carbs of all manufactures, the A/F ratio is fixed with BOTH pilot fuel and pilot air jetting.
on carbs with BOTH pilot jets the needle will simply control how much of the predetermined mix is allowed into the intake when the shutters are closed.

pretty simple isn't it?

[boscoe99]

I have had no problems with the anti-tamper device.
occasionally they break if your not careful but no worries.

easiest way to tell is LOOK at the parts diagram.
on the 25 2cyl you will notice a PILOT fuel jet and NO PILOT air jet.
guess what the needle controls?

on some carbs of all manufactures, the A/F ratio is fixed with BOTH pilot fuel and pilot air jetting.
on carbs with BOTH pilot jets the needle will simply control how much of the predetermined mix is allowed into the intake when the shutters are closed.

pretty simple isn't it?

I am guessing that the pilot jet controls the idle fuel flow and the air adjusting screw controls the amount of idle air. Not like some F25's where the screw adjusts the fuel as well.

[rodbolt17]

there are about 3 ways that have been used to control A/F at idle since the 50's.
1 is a two stepped needle that con*****ed BOTH.
that hasn't been used by any manufacture in many many moons.

2 is a fixed orifice or pilot fuel jet and an adjustable air needle.

3 is a fixed pilot air AND a fixed pilot fuel, this method may be used with a needle, most jap stuff or without,,jonnyrudes for many many years.

fastest way to make most late 80's and up fixed jet jonny/rudes idle well is to play with the pilot or idle air jetting.

typically small yamahas use a fixed fuel jet for idle,hence the translation to a PILOT jet.
that means FUEL at idle is NOT adjustable. only air.

if BOTH air and fuel pilot jets are used there is NO A/F adjustments only how much or how little amount is allowed to pass. and not all carbs even have that adjustment.


it all comes down to a 14.7 mile high colum of air pushing down on the fuel in the bowl.

we wont even get into backdraft style carbs .

[boscoe99]

God forbid that Yamaha were to put theory of operation in a Service Manual for the particular product covered by that SM.

God forbid that Yamaha would be consistent with their methodology.

Oh, and let's not talk about wire colours changing across connectors.

[YammyMan]

Thanks for the replies. This is a 2 cyl. 2 stroke. Each carb has an adjustment screw that plunges straight down into the "barrel" or throat of the carb. So I would guess it only allows air in. With the carbs removed and the adjustment screw bottomed out, you can stick your finger into the carb throat and feel the tip of the adjustment needle with your finger. I bought this motor with the top mixture screw totally missing and the bottom one was not the correct one. So out of curiosity, I pulled the plugs to see what was happening. The top plug, which was the one where the mixture screw was missing was bright white. So it was running super lean. The bottom plug, which didnt have the correct mixture screw, seemed to be burning properly. It was a light brown color. First of all, I want to get these carbs set up correctly. Secondly, if the motor is trimmed all the way down, it smokes and runs super rich and choppy. When i have the motor tilted up, she purrs like a kitten. So i figure someone went into the carbs and didnt set the floats properly which I hope to correct when I get time. Does this sound logical? Thanks again for the replies.

[99yam40]

I bought this motor with the top mixture screw totally missing and the bottom one was not the correct one.

So i figure someone went into the carbs and didnt set the floats properly which I hope to correct when I get time. Does this sound logical? Thanks again for the replies.

Seems like 2 different stories on the needles if one was missing and you 1st said " I purchased a 2007 25 hp 2 stroke yamaha msh and it had the EPA covers on the air pilot screws"

If there were missing parts then who knows what all is messed up in there.
If the wrong one was used and/or they were run in too hard the seating area may be damaged
I would open the carbs up and go through them to makes sure all is to spec

I do not see how tilting motor up would make motor run better even with float setting wrong, but who knows what is going on

[YammyMan]

Yeah I found the needle and the epa cover in the bottom on the cowling. This motor is one that used the epa caps I should have said. The needle was bent and the cap was broken. So I figured it happend when someone tried to remove it. Running rich when trimmed all the way down is confusing to me. But last night I went out and tinkered with it some. I was messing with the spark plug wires and the top boot shocked me. So I gues I need to replace the boot or the whole coil and wire. Going to be a project, but I'm looking foward to figuring this one out.

[99yam40]

I always start with basics on a new to me motor just to make sure all is good
compression, spark , timing, fuel pump, and then clean and adjust carbs to specs.
I have cleaned plug caps before to make sure there was no salt residue. But once they start to arc where they should not they may be shot
A lot have ohm readings in specs