Carburetor air and fuel flow

[99yam40]

lets say that the ratio of the mix from the pilot FUEL and pilot AIR is 14.7/1.

when you move that screw in or out you decrease or increase the amount of that 14.7/1 mix.
not enough of that mix and that cyl starts burning the limited amount in the crankcase.
that is what makes it "sneeze" and can shut it off.
it is an explosion under a piston on it's down stroke and tries to reverse the crankshaft direction.
do it violently enough and it can shear flywheel keys and pop fuel pumps.
at idle it has two fixed air sources and one fixed fuel source.


unlike some carb designs that actually meter fuel or air with that screw Yamaha did not choose to do it that way.
some carbs are set up with no idle speed screw and no A/F screw and it is all fixed jet.

OK, seems we are not communicating well on which air to fuel ratio we are talking about.
I believe you are talking about the ratio of air/fuel going thru the pilot screw/ jets/ passages. And that ratio cannot be changed by turning the screw

I am talking about the air to fuel ratio going into the cylinders to be burned, which can be changed by lessening or increasing the amount of pilot screw turns allowing more of less of that mix into the motor to mix with the air going in also.

If the air fuel ratio is 14.7-1 going thru the pilot screw passage and it mixes with the other air coming thru the carb throat it will be way to lean would it not./

I have seen throttle plates with holes in them to meter the air in instead of cracking the plates open just a little.
And I can understand them allowing a fixed amount of air when the plates are closed completely but most carbs I have seen use the idle speed screw to adjust the speed/amount of air in.

On the small 2 stroke motors I prefer the ones with high and low speed screws to adjust the amount of fuel

[boscoe99]

On the small 2 stroke motors I prefer the ones with high and low speed screws to adjust the amount of fuel

Me also. And what I can see immediately is that tweaking the lean mixture screw outward a tad makes an all but worthless motor purr like a kitten. Starts and idles a bit easier and the throttle response is much nicer. Little to no bog upon a quick acceleration.

Now how about a carburetor where the air/fuel mixture can be adjusted from idle through wide open throttle? Such as that used in an aircraft.

Would there be too much risk in the untrained causing a motor to become damaged by running it too lean?

On my old Mercury there was an idle mixture screw right at the front of the carburetor right at the front of the motor. I was tweaking that thing all of the time just to get the motor to idle a bit nicer. It seemed to be very sensitive to normal pressure changes and small elevation pressure changes such as from going from sea level to maybe a thousand foot altitude.

[boscoe99]

So yes that statement is correct.

As for the rest of it...I will have to think on that a bit.

Did you ever have to adjust the idle air mixture on a carburetor during your training or once you were licensed to work on them?

Pull out the mixture cutoff screw and watch for the rise in engine RPM as the mixture changes. Too much rise and you turned the idle mixture screw inward to make it a bit less rich. Too little rise and you turned the idle mixture screw outward to make it a bit richer. Looking for the magic 50 RPM rise as I recall.

Carburetor Maintenance (Part Two)

Hey (as we say down here) do you know of any good aeroplane mekanik forums where folks are genial and like to contemplate mechanical matters without getting all pissy over answers they read?

I gots some questions about torque meters and dynamometers.

[99yam40]

I bet those newer little 2 strokes with no adjustments are a pain if used in different altitudes

[pstephens46]

I bet those newer little 2 strokes with no adjustments are a pain if used in different altitudes

I have two echo blowers. One made in 1998 and the other in 2004. Same model. Older one screams. Newer one runs ok but not nearly as powerful.
Carb on newer model has but one adjustable screw, I believe the high range.

I blame California.

Never thought about it until now but I wonder if I could replace the fixed idle "plug" with an older model adjustable screw?

[panasonic]

Did you ever have to adjust the idle air mixture on a carburetor during your training or once you were licensed to work on them?

Pull out the mixture cutoff screw and watch for the rise in engine RPM as the mixture changes. Too much rise and you turned the idle mixture screw inward to make it a bit less rich. Too little rise and you turned the idle mixture screw outward to make it a bit richer. Looking for the magic 50 RPM rise as I recall.

Carburetor Maintenance (Part Two)

Hey (as we say down here) do you know of any good aeroplane mekanik forums where folks are genial and like to contemplate mechanical matters without getting all pissy over answers they read?

I gots some questions about torque meters and dynamometers.

I never worked on piston airplane's, only in school. Went right to a company with all turbine engines. My licence is M2 which is 12,000 lbs and above. M1 licence is for small aircraft 12,000lbs and below..this is where you will find most piston powered planes.

And I remember something about that idle cut off mixture stuff while running small aircraft and engines in test cells. Instructors used to change settings on carbs, turbo's, ect and then see if we could find the trouble and fix it.

Know lots of guys who worked on pistons for years and still do. You can smoke an Aircraft piston engine in very short order, like in a few minutes if you do not keep and eye on the cylinder head temp gage while leaning out the engine for different altitudes and power settings. Some use both cylinder head temp and EGT's to keep engine from running to lean and burning it up.

Good thing they make those engines so tough, they can blow a cylinder or two and still get you back down. But there are people who get killed or wreck their airplanes and engines all the time.

You can wreck a turbine engine as well with one hot start, you have to keep a eye on that EGT gauge like a hawk when starting or you can melt the turbine blades right out of it. $1,000,000 mistake....seen it happen.

All forums are the same.. I only go on this one for outboards and a few motorcycle ones. Everybody gets their pee hot at times, including myself.

[Jason2tpa]

Now how about a carburetor where the air/fuel mixture can be adjusted from idle through wide open throttle? Such as that used in an aircraft.

Would there be too much risk in the untrained causing a motor to become damaged by running it too lean?

Damage, nooo. That's because the untrained (like myself) would have his handy dandy yamaha service manual. And as you know, the SM provides the operator with all of the clear and concise procedural steps required to perform a successful repair.

[99yam40]

Damage, nooo. That's because the untrained (like myself) would have his handy dandy yamaha service manual. And as you know, the SM provides the operator with all of the clear and concise procedural steps required to perform a successful repair.

I do not think any repair manual would tell you how to run the motor to keep from hurting it

[panasonic]

Answer for Boscoe

Did you ever have to adjust the idle air mixture on a carburetor during your training or once you were licensed to work on them?

Pull out the mixture cutoff screw and watch for the rise in engine RPM as the mixture changes. Too much rise and you turned the idle mixture screw inward to make it a bit less rich. Too little rise and you turned the idle mixture screw outward to make it a bit richer. Looking for the magic 50 RPM rise as I recall.

Carburetor Maintenance (Part Two)

Hey (as we say down here) do you know of any good aeroplane mekanik forums where folks are genial and like to contemplate mechanical matters without getting all pissy over answers they read?

I gots some questions about torque meters and dynamometers.

I got a text from a friend of mine with his own Cessna 150 and indeed at idle cutoff the engine RPM rises slightly. Got to be paying attention to the tach closely to see it.