YF problems, continued...

[grocery getter]

you're on the right track. I also thought about clipping the tip, but came up with the same thought about it finding it's way back into the jet.


what about a cheap eastwood plating system ? plate the first two steps...

that's completely out of the box. i'm just throwing darts

 

[Bort62]

I actually like the idea of opening up the jet and increasing the mid-rod width in order to reduce flow area @ part throttle more.

 

[falcon60]

"I did not however notice any mechanism by which vacuum would pull the rod down, it appears to be fully mechanical."

I too am using a YF from a 300 on my Falcon/200. The bottom side of the diaphragm on the metering rod/accel pump apparatus is exposed to manifold vacuum.Vac holds it "down" and when you step hard on the throttle the spring and mechanism raises the assembly since the vac drops and can no longer hold it down. The top side of the diaphragm is accel pump (as I'm sure you know) .This arrangement is why the accel pump seems weak when the engine is not running.....no vac to fully return the diaphragm on the "fill" stroke.
The main purpose of the check ball and weight is to keep fuel from being forced through the accel pump circuit when high airflow conditions exist in the venturi. It also helps keep the delivery channel full and ready for the next "shot".
Almost forgot......Have you considered restricting the main jet air bleed to compensate for increased air density under boost?

Terry

 

[Bort62]

"I did not however notice any mechanism by which vacuum would pull the rod down, it appears to be fully mechanical."


Almost forgot......Have you considered restricting the main jet air bleed to compensate for increased air density under boost?

Terry

No I have not terry... this is the sort of advice I am looking for, please elucidate...

 

[Bort62]

As far as the vacuum acting on the diaphragm... I don't see any passage that would facilitate this, am I missing it ?

The only passage on the bottom side of the diaphragm is the one which leads up into the ACC pump squirter. That would, however, make sense as to why I had acc pump problems with the engine off but didn't seem to have any while driving. I placed washers between the linkage arm and the pump plunger, holding it in the down position when the throttle was closed - and got a very strong pump shot with the engine on or off.

 

[Lazy JW]

There is a small passageway under the diaphragm that leads to the bottom of the carb right beside the throttle butterfly. It is exposed to manifold vacuum.

I have exactly zero experience with turbos but I thought that blow-through required a can built around the carb to equalize pressures because raising the pressures upset the equilibrium that carbs need to work. A draw-through works fine though.

Tractor carbs are "balanced" so blow-through "should" work fine. I've never seen it done though.
Joe

 

[Lazy JW]

Ok, been ponderizing on this all day.
Blow-through raises pressure inside the venturi. A carb relies on a pressure DROP (Bernoulli Principle) to lower the pressure, then the higher pressure (atmospheric) inside the float bowl pushes fuel into the airstream.

You have raised the pressure in the venturi but left the float bowl at atmospheric so the venturi cannot create enough of a pressure drop to "suck" the fuel in, or at least not ENOUGH fuel. Lean condition is the result.

Until you can raise the float bowl pressure it's gonna be tough to get enough fuel.
Joe

 

[falcon60]

Joe,I believe that the bowl should be at the same pressure as the venturi due to the bowl vent tube connecting to the upper venturi area.I need to have another look at my YF but it's cold and snowing out there,lol.
Ian- The air bleed will be in the upper venturi below the air cleaner flange.Like I was saying to Joe,I need to look closer to refresh my memory as it's been a while.....but if you blow WD40 or carb cleaner through the main jet it will emerge from the air bleed and then you will see it.Restricting the air bleed is a good way to increase fuel delivery at open throttle without affecting it so much at lesser throttle openings.Many times a small diameter piece of wire temporarily inserted in the bleed will allow a good test.

Terry

 

[Bort62]

The bowl is referenced to the upper venturi. The pressures will be equal. The venturi effect of the carburetor is unchanged.

I will have to look for that air bleed... It's not obvious to me.

 

[falcon60]

Ok,I looked at a spare YF. The air bleed is a small hole (maybe .030"?) in a pressed-in aluminum disc right where the booster joins the main carb body. You can see where the connecting channel joins up with the booster's fuel delivery tube. With the top off the carb the air bleed will be right on top.
Oh....I was thinking about what Joe said and you know,some fuel bowls have an external vent to the evap canister.If that's open under boost you might not have equal pressures. Anyway,it might pay to look and see if the bowl has any external vents,and plug them if necessary.

Terry

 

[Bort62]

Ok,I looked at a spare YF. The air bleed is a small hole (maybe .030"?) in a pressed-in aluminum disc right where the booster joins the main carb body. You can see where the connecting channel joins up with the booster's fuel delivery tube. With the top off the carb the air bleed will be right on top.
Oh....I was thinking about what Joe said and you know,some fuel bowls have an external vent to the evap canister.If that's open under boost you might not have equal pressures. Anyway,it might pay to look and see if the bowl has any external vents,and plug them if necessary.

Terry

Off the top of my head, I think I know what you are talking about. Would you mind explaining to me the fucntion of the air bleed so I can better understand it... facilitates tuning.

As far as the evap bowl vent, I think I am in the clear there. I did a pretty good job sealing the carb.

