28AV11 / 9778 Carb rebuild
Posted: Sun Nov 28, 2010 1:11 amIts a slow weekend and I need to clear off some projects.. so here goes a carb rebuild.
First we start with two 'mutt' carbs. Although one has an ID disk with the proper numbers, the throttle body (the big cast iron part where the throttle shaft is located) is not a 28AV11 part. How do I know this? Simple. The 28AV11 used in the CCKW has a fixed idle discharge port. The throttle body I had had an adjustable set screw, also the accelerator linkage was different.
Here is a pic of the two carbs I will combine back into an 'original' CCKW carb:
On the left is the carb off my CCKW. Its got the correct Air Intake body (it has the ID disk) but the fuel bowl is wrong as is the throttle body. By using TM9-1826C figure 13 "Series 28 Specifications" I could easily find out what the proper jets/venturi parts are. For a 28AV11 pn 9778 I need the venturi #29, discharge jet #70, and fuel valve seat #40. There are other jets involved, but the rebuld kit is assumed to be 'correct' (it was by the part number and the list of enclosed parts) and I was simply not going to tear the carbs apart to verify them. If my CCKW suffers from poor performance then I will go back and verify them. And the simple truth is that even if I did discover that the well vent is wrong.. where could I get another one?
Rebuilding the carb is pretty straightforward. The TM give the steps. One problem is that you most likely will not have the tools called for. I don't. Most jets can be removed with standard or deep well sockets. For the screwed in jets a screwdriver ground 'straight' to remove the bulge after the blade. There is one tricky part, the check valve in the accelerator circuit, under the pump:
The TM says drill then remove with a tap like device. My substitute is a 4 inch drywall screw:
I did not remove the casting plugs to clean the passages, but they did get a good soak in xylene.
One last part that I needed to verify was the venturi. The venturi for a 9778 carb is a number 29. Luck for me that the size is cast into the venture so IDing the proper one was simple. Look at the number, insert the proper one into the fuel bowl casting:
The one on the right is the #29. The other is a #33 from the other carb.
Some other minor items is that the accelerator pump linkage is exposed and can rust.. so that may need some sanding to get it to go through the casting to remove.
Note the fuel bowl sight plug.
I am still missing the choke cable return spring and bracket.. I have it, just not installed. Also note the pipe plug on the throttle body. Beneath the plug is a vacuum port used for vacuum operated devices like distributor advance or governors. The CCKW's port should be capped with a plug. Just to the right of the plug is the fixed idle port. The casting will accept a needle screw (as one carb had) but the CCKWs design was a fixed idle port plug. Its one tell for an original carb.
Note the fuel bowl is simply ZENITH over Made in the U.S.A. No bendix or script font. Also note the throttle body has a 20 cast into it. There is also a 7 under the boss where the fixed idle port is pressed in.
And here is the ID disk.
I did try and polish it up with a wire wheel and a dremil tool. that's why the pictures look 'wavy'. The throttle body was going to be bead blasted and painted black but its to cold to be out in the garage blasting and painting.. but it should be a satin black. I also plan to get the proper fillister head screws to replace the rusted ones... but the really do look fine... its just the camera and the flash make it look worse than it really is.
First we start with two 'mutt' carbs. Although one has an ID disk with the proper numbers, the throttle body (the big cast iron part where the throttle shaft is located) is not a 28AV11 part. How do I know this? Simple. The 28AV11 used in the CCKW has a fixed idle discharge port. The throttle body I had had an adjustable set screw, also the accelerator linkage was different.
Here is a pic of the two carbs I will combine back into an 'original' CCKW carb:
On the left is the carb off my CCKW. Its got the correct Air Intake body (it has the ID disk) but the fuel bowl is wrong as is the throttle body. By using TM9-1826C figure 13 "Series 28 Specifications" I could easily find out what the proper jets/venturi parts are. For a 28AV11 pn 9778 I need the venturi #29, discharge jet #70, and fuel valve seat #40. There are other jets involved, but the rebuld kit is assumed to be 'correct' (it was by the part number and the list of enclosed parts) and I was simply not going to tear the carbs apart to verify them. If my CCKW suffers from poor performance then I will go back and verify them. And the simple truth is that even if I did discover that the well vent is wrong.. where could I get another one?
Rebuilding the carb is pretty straightforward. The TM give the steps. One problem is that you most likely will not have the tools called for. I don't. Most jets can be removed with standard or deep well sockets. For the screwed in jets a screwdriver ground 'straight' to remove the bulge after the blade. There is one tricky part, the check valve in the accelerator circuit, under the pump:
The TM says drill then remove with a tap like device. My substitute is a 4 inch drywall screw:
I did not remove the casting plugs to clean the passages, but they did get a good soak in xylene.
One last part that I needed to verify was the venturi. The venturi for a 9778 carb is a number 29. Luck for me that the size is cast into the venture so IDing the proper one was simple. Look at the number, insert the proper one into the fuel bowl casting:
The one on the right is the #29. The other is a #33 from the other carb.
Some other minor items is that the accelerator pump linkage is exposed and can rust.. so that may need some sanding to get it to go through the casting to remove.
Note the fuel bowl sight plug.
I am still missing the choke cable return spring and bracket.. I have it, just not installed. Also note the pipe plug on the throttle body. Beneath the plug is a vacuum port used for vacuum operated devices like distributor advance or governors. The CCKW's port should be capped with a plug. Just to the right of the plug is the fixed idle port. The casting will accept a needle screw (as one carb had) but the CCKWs design was a fixed idle port plug. Its one tell for an original carb.
Note the fuel bowl is simply ZENITH over Made in the U.S.A. No bendix or script font. Also note the throttle body has a 20 cast into it. There is also a 7 under the boss where the fixed idle port is pressed in.
And here is the ID disk.
I did try and polish it up with a wire wheel and a dremil tool. that's why the pictures look 'wavy'. The throttle body was going to be bead blasted and painted black but its to cold to be out in the garage blasting and painting.. but it should be a satin black. I also plan to get the proper fillister head screws to replace the rusted ones... but the really do look fine... its just the camera and the flash make it look worse than it really is.