22R Aisan Carburetor Suddenly Running Rich

[RKCRLR]

My 22R Aisan carburetor suddenly started running rich at idle and part throttle. Engine dies at idle and runs rough at part throttle. It clears up when given more throttle but probably still running rich.

Turning the idle mixture screw all the way in helped a little. I had to set the throttle plate further open to keep it running (roughly) at idle speed. I can see fuel dribbling out the primary center venturi when running. Choke is fully open and fuel level in bowl window is good.

If it had a diaphragm for the power enrichment valve I'd suspect a torn diaphragm but Aisin power valve design doesn't have a diaphragm.

Any suggestions before I blindly start tearing into it?

 

[Mudder]

Have you checked the FSM to see if it gives a flow chart? There’s a 20r tune up one thanks to @clx16

https://forum.ih8mud.com/resources/categories/hilux-minitruck.17/

 

[ttrmike]

Check the auxiliary accelerator pump diaphragm

 

[RKCRLR]

Have you checked the FSM to see if it gives a flow chart? There’s a 20r tune up one thanks to @clx16

https://forum.ih8mud.com/resources/categories/hilux-minitruck.17/

Thanks, I have a 1980 FSM that covers the 20R carburetor. The troubleshooting section was pretty basic (i.e., repair carburetor). The 1983 manual in the link that covers the 22R carburetor is similar but it has the carburetor overhaul section for the 22R carburetor which will be helpful.

 

[RKCRLR]

Check the auxiliary accelerator pump diaphragm

Thanks, I'll do that.

 

[RKCRLR]

Check the auxiliary accelerator pump diaphragm

The AAP is definitely bad. Fuel came out when I pulled the vacuum line. It didn't hold vacuum when I tested with my Mityvac and fuel came out of the Mityvac after several pumps.

I pulled the carb and removed tha AAP. There is no visible damage to the AAP diaphragm so it must be leaking at the attachment to the rod. Hopefully that is the root problem.

 

[RKCRLR]

I disassembled the carburetor. The only thing abnormal I saw was the oblong o-ring between the secondary venturi fuel rod and body was ruptured but this shouldn't cause a rich mixture at part throttle. Unfortunately it doesn't appear that these o-rings are included in a rebuild kit and they don't appear to be available anywhere. Hopefully a standard o-ring can be made to fit. My accelerator pump boot is torn but it looks like one comes in a rebuild kit.

My main concern is while the leaky AAP diaphragm can cause a rich mixture I don't see how the bad AAP diaphragm could cause fuel to dribble out of the primary boost venturi. This means that fuel dribbling out the boost venturi is normal, the leaky AAP diaphragm can somehow cause fuel to dribble out the boost venturi, or there is another problem.

 

[RKCRLR]

Just wanted to follow up with the results. I rebuilt the carb and the original problem is fixed. Idles great and doesn't run rich at part throttle. I haven't taken a test drive yet to make sure I didn't screw up something else though.