Carb Sync - Spec Pilot Screw

[Jason2tpa]

'98 S115TLRW

New kits installed in both carbs, much needed. My manual states the spec pilot screw (turns out) is 5/8 (+/-) 1/4. All four screws were set at 1/2, but I don't want to use this as the default because the engine wasn't purring due to bad carbs.

I know every engine is different, therefore there is NO one way to set your idle for all engines. But 5/8 seems a bit rich to begin with, right? Majority of articles, forum post, etc seems to start the sync process around 1.25 to 1.5 turns out, then adjust from there turning in or out 1/4 turn.

Syncing carbs from what I've learned is like an art form. And me, I'm finger painting over here. So feel free to share your pearls. Much appreciated.

[amahaork]

Better rich than lean. Begin with all to
Minimum setting, see how it works and increase to max setting, allowing some stabilization time. If not happy you are, increase all quater more. If require to open too much in order to run, carbs are not your problem.
So, start with EXACT ignition timing setting. And run from new gas in portable tank.

[boscoe99]

When you say that 5/8 seems a bit rich are you basing that on what you are seeing, smelling, hearing, feeling when the motor idles or is this just an intuition?

What do you mean that there is no one way to set the idle? The SM only provides one way to perform the adjustments. What's is wrong with the Yamaha published procedure?

Some like to run a two stroke a bit on the rich side to prevent bad stuff that can happen if and when the motor runs lean. Maybe better to err on the rich side than the lean side.

If turning the idle mixture screw in a 1/4 turn makes the motor idle better, or you feel better, you are free to do so. Even moreso if you want.

[Jason2tpa]

When you say that 5/8 seems a bit rich are you basing that on what you are seeing, smelling, hearing, feeling when the motor idles or is this just an intuition?

Similar to intuition, though I wouldn't dare to say I have good motor intuition. I'm saying comparatively, with what I've read on this forum and other informative resources.

What do you mean that there is no one way to set the idle? What's is wrong with the Yamaha published procedure?

Meaning from what I've learned, there is no one specific carb setting for every model of that motor because every motor has its quirks. Some motors may run better on lean side, some on rich side. The SM does provide steps, though it doesn't embellish with finer details.

Guess I'm trying to learn more of the relationship between the pilot screw and idle adjust screw. Manual says before syncing carbs to loosen idle screw, closing throttle valves. But if my goal is to fine tune the pilot screws to get motor idling good, then why is the idle adjust screw needed? Or should idle adjust screw always be engaged, even if only a little.

That's where it's get a little confusing...balancing the pilot screw--idle adjust screw combo.

[boscoe99]

The idle adjustment screw opens and closes the throttle valves and in so doing changes the idle RPM. As the throttle opens more air is allowed in. Fuel gets added to that air via the idle mixture system.

Now turning the idle mixture screw (aka pilot screw) allows more or less fuel to get to the air that the throttle valve is allowing into the motor at idle. More fuel (actually a very rich mixture of air and fuel through the idle mixture circuit) makes the motor run on a richer air/fuel mixture. Less fuel does the opposite.

You can have a motor running with a lean air/fuel mixture (pilot screw inward) but at a fast idle (idle RPM screw turned in) or you can have a motor running with a rich air/fuel mixture (pilot screw outward) but at a low idle (idle RPM screw turned outward) or any number of combinations between the two.

Now keep in mind that the height of the float in the float bowl will affect the idle mixture. Ethanol being added to gasoline will affect the idle mixture.

Ignition timing will also affect the idle RPM.

A whole lot of stuff going on at the same time in a motor.

[Jason2tpa]

Boscoe...once again you give details that fit my brain. And now it seems simple to me. Tuned it up, idling down a channel now. Sounds and runs strong.

As always, much appreciated.

[99yam40]

I believe you are not reading far enough into the procedure.

you back off the idle speed screw to let the throttle plates close completely and set all of them to the completely closed position and sync them together at that point.

then you adjust them open a little before starting the motor.

You never try to start motor with the throttle plates completely closed

you set the pilot(idle mixture) screws to factory specs befor starting the motor.

