Carb Circuits - Aisin 2F FJ60 USA - Troubleshooting (2 Viewers)

[Slow Left]

 

[Slow Left]

Fuel Bowl
The level of fuel in the fuel bowl is regulated and adjusted by the Needle Valve assembly (#9) and Float.
Since both the 1st Main Jet (#17) and 2nd Main Jet (#10) are always open, they allow fuel to pass from the fuel bowl into the circuits behind each Main Jet, filling the level of fuel in those circuits matches the level of fuel in the fuel bowl.*

Circuits

1st Slow (Idle) Circuit (#16)
The fuel bowl level fills up the 1st Slow Jet (tube)(#16) through the 1st Main Jet (#17).
When the ICS Valve (#7) is open, fuel is metered by the Idle Mixture Screw (#15) and drawn out a port below the throttle plate into the 1st barrel. Because the port is below the throttle plate, it sees manifold vac with the plate closed, allowing the engine to draw enough fuel to maintain an idle.

1st Main Circuit (Nozzle (#5))
Uses venture vac, which is created when fast moving air (under throttle) passes the venture port in the 1st nozzle, drawing fuel from the fuel bowl through the 1st Main Jet (#17) (and presumably also through the Power Valve (#18) when it is open).

Power Piston (#8)/Power Valve (#18)
At idle and conditions above 3 inHg of manifold vac (i.e. idle or 'cruising'), manifold vac applied to the Power Piston (#8) keeps Power Piston retracted from the Power Valve (#18), and the spring internal to the Power Valve keeps the Power Valve closed.

The manifold vac is generated in the 1st barrel, below the the 1st throttle butterfly valve. The vac circuit leads from the Power Piston, through the Air Horn housing, down through the hollow bolt that holds the carb plates together and through a hole in the throttle body housing between the 1st barrel and the hole in the housing in which the hollow bolt sits.

When the manifold vac decreases below 3 inHg (under certain load conditions which could include, but not exclusively WOT), the vac keeping the Power Piston retracted goes away, and the Power Piston return spring pushes the Power Piston on the Power Valve, causing the Power Valve to open.

When the Power Valve opens, additional fuel flows (venturi vac) from the fuel bowl into the 1st Main Nozzle circuit, through the 1st Main Venturi and into the 1st barrel.

Once the throttle is released (under cruising conditions or return to idle conditions), manifold vac returns to overcome the return spring force on the Power Piston and the Power Piston once again retracts and the spring internal to the Power Valve forces the Power Valve to close…shutting off the additional flow of fuel to the 1st Main Nozzle circuit...and full control over fuel delivery from the fuel bowl to the 1st Main Nozzle circuit is returned to the 1st Main Jet.

Notes
*Changing the size (ID) of the Main Jet orifice changes the maximum rate that fuel can be drawn from the fuel bowl into the 1st and 2nd carb circuits.

OEM 1st Main Jet size: 147 (1.47mm)
Recommended 1st Main Jet size (smogged): 150 (1.5mm)
ref. Pin_Head post #11 below
Recommended 1st Main Jet size (desmogged):


OEM 1st Slow Jet size: 65 (0.65mm)
Recommended 1st Main Jet size (smogged):
Recommended 1st Main Jet size (desmogged):


OEM 2nd Main Jet size: 200 (2mm)
Recommended 2nd Main Jet size (smogged):
Recommended 2nd Main Jet size (desmogged):

OEM 2nd Slow Jet size: 90 (0.9mm)
Recommended 2nd Main Jet size (smogged):
Recommended 2nd Main Jet size (desmogged):

Drill Bits: To drill OEM 147 Main Jet to 150, use 1.5mm diameter drill bit
Commercially available Jets: Keyster Carb Rebuild Kit has: 150 and 200 Main Jets (Retail: Kurt at Cruiser Outfitters, )

 

[Slow Left]

After removing my 12/1981 FJ60 2F carb (PN #21100-61141), I think I have identified a leak from the 1st throttle shaft in the front where it goes into the throttle body housing of the carb.

With the carb removed from the vehicle, if I twist the primary throttle shaft, fuel will squirt out of the Pump Jet (#4) and flow down the walls of the 1st barrel…and will seep out in the front between the 1st throttle shaft and the housing it goes through.

So, I’m thinking that the loose shaft might be a vac leak source when the engine is running and under load…and if so, could the vac leak be large enough to explain the symptoms I am seeing (flat spot around 1,800 rpm in the acceleration band, poor gas mileage and stinky exhaust).

