air screw 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..)

[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.

[boscoe99]

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

Engine vacuum, as the piston goes down (in a two stroke the low pressue is created as the piston goes up) and the intake valve opens (I think we are talking about a two stroke with no intake valves, unless you consider the reed valves to be intake valves), it creates low pressure in the combustion chamber and PULLs air (and fuel) (does a low pressure area pull air towards itself or does the high pressure push the air towards the area of low pressure) 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..If there is a perfect vacuum (zero air pressure) in the intake, but then the 14.7 psi of ambient air pressure is removed, what will happen to air movement?

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..) I believe they can. So does Yamaha. Some mechanics apparently don't.

See comments above. I like posts like this. They cause me and other folks to think.

[mygrady1]

On my tach or my ear, I can hear no change in rpm. From questions B4 there is no way to hook up my analog tach/dwell meter to my motor for slight differences in rpm. The yama tach is digital and does bounce around somewhat not giving very accurate small changes.

[TownsendsFJR1300]

Any internal combustion engine, two or four stroke (re the original carb post) all need a vaccum for the CARB to operate.

(We all know FI is a different animal altogther, it'll shoot fuel when the injectors are activated)

Yes, on the 4 stroke, we're talking the intake valve opening, the piston goes down and sucks air thru the carb which in turn sucks fuel from the carb body(sometime referred to as the venturi) and into the combustion chamber.

On the 2 strokes, same thing but as you noted, there's no cam/valve like a 4 stroke, but as you stated, a REED valve (opens with vacuum) is the most common by far.

piston port (mostly used in weedeaters, chainsaws, etc).

Some other designs I'm familar with are 2 strokes that used a rotary "DISC" (Kawasaki 350cc, 2 stroke, late 70's,SINGLE cylinder, rare now). The disc was attached to the rt side of the crank with an opening cut in it, set a specific open / close. It opened and closed ata set rate (when there was vacuum to pull air'fuel thru the carb. The carb was actually on the side of the engine, under the side cover. The cylinder had an exhaust pipe out the front, the intake, again, down inside the engine cases(NOT in the crankcase).
Its not as efficient as a reed valve as the reed will open WHENEVER theres a vacuum.

Re the atmosphere, right now we're talking normally aspirated engines, no forced induction-super charger, turbo, etc.

At sea level, we have one atmosphere (or column) of air pressing down on us. That has a certain density. At 33' deep, under water, that pressure DOUBLES to 2 atmospheres. At 66', 3 atmospheres, etc, (basic scuba diver training.)

Water is MUCH DENSER, again 33' is equal to one entire column of air from sea level to the edge of out atmosphere. Critical to know when diving as your lungs to take in TWICE AS MUCH AIR AT 33' OR, they will COMPRESS to half thie rnormal size (not good)..

The higher you go, ie, mountains, the air density decreases, the engine will lose power as the air is NOT as dense.

But, as Rodbolt posted, unless I misunderstood, the atmosphere does have pressure on your body and everything else around you, however, its not currently pushing air and fuel thru your engine as it sits, turned off.

[TownsendsFJR1300]

On my tach or my ear, I can hear no change in rpm. From questions B4 there is no way to hook up my analog tach/dwell meter to my motor for slight differences in rpm. The yama tach is digital and does bounce around somewhat not giving very accurate small changes.

I don't know how much (apparently not much) that screw allows for adjustment.

On my FI'ed motorcycle, 4 banger (Yamaha BTW), you do your throttle sync using 4 air screws. This engine has two, "two barrel" throttle bodies. It does have one sync screw for the butterfly shaft (all TB's are in a straight row) however that screw is preset at the factory and is a no no to adjust.

Now the air screws, once hooked up to a manometer, they are EXTREMLY SENSATIVE, 1/16 of a turn makes a huge difference if adjusted correctly or incorrectly. Yamaha allows for 10mm's difference between all 4 cylinders. Doing some fine tuning not only at idle but at 4K RPM's(not in the manual), I have them within 3 mm's. The engine is much smoother, the typical buzziness of the engine, 90% gone...