If all passages and jets are clean and no other problems with the motor are present the motor should run well when started at this point.

once warmed up you adjust the idle speed to spec and then you can adjust the pilot screws a small amount to make it better if needed

[rodbolt17]

once again.
you CANNOT alter the A/.F ratio with the screw on those carbs.
you can ONLY allow more or less of the predetermined mix.
the pilot fuel jet and pilot air jet fix the A/F ratio.
set them where the SM says and roll.

[boscoe99]

Some information from a Mikuni carburetor tuning book:

IDLE CIRCUIT (PILOT SYSTEM)
The idle circuit supplies fuel at idle speeds and has a major influence on fuel flow up to 1/4 throttle. There are three tunable parts in the idle circuit:
1) PILOT JET --- controls maximum fuel flow through the
idle circuit.
2) PILOT AIR JET--- controls the maximum amount of fuel that will flow through the pilot jet by allowing a higher (smaller air jet) or lower (large air jet) vacuum signal at the pilot jet.
3) PILOT SCREW --- controls how much fuel is allowed to enter the carburetor venturi.

The pilot screw is used to control idle mixture. Turn the screw out to richen the idle mixture. Turn it in to lean the mixture. The engine should have a smooth, steady idle with the screw between 1/4 and 3-1/2 turns out from fully bottomed (gently!). If the engine requires more than three turns out, the pilot or pilot air jet may be too lean. If it requires less thatn 1/4 turn, it may be too rich. As the throttle is opened the pilot screw’s position becomes less important than the sizes of the pilot and pilot air jets. A larger pilot jet richens the mixture from just off-idle to 1/4 throttle. A smaller one leans it. A change in pilot air jet has the reverse effect. A larger pilot air jet leans the mixture and a smaller one richens it. The pilot jet and pilot air jet have slightly different effects on mixture strengths. These effects are discussed in the “General Tuning Procedure” portion of this manual.

[boscoe99]

Time for the ole noggins to get used.

When the motor is running at idle let's assume that 12.5 parts of air are being allowed across the throttle valve, and 1 part of gasoline, mixed with .01 part air, is added to the 12.5 parts of air via the idle mixture jet. The motor will be running on an air fuel mix of 12.51/1.

The idle mixture screw is opened up a bit so that now some more of that exact same 1 part gasoline/.01 part air idle fuel/air mix is provided to the air coming in through the throttle valve. Let's say that the idle fuel/air mixture being provided is now 1.1 part gasoline and .02 parts air. The motor is now running on an air fuel mix of 12.52/1.1.

Which is the richer air fuel mixture?

[Jason2tpa]

There are three tunable parts in the idle circuit:
1) PILOT JET --- controls maximum fuel flow through the
idle circuit.
2) PILOT AIR JET--- controls the maximum amount of fuel that will flow through the pilot jet by allowing a higher (smaller air jet) or lower (large air jet) vacuum signal at the pilot jet.
3) PILOT SCREW --- controls how much fuel is allowed to enter the carburetor venturi.

Perplexed. To my knowledge, of the 3 listed above only the pilot screw is tunable.

[boscoe99]

Perplexed. To my knowledge, of the 3 listed above only the pilot screw is tunable.

Readily tunable yes. However, in many carbs the actual jets themselves can be removed and replaced with larger and smaller sizes to better "fine tune" a motor. That is what a true motorhead will do. Also, if taking a carbureted model to a lake at a high altitude it might be desirable to go in and replace the jets for better tuning.

Some automobile carburetors are designed so that a Rodnut can simply bend over and remove and replace a jet very simply. You can buy kits with a bunch of jet sizes in them and spend all day playing with jets to find out what works best for a particular motor. Some get more pleasure out of playing with a motor than they do from using the automobile.

[Jason2tpa]

Ah ha, thanks. Neither of which apply to me.

[99yam40]

did you get enough info to understand the need of the idle speed screw and the purpose of backing it off when syncing the carbs?

[Jason2tpa]

Sure did. And I can tell you why I was getting confused. Referring to rodbolts post, adjusting either the idle screw or pilot screw does not affect the A/F ratio.

My concern was adjusting either changes the mix ratio, and now i know that can't be done. So long as the pilot screws are within spec, adjustment of either screws soley affects idle speed