I should also note, that I’ve got the base timing set to ~10* BTDC (bb in the bottom of the viewing window) and the Idle Mixture screw set ~1 ½ turns out from seat.
Manifold Vac (measured at the A/C Idle Up union) is ~22 inHg at idle.
Engine is stock (fully smogged).


Questions:

1) If this is a source for a vac leak internal to the carb, how could I measure/know what it’s affect is?
And can it be fixed?
Could this be the reason my choke breaker/opener isn’t working?
Would this affect the vac being pulled on the center hollow screw and what are the implications of that?

2) When I set the Idle Mixture screw to ~1 ½ turns out from seat, this is the most obvious point where I see RPMs ‘stop’ increasing (if I screw it back in from 1 ½ turns out, there is a noticeable decrease in idle/RPM)…but I understand that for best performance, the Idle Mixture screw should be about 4 turns out…
so, If I continue to turn the Idle Mixture screw out beyond the 1 ½ setting, how do I know when to stop?
The rate of RPM increase beyond 1 ½ turns out is a lot less, so perceiving what max RPM is isn’t so apparent…but is this the correct way to set for max RPM?

3) If I have the Fast Idle speed screw set so that when the choke handle is pulled out all the way, the engine idles at 1,800 RPMs (peak torque for the 2F), then can I use the choke to hold the engine at 1,800 RPMs while rotating the dizzy cap back and forth until I find that ‘happy’ sounding spot where it is making ‘max torque at it’s max torque RPM’?

 

[LAMBCRUSHER]

assuming your carb is running desmogged and with the vac advance hooked up, I'd suggest plugging the ports to the vac advance cans and get a baseline with mechanical advance only and see if you still have the flat spot...(if gone, then vac advance canisters could be source of vac leak at 1800 revs...)

 

[Slow Left]

assuming your carb is running desmogged and with the vac advance hooked up, I'd suggest plugging the ports to the vac advance cans and get a baseline with mechanical advance only and see if you still have the flat spot...(if gone, then vac advance canisters could be source of vac leak at 1800 revs...)

It's fully smogged (stock) with both the primary (ported) and secondary (HAC) vac advance hooked up.

So if I plug both primary and secondary vac lines, is the best way to set the base timing to 'max torque at it's max torque RPM' to pull the choke so it holds steady at 1,800 RPM while rotating the dizzy cap until I find the middle spot that sounds the best?

 

[Pin_Head]

Fuel seepage is not unusual.
Throttle shaft bushing wear is not common for Aisan carbs like it is for Q-jets.
Is there any slop noticible?
Spray some carb cleaner around the shaft while idling and see if the idle speed increases. This would indicate that there is some vacuum leakage.

 

[Slow Left]

Fuel seepage is not unusual.
Throttle shaft bushing wear is not common for Aisan carbs like it is for Q-jets.
Is there any slop noticible?
Spray some carb cleaner around the shaft while idling and see if the idle speed increases. This would indicate that there is some vacuum leakage.

OK. Thanks Pin_Head.

Yeah, there is some slop in the shaft...if I wiggle it back and forth or in and out I can feel it and see it move.

When I reinstall the carb tomorrow I'll do spray around it at idle.

Could a leak like this at the primary throttle shaft be enough to cause the symptoms I've been seeing?

Also wanted to ask you what the hollow bolt (the one that screws underneath the bottom of the carb) draws/holds vac for (I reused that gasket when I did the rebuild and I'm wondering if that could be causing an internal vac leak).

Thx.

 

[Pin_Head]

Throttle bushing wear usually causes a vacuum leak at idle, but you don't seem to have a problem with this. A stumble at 1,800 sounds like you might need a little larger primary main jet.

The hollow bolt provides manifold vacuum to the carb to run the power valve among other things.

 

[Slow Left]

Throttle bushing wear usually causes a vacuum leak at idle, but you don't seem to have a problem with this.

Yeah, my vac at idle is terrific, but at idle the fuel is being drawn through the 1st Slow circuit (1st Throttle Plate is closed). So what I'm thinking is once that 1st throttle plate moves off idle, air flowing down the primary barrel might also draw air in through that sloppy primary throttle shaft...(Bernoulli Effect)...

A stumble at 1,800 sounds like you might need a little larger primary main jet.

Right, and then once the secondary kicks in it improves a little, but still not what it should be...