 

[falcon60]

The air bleed is just a calibrated air "leak" into the delivery circuit of the main jet. It delays the activation of the main metering circuit since air flows easier than fuel. When enough airflow is present to overcome the flow capacity of the airbleed,fuel begins flowing in the circuit,but also air is still being drawn through the bleed orifice and acts to lessen the flow through the main jet. The higher-end performance carbs have screw-in replaceable air bleeds to be used for tuning,and can be very helpful in boosted applications according to good carb guys I've talked to. A smaller air bleed will effectively increase jet flow,and it seems like it would increase at a rising rate as pressure differential increases. I'm thinking maybe it will be possible to better tune the cruise mix with the needle if tuning the air bleed can help the main jet at WFO.

Terry

 

[grocery getter]

The air bleed is just a calibrated air "leak" into the delivery circuit of the main jet. It delays the activation of the main metering circuit since air flows easier than fuel. When enough airflow is present to overcome the flow capacity of the airbleed,fuel begins flowing in the circuit,but also air is still being drawn through the bleed orifice and acts to lessen the flow through the main jet. The higher-end performance carbs have screw-in replaceable air bleeds to be used for tuning,and can be very helpful in boosted applications according to good carb guys I've talked to. A smaller air bleed will effectively increase jet flow,and it seems like it would increase at a rising rate as pressure differential increases. I'm thinking maybe it will be possible to better tune the cruise mix with the needle if tuning the air bleed can help the main jet at WFO.

Terry

i'm not bort62...but thanks for the explanation. it sounds like it acts like a balancer or a filter to smooth the flow out. so it's not so reactive that you constantly feel the change.

 

[Bort62]

That makes sense, thanks.

Problem is, the volume of air flowing through a .030 orifice isnt going to displace a lot of fuel @ full flow, and as a result I am not sure how much effect I could realistically get by restricting it. Worth a try I guess...

Just to confirm - is this air bleed what prevents the main from flowing fuel @ idle? - @ idle flow rates, the air bleed is sufficient to displace 100% of the fuel that would flow otherwise?

I think my first approach is going to be to open up the jet until I get it running ok at WOT (damn JAW isnt here yet). At that point, I will re-evaluate the situation and look at either:

a.) Increase the diameter of the metering rod @ part throttle. (this will be challenging to accomplish)
c.) Decrease the diameter of the metering rod @ WOT
d.) Adjusting the pull-out distance of the metering rod
e.) Alt methods IE fiddling w/ air-bleed restriction and shrinking the jet back down.

I've posted this before, but incase anyone didn't see it - the carter YF accepts edlebrock AFB jets.

 

[Lazy JW]

....

a.) Increase the diameter of the metering rod @ part throttle. (this will be challenging to accomplish)....

Maybe try building it up with some Gear-Kote® or Gun-Kote® stuff. You spray it on then bake it in an oven. Probably could increase diameter by .005" or so.
Joe

 

[falcon60]

"Just to confirm - is this air bleed what prevents the main from flowing fuel @ idle? - @ idle flow rates, the air bleed is sufficient to displace 100% of the fuel that would flow otherwise?"

Yes,sir.....that and the fact that pressure diff is low at the venturi at idle. The air bleed does keep away that nasty "drip" at idle.

Terry

 

[Bort62]

....

a.) Increase the diameter of the metering rod @ part throttle. (this will be challenging to accomplish)....

Maybe try building it up with some Gear-Kote® or Gun-Kote® stuff. You spray it on then bake it in an oven. Probably could increase diameter by .005" or so.
Joe

I was thinking along these lines... I could also use JB weld and then turn it down in a lathe.

In the absense of a lathe, I could chuck it up in a drill and lay into it w/ sandpaper.

 

[Lazy JW]

Still puzzled with this challenge

Could the higher pressure in the float bowl be affecting the fuel level? Seems that a hollow float surrounded by increased pressure "might" ride higher, resulting in a lower fuel level.

Just stabbing around in the dark looking for a solution
Joe

 

[Bort62]

Still puzzled with this challenge

Could the higher pressure in the float bowl be affecting the fuel level? Seems that a hollow float surrounded by increased pressure "might" ride higher, resulting in a lower fuel level.

Just stabbing around in the dark looking for a solution
Joe

There is nothing especially challenging about it, the problem makes perfect sense.

A carburetor delivers X amount of fuel for Y amount of volumetric airflow through the venturi. (Z mass flow of air @ ambient pressure)

Under boost, say 14.7 psi, Y amount of volumetric airflow is 2*Z mass flow, as the air is twice as dense at that pressure. Constant velocity through the venturi.

So, since you have twice the mass of air, you need twice the fuel.

Hence the need to richen up the carb, dramatically.

I am using a solid float. Bowl pressure is equal to inlet and + venturi pressure, same as if the carb were N/A.

 

[66 Fastback]

The drill & sandpaper method of "machining" the metering rods is a method that is listed in my Carter Carburetor book that I got yesterday.

I too am ignorant of turbos. But, has the turbo altered the vacuum readings of the car significantly from a stock engine? My gut feeling is that since more air is forced into the manifold, then the manifold vacuum properties are different. I bring this up because the metering rod springs are typically calibrated to different levels of manifold vac and the rods are held in the jets by increasing manifold vacuum. So with the cam changes and turbo, is it possible that the metering rod is not allowed to pull out at WOT?

My experience with the metering rods is on the 4 bbls, so maybe the 1 bbl's are set up differently. One of my AFB equipped cars had a lumpy cam and poor vacuum at idle which was not strong enough to pull the metering rod down at idle and the engine ran rich.
Doug