Carb Sync - Spec Pilot Screw

[boscoe99]

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

Some of us will disagree.

[99yam40]

Some of us will disagree.

I am with him^^^

I do agree that opening the throttle plate more adds more air.
I do not understand how the pilot screw set in a certain place could keep that same air to fuel ratio when the throttle plate is opened more and the RPM increases.
I know that the more air coming threw/by the plates should suck more of the fuel/air mix out of the pilot jets, but do not get how it would be in the same ratio as the holes are only so big

[99yam40]

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.

This question about closing the throttle plates is what I was trying to address.
the motor will not run with the throttle plates closed completely, unless there are holes drilled into them to let air in.

[scofflaw]

I am with him^^^

I do agree that opening the throttle plate more adds more air.
I do not understand how the pilot screw set in a certain place could keep that same air to fuel ratio when the throttle plate is opened more and the RPM increases.
I know that the more air coming threw/by the plates should suck more of the fuel/air mix out of the pilot jets, but do not get how it would be in the same ratio as the holes are only so big

When the throttle plates open the main jet starts adding the fuel. Pilot jets are an idle thing.

[boscoe99]

Let me try to explain my point of view another way.

Rodbolt says that as the pilot screw/idle mixture screw is turned outward it only lets more of the same air/fuel mix into the motor. I agree with that. But it is a super concentrated mix. That super concentrated mix gets added to pure air coming in across the throttle valve and in so doing gets really diluted. Down to about 12.5 parts air to 1 part gasoline.

Now if and when the idle mixture screw is turned outwards more of the same super concentrated air/fuel mix gets added to the same amount of pure air, because the throttle valve is still in its same position.

The greater amount of the same super concentrated mix being added to the same amount of air causes the air/fuel ratio that is ingested into the motor is now richer, because it has a greater amount of fuel to air.

In a motor with a single carburetor this is very easy to see in the form of the RPM changing solely as a function of the different burn rate of a rich mixture versus a lean mixture. But in a multiple carburetor model it is not so easy to see. The carburetors not being changed help to mask the change that is being made when just one of the carburetors is adjusted.

[99yam40]

When the throttle plates open the main jet starts adding the fuel. Pilot jets are an idle thing.

I was talking about at idle.
adjusting the idle speed screw opens or closes the throttle plates letting in more or less air into the motor to mix with the fuel/air mix coming threw the pilot screw and jets.
open them very far and some uncover off idle ports to get more in during transition from idle

[scofflaw]

The pilot jets, air and fuel, are so freakin tiny compared to the throttle plates [air] and the main jet [fuel] it's insignificant in the final fuel air ratio at any speed above idle. OK, pilot screw adjustment could help in the transition from idle circuit to main circuit

[99yam40]

this whole thread is about idle
not off idle or higher RPMs

[Jason2tpa]

Let's say the motor idles and runs better with the pilot mix screws turned outside of manufacturer specs? For example, the spec for my motor is 5/8 (+/-) 1/4. Mind you there is no "spec" for the idle adjust screw (throttle plates), so let's just say this screw is engaged enough to open plates and pilot screws are set at 1 turn out. Result is a good idle.

But if I were to turn in the pilot screws to within spec, rough idle. My choice would be for the out of spec, good idle sync.

But this would mean the carb isn't set to spec...in this circumstance, would it have a bad a result down the road?

[boscoe99]

There is a spec for the idle adjustment screw. It is 750 RPM when the motor is idling in neutral. After having first made sure that all of the throttle plates are closed at the same time and open at the same time.

If you have to turn the idle mixture screw outward more than the specification to get a good idle quality then there should be no harm, no foul to the motor. Turn it inward to less than minimum specification might not be so good.

But if you have to turn the screw outward more than normal then I am thinking that the carburetor is not thoroughly clean inside. You are trying to fix a problem by turned the mixture screw outward beyond the specification.

There is more to getting a good idle than just the idle RPM and the mixture setting however. There is a complete set up procedure that should be followed.

[boscoe99]

[Jason2tpa]

There is a spec for the idle adjustment screw. It is 750 RPM when the motor is idling in neutral.

My fault, I should have been more clear. I was implying the idle adjust screw does not have a specified turn out, as the pilot screw does.

I took the boat out this evening for a short cruise up the intercostal. Mostly idling. While the rpms were in range compared to where the throttle control level was, i could feel just a slight hiccup. A little shimmy, if you know what I mean. When I got on land and flushed at idle speed, the motor would give a lean misfire (lean sneeze?). Hope I'm describing correctly what was occurring. Made an adjustment to the pilot screws (turned out) and it idled more smoothly.

Both carbs were just cleaned thoroughly, new kits installed. Each jet was removed and cleaned. So it shouldn't have to do with that. I'm hoping to do a little more fine tuning tomorrow.

Appreciate all the good info!

[99yam40]

a lot of people say the carbs are clean, but after all is said and done they end up cleaning them again to solve all of their problems.

At times a heated sonic bath is needed as it does a lot better job on the hidden passages.
final adjustments should be done with warmed up motor in the water so back pressure on motor exhaust is at its maximum.

[Jason2tpa]

A lot of people refer to carb cleaning as spraying cleaner down carb throat while reving motor in idle. And even more unfortunate, there are some outboard shops you bring in for carb service and thats what they do.

[walleyehunter13]

That sounds more like decarbing than carb cleaning. You aren't getting anything cleaned at all in the carb by spraying down the throat while idling...