[rodbolt17]

townsend. your simply wrong.
its air pressure on the fuel bowl that dictates fuel movement, at or near WOT there is NO or very little INTAKE vacuum.
it basically drops to baro pressure.
that's why EFI uses manifold absolute pressure and not intake vacuum.
this is where the venture principle takes over and uses the air moving past the venture to draw air out of the bowl as the fuel in the bowl is still under atmospheric pressure.
here is where certain back draft style carbs come into play. the 115 and the 2.6L carbs since the lte nineties anyway are backdraft models.
merc used back on the inline 6 motors of years past.
the 150 did do so well.
its also why carbs stuggle more with altitude changes.
but it comes down top that simple 14.7 PSI pushing down from above to move fuel in most carbed engines.
if you don't believe it try sucking milk in a vacuum from a straw.
not gonna happen.
at se*****l there will be about 14.7 psi pusing down on the liquid.
any lowering of the surrounding air pressure will cause the liquid to rise.
its also how a manometer works.
its also why the low pressure center of a hurricane has a larger storm surge than the outlying area.
say for 100 mile diameter we lower the air pressure at the center of the altantic to about 12.5 psi, its a verly low pressure system, yet the rest of the atlantic has 14.7 PSI on it.
it will cause a several foot rise in that 100 mile circle with no winds.

going back to the V4 and 2.6L carbs.

next time you have one apart spray caarb cleaner through the pilot air jet.
you will notice it only comes out in two places.
one is via the needle outlet the other at the transfer point from the body to the bowl over the pilot fuel jet.
now spray through the pilot fuel jet.
here again one inlet on the fuel bowl and one outlet to the transfer port.

if the bowl gasket doesn't get a decent seal it will leak air and toss everything out of whak.

like I said adjusting the needle does NOT affect air to fuel ratio.
simply allows MORE OR LESS of the ratio mixed by the pilot air and pilot fuel jets.
the only way to change the A/F ratio,other than leaking gaskets, would be to alter one of the jet sizes.

that's why ,especially on the old crossflow and 90* looper jonnyrudes we got good at playing with the idle air jets.

I have done many carb and EFI schools for Bosch,delco,rochester,holley,merc,volvo,tohatsu,su zuki,yamaha, and several others in the past 35 years or so of dealing with this plus playing trying to modify many carbs.
some way more succesfull than others.

google back draft carburetor theory and you will see what those two jets under that stainless plate in the V4 and 2.6L v6 carbs actually do.

[mygrady1]

Use English, I have no idea of what you just said.
I am not well versed in electronics or the theory of what ever you said.
What is a manometer?? I am apparently a dumb 70 year old looking for a
simple answer in terms that I can understand.
No offence intended but most of use do not understand these tech terms.

[mygrady1]

Rodbolt: please answer my question.
Thanks

[TownsendsFJR1300]

Use English, I have no idea of what you just said.
I am not well versed in electronics or the theory of what ever you said.
What is a manometer?? I am apparently a dumb 70 year old looking for a
simple answer in terms that I can understand.
No offence intended but most of use do not understand these tech terms.

Sorry,

A manometer measures the vacuum a cylinder is pulling while running.

On a single carbed engine, its not needed (there's no other cylinders to sync to). A Harley has two cylinders, BUT one carb. In that case, again there's only one carb, a common intake manifold, nothing to sync...

It lets you see how far off or close Each cylinder is pulling a vaccum compared to the others. This particular unit uses rods, no liquids and has markings on it to see how far or close each cyclinder is to each other.

You want the cylinders pulling as close a vacuum to each other as possible for the best efficency..

Here's a link: www.carbtune.com

[TownsendsFJR1300]

Rodbolt: please answer my question.
Thanks

Ya gotta give me a couple of minutes to type..

[mygrady1]

As I am constantly reminded, I own an antique. I really don't think a manometer is in my future. Just poking. however, with your knowledge, how would I hook up my analog tach/dwell meter to my motor??

[TownsendsFJR1300]

As I am constantly reminded, I own an antique. I really don't think a manometer is in my future. Just poking. however, with your knowledge, how would I hook up my analog tach/dwell meter to my motor??

