air screw adjustment

[mygrady1]

225TLRS: I had been trying to adjust the air adj screws and was advised to set and forget trying to dial them in. I had been trying to adj at idle but could not notice any change in rpm. so I set them all at 1 1/8 turns. AM I wrong??

Rodbolt: Please correct me if I'm wrong but many years ago on that other site, you said my motor liked to run on the rich side and to adjust these (I think) around 5/8 to 3/4 turns. So am I wrong or how important is this adjustment.

[TownsendsFJR1300]

I have to disagree with at least part of the above..

Engine vacuum, as the piston goes down and the intake valve opens, it creates low pressure in the combustion chamber and PULLs air (and fuel) thru the only opening, the carb. One atmosphere of pressure, 14lbs or so, is NOT forcing fuel into the engine nor is it moving air thru the engine on its own..

Agreed the idle and main jets are the main A/F componants. You may have seen my posts re cleaning them, pulling them and actually looking thru them for cleanliness.

In your other post re the Olds 442 carb with the mechainical secondaries (like a GM Quadrajet), at the front base of that carb had two idle air adjustment screws.

The Quadrajet had main jets, secondary jets that the butterfly needles inserted. I don't know if the holly knock off Q-=jet had those air screws, I'd think so..

With that said, the carb is basically jetted for a certain A/F ratio however, GW gave you those two lower air screws to fine tune the idle. You turned them in slowly till the engine ran rough, then backed them out until again the engine ran rough. The final setting was in the middle of the two.

On my older motorcycles (carb'ed) when the government leaned out engines to excess and covered the air screw with a cap (to prevent consumer access) the engine often had an off idle stumble. A lean stumble to be exact. Removing that cap, accessing the singe air screw (for a single throat carb), opening or richening that air screw, fattened up the idle mixture just enough (most of the time) to remove that stumble. Really bad cases, the idle jet needed replacement to one slightly richer..

I've read posts here on the forum, about the air screws on the OB's and what to set them at. Comments made, they must be set at x amount turns out..

I know Yamaha will have a factory set # of turns out, but I don't understand, why they can't be slightly adjusted for more or less fuel AT IDLE/ just off idle for a smoother running engine.. (just like the Q -Jet..)

[rodbolt17]

I know what it says, Yamaha also calls the 3 switchs in the oil tank a sensor.
the translation wasn't so good.

if you carefully study the design of that carb you will notice it has a set of back draft jets as well.
no one in any class ever mentions them as most don't understand the principle.

anyway back to the idle circuits.

you will notice the carb has a PILOT(idle) air jet AND a PILOT fuel jet for each venturi.
this controls the air to fuel ratio.
the adjustment screw simply allows more or less of this mix to pass.

its why if all else is correct you can set them at the factory spec and it works.

not all carbs work this way but most the twin float and most carbs that use a pilot air and pilot fuel jet will.

unlike the old jonny rude carbs that simply ran an orifice tube for fuel and a pilot (idle) air jet.
those we typically had to play with jet size to get a stable idle.

remember, its not engine vacuum that moves fuel, its that 14 mile coluom of air pressure on the fuel in the bowl that moves fuel.
don't belive it, change the air pressure in the bowl.

[TownsendsFJR1300]

2001 115TLRZ Yamaha Outboard CARBURETOR Diagram and Parts

Part 20 , per the above parts manual is: "screw, air adjusting".

IME, the adjustment screws, closer to the intake( between the butterfly and intake manifold) adjust the amount of fuel to be released/adjusted while at low speed.

The adjustments are generally for fine tuning low speed, idle. In the higher RPM's, the main jets take over for the overall A/F mixture.

However, I'll bite as well..

[boscoe99]

Early engines did not have the plug. Later models got it.

Thanks to the EPA.

Who did not want folks changing the idle RPM air/fuel mix to get their models to start and run better.

[countrytn]

yes that is the screw, mine don't have tamper covers

[boscoe99]

And since it does not affect the air/fuel ratio why does Yamaha install an anti-tamper EPA plug?

Is this the screw of which you speak?

[countrytn]

What does the adjustment do?

I want to know also

[boscoe99]

double chicken dinner.
those screws DO NOT alter A/F.
I win.
I prefer beef.

What does the adjustment do?

[rodbolt17]

double chicken dinner.
those screws DO NOT alter A/F.
I win.
I prefer beef.

[almetelo]

Chicken!!! Again!? lol

To make sure burning (A/F) is good have look on spark plugs, diferent settings doesnt mean is wrong)

[countrytn]

thanks guys, every screw was at a diff setting

[boscoe99]

5/8 turns out after lightly seated +/- 1/4 turn (if I'm not wrong)

We have a winner.

Free chicken dinner.

[almetelo]

5/8 turns out after lightly seated +/- 1/4 turn (if I'm not wrong)

[Yamaha 50TLRA]

I dont know for sure about yours but from my experience in all the past outboards I have had they all averaged about 1 1/2 turns out from completely seated in and then fine tune from there.

My 02 Yamaha 50tlra is 1 3/8 - 1 5/8 out