OEM 1st Main Jet size: 147
Recommended 1st Main Jet size (smogged):
Recommended 1st Main Jet size (desmogged):

OEM 1st Slow Jet size: 65
Recommended 1st Main Jet size (smogged):
Recommended 1st Main Jet size (desmogged):

OEM 2nd Main Jet size: 200
Recommended 2nd Main Jet size (smogged):
Recommended 2nd Main Jet size (desmogged):

OEM 2nd Slow Jet size: 90
Recommended 2nd Main Jet size (smogged):
Recommended 2nd Main Jet size (desmogged):

Any recommendations on where to buy the correct sized jets or what drill bit sizes to use if DIY drilling out the jets?

A stumble at 1,800 sounds like you might need a little larger primary main jet.

This thread address similar symptom
https://forum.ih8mud.com/60-series-wagons/25963-power-valve.html

"As I understand the function of the P valve in the Aisin carb it only comes into play when vaccum drops real low. A steady state acceleration should not open the P valve. The check to see if the P valve is working is to disconnect the arm of the accelerator pump and then "goose" the engine. A proper working P valve should let the engine die for about .5 seconds then kick in."

Is this a valid description of the conditions under which the Power Valve operates?
And is this a valid field test to determine whether or not the Power Valve is operating correctly?

The hollow bolt provides manifold vacuum to the carb to run the power valve among other things.

Like the Secondary Diaphragm?

What about the Power Piston (#8 in image above), does that also operate based on vac?
I guess I'm not seeing how vac operates the Power Piston/Power Valve assemblies...
So vac draws on top of the Power Piston which the opens the Power Valve allowing fuel from the fuel bowl to flow to the 1st Venturi?
If so, then where does it get it's vac from (if I follow the circuit from the top of the Power Piston, it leads down below the Idle Mixture Screw, but doesn't show if it ports into the wall of the primary barrel...or?)?
And under what conditions does it see vac and open?

 

[Slow Left]

The hollow bolt provides manifold vacuum to the carb to run the power valve among other things.

I'm still trying to decide if I should risk backing out this hollow bolt to replace that gasket...I reused the OEM crush washer and am wondering if that might be giving me a vac leak.

But when I did the rebuild, I had a difficult time getting this bolt out and chewed up the single slot a little...the last thing I'd want to do is break this bolt.

 

[Pin_Head]

Oy, so many questions I might forget a few.

1. Off idle fuel comes from the transition slot just above the throttle plate that is fed from the idle circuit. When people have a manifold vacuum leak, they increase the idle speed to compensate, pulling the fuel from the transition slot and causing off idle bogging down.

2. I think that 150 and 200 jets come in the Keyster carb kits for $25. Call Kurt to confirm. 150 is a 1.5 mm hole in case you want to drill.

3. Manifold vacuum keeps the power valve closed and when it drops below about 3 inches of water, it opens and richens the primary. I don't really know how to test it other than to verify that the piston moves freely.

4. The vacuum secondary does not run on manifold vacuum. It gets vacuum from high up in the primary and secondary venturis. You need vacuum in both bores, which is why it is important that the linkage kicks the secondary open enough to get some vacuum in the secondary venturi.

5. The power piston operates the power valve. Manifold vacuum retracts the pistion which keeps the valve closed by its spring.

6. The hollow bolt can be a pain. If you got it out once, chances are it will come out again. Use a jewlers file to repair the slot or cut a new slot 90 degrees to the other one.

 

[lovetoski]

Nothing to contribute, other than to admit that I now realize I know way less than I thought I did about how a carb works. Very humbling.

 

[Slow Left]

Oy, so many questions I might forget a few.

Ha Ha! Sorry Pin_Head...I'm just trying to make sense of this...

I'm going to start logging how each circuit works based on contributions/clarifications that MUDers make, so if you see and explanation that doesn't make sense, please chime in so it's right...

1. Off idle fuel comes from the transition slot just above the throttle plate that is fed from the idle circuit. When people have a manifold vacuum leak, they increase the idle speed to compensate, pulling the fuel from the transition slot and causing off idle bogging down.

Ok. So this means that setting the Idle Mixture screw to 4 turns out will not address fuel starvation issues I might be having at 1,800 RPM. Instead, making sure the Power Valve is working correctly will and/or increasing the 1st Main Jet size.

4. The vacuum secondary does not run on manifold vacuum. It gets vacuum from high up in the primary and secondary venturis. You need vacuum in both bores, which is why it is important that the linkage kicks the secondary open enough to get some vacuum in the secondary venturi.

Yeah, and looking at the cross-section image of the carb above, it shows a vac circuit from which the 2nd diaphragm draws from that runs up the middle of the carb.
I wonder if that has anything to do with the hollow bolt...