I'm not much younger...

If we're setting the dwell on your points equipped car, you need to hook up your meter to the ignition coil (+ side) and ground. An old style GM, most you could adjust with an allen key. The old Fords, the cap had to come back off, re-adjust and re-check. Generally 30 degrees as I re-call

Your OB engine, no dwell to speak of, a tach, no idea where it hooks up to...

Re the Manometer, I've had this one, and learned to use it maybe 5 years ago when the bike needed syncing. Much cheaper than paying a shop to screw up your bike...

[rodbolt17]

a manometer is a simple device that measures air pressurevs air pressure changes.
been around in use as a barometer for several hundred years.
next time you look at an old barometer you will notice 2 needle,one is manually settable the other moves by the manometer fluid pressure.
that's why when you check the baro pressure you set the needle to line up with the other.
next time you check it you can see did baro pressure fall, bad weather comeing,rise, good weather, or remain steady.

some are calibrated in "Hg,some in millibars and some in PSI units.

but in any carbed engine with a throttle plate you will have aa low pressure on one side and atmospheric on the other.
as the throttle plate is moved to the WOT position both sides of the throttle plate are now equal.
as ,in a 4 stroke, that piston moves down you now have barometric pressure at 14.7 psi or so trying to fill that void left behind.
in most two strokes, its as the piston moves up but at or near WOT there is no,or negligable intake vaacum.
both sides of the throttle plate are at the same pressure.

as EFI does NOT require any air flow to move fuel the engineers used manifold absolute pressure instead of intake vacuum to determine engine load.

on the typical carbed 4 stroke you will see at say 32"Hg baro pressure, about 18-20"Hg of intake vacuum.
now say subtract the 20"Hg from 32"Hg and you get about 12"Hg.
that is the pressure differential between the air on the airfilter side of the throttle and the intake side of the throttle.

now on EFI, at key on, most marine EFI system lock in barometric(atmospheric pressure) and retain this as a standard for the rest of that key on cycle.
boats typically don't change altitude much during a single key on cycle.

now we start the engine, look at the laptop and our manifold absolute pressure(MAP) is roughly 12"Hg.
whaaaaaat.

do the same math,subtract 12 from 32 and you typically get about 20.

measuring VACUM in the intake is the inverse of measuring pressure in the intake.

as the throttle plate is opened the vacuum measurement will go from about 20"Hg to about 0"Hg.

on most laptops you will see the map go from about 12"Hg to about 32"hg as the plate nears its wide open position.

confusing aint it .
then toss in each manufacturer likes to change the units its scaled in and this gets fun.

but both units whether measuring vacuum or pressure work, they are inversely proportional.

this is why most carbs utilize whats called the venturi principle.
that's that funky necked down area that the emulsion or main jet tube sits in.

mow we have established that intake vacuum is the same on BOTH sides of the throttle at WOT.
we have established that barometric pressure is the SAME on BOTH sides of the plate at WOT.
now why does the gasoline jump out of the fuel bowl.
its called the venturi principle again.
its also why older non efi motors used an ex crossover to heat the carb.
its also why various outboard manufacturers offered carb heater kits.

as the air passes through that venture its sped up and compressed, as it exits the venture a couple funny things happen due to some gas laws.
one is the pressure drops aas the air expands,this draws fuel from the bowl,still at baro pressure,into the air stream.
the other is a rapid temperature decrease.
when running in cold weather and relatively high humidity this can and does lead to carb throat icing.
on the humidity think of it as duck hunting when the ambient air temp is 25*F yet the water temp is in the 40's.
means the air,at or near the waters surface is cold yet still contains a lot of water.

now look at the throttle body of an EFI motor.
note the lack of a venture?
that's why EFI cylinder heads contain no ex gas crossover.
not needed nothing to ice.
fuel is moved with a pressure pump and injectors.

any clearer ?
other than I type slow and sometimes cant spell.

[countrytn]

Rodbolt, you will have to excuse me but the mechanics in my area would not make a scab off your ass