6. The hollow bolt can be a pain. If you got it out once, chances are it will come out again. Use a jewlers file to repair the slot or cut a new slot 90 degrees to the other one.

Thanks for the great tips! Right now I'm thinking unless I have a solid reason to pull it, I'll just leave it. I used the Hygrade Kit, so I'm not sure there was a new gasket that came with the kit for that hollow bolt.

 

[LAMBCRUSHER]

I'm going to go out on a limb ans step on some toes...the power valve is only active under WOT(less than 3" vacuum); won't fix the off-idle bog. the larger primary main jet will help, allowing more fuel to air in at lower speeds. more important to make certain that you aren't pulling from the transition slot at idle speeds(back off the idle speed set screw...) have you tried bypassing the egr valve?

 

[Slow Left]

3. Manifold vacuum keeps the power valve closed and when it drops below about 3 inches of water, it opens and richens the primary. I don't really know how to test it other than to verify that the piston moves freely.

5. The power piston operates the power valve. Manifold vacuum retracts the pistion which keeps the valve closed by its spring.

I'm going to go out on a limb ans step on some toes...the power valve is only active under WOT(less than 3" vacuum); won't fix the off-idle bog.

At idle and conditions above 3 inHg of manifold vac (i.e. idle or 'cruising'), manifold vac applied to the Power Piston (#8) keeps Power Piston retracted from the Power Valve (#18), and the spring internal to the Power Valve keeps the Power Valve closed.

The manifold vac is generated in the 1st barrel, below the the 1st throttle butterfly valve. The vac circuit leads from the Power Piston, through the Air Horn housing, down through the hollow bolt that holds the carb plates together and through a hole in the throttle body housing between the 1st barrel and the hole in the housing in which the hollow bolt sits.

When the manifold vac decreases below 3 inHg (under certain load conditions which could include, but not exclusively WOT), the vac keeping the Power Piston retracted goes away, and the Power Piston return spring pushes the Power Piston on the Power Valve, causing the Power Valve to open.

When the Power Valve opens, additional fuel flows (venturi vac) from the fuel bowl into the 1st Main Nozzle circuit, through the 1st Main Venturi and into the 1st barrel.

Once the throttle is released (under cruising conditions or return to idle conditions), manifold vac returns to overcome the return spring force on the Power Piston and the Power Piston once again retracts and the spring internal to the Power Valve forces the Power Valve to close…shutting off the additional flow of fuel to the 1st Main Nozzle circuit...and full control over fuel delivery from the fuel bowl to the 1st Main Nozzle circuit is returned to the 1st Main Jet.

Does this accurately explain how this circuit works? If not, what corrections need to be made?




Where does the manifold vac that acts on the Power Piston originate from? If the Power Piston sees manifold vac at idle, I would assume that there is a port in the wall of the 1st barrel below the 1st throttle butterfly valve…?

 

[LAMBCRUSHER]

IIUC, the power valve gets its vacuum internally via that hollow bolt...3" vacuum is pretty low for a mid throttle condition...

 

[Slow Left]

I don't really know how to test it other than to verify that the piston moves freely.

I remember reading a thread once recommending stretching the Power Piston spring about 1/4" during rebuild (new springs don't come in the kits) to give it a little more 'springiness'.
Is this a good idea or is there a better way to more accurately make sure that the spring constant is to spec?

...with a new Power Valve installed (came in the Hygrade kit), if there is a way to test for Power Valve functioning with the carb installed on the vehicle, that might be away to make adjustments to the Power Piston spring constant (a new Power Valve allows one to assume that the Power Valve is not at fault for the system not working).

 

[Pin_Head]

It is not just WOT, but also the load on the engine that determines the manifold vacuum. WOT at 4,000 PRM could still have a manifold acuum of greater than 3 inches, while WOT at 1,000 RPM the vacuum may drop to less than 1.

Manifold vacuum is anywhere below the throttle plates. In the case of Aisan carbs it is obtained through the hollow bolt.

I never stretch the PV spring, but I doubt that stretching it will make much of a difference in its spring rate.

 

[Slow Left]

more important to make certain that you aren't pulling from the transition slot at idle speeds(back off the idle speed set screw...)

I'm good on the Idle Speed screw setting...it is just in from the contact point on the linkage, so I'm pretty sure that I'm not using the throttle (fuel through the 1st nozzle instead of the 1st slow circuit) in order to idle (compensate for a vac leak, for example)

have you tried bypassing the egr valve?

I haven't tried this. What's the best way to do this?

 

[LAMBCRUSHER]

disconnect and plug lines at modulator.