Carb tuning and tweaking.
04-12-2017, 06:59 PM
#1
Registered User
Thread Starter
Carb tuning and tweaking.
So there's still a few bugs in my system.
In what position do I put the electric choke so it works right? I know I've got it close to the right position.
The primer doesn't squirt gas into the intake when you push the pedal down like it used to. It makes cold starts take a long time.
Also out of curiosity, how does the carb adjust for air density, like for elevation or in winter?
This is on a 22R, it's an Aisin carburetor.
Thx.
Edit: other problems fixed.
In what position do I put the electric choke so it works right? I know I've got it close to the right position.
The primer doesn't squirt gas into the intake when you push the pedal down like it used to. It makes cold starts take a long time.
Also out of curiosity, how does the carb adjust for air density, like for elevation or in winter?
This is on a 22R, it's an Aisin carburetor.
Thx.
Edit: other problems fixed.
Last edited by Johnsoline; 04-15-2017 at 04:23 AM.
04-20-2017, 11:10 PM
#2
Registered User
The last time I rebuilt an Aisin carb and adjusted the choke, I replaced the 3 rivets with screws so I could easily adjust it on the truck. Here is basically how I did it, and it seemed to work well: With the truck cold, hold the throttle open part way so it does not lock the choke, then slowly turn the choke heater until the choke is lightly closed. I marked the choke housing once I was close so I could see how far I was adjusting it when I fine tuned it. Keep in mind that if the linkage is operating properly, the choke heater also controls the fast idle, which steps down in notches as the truck warms up. That can be another way to gauge how close the choke is to being in the right place. If the truck immediately kicks down to warm idle after tapping the throttle, even when cold, the choke needs to be more towards the closed position. If it stays on the highest idle and does not kick down to a lower notch even after the truck has run a while, then it needs to be adjusted closer to the open position.
The acceleration pump needs to be fixed to get a fast cold startup. Without that being fixed, it is going to be difficult to fine tune the choke because it will always be hard to start when cold. When everything is adjusted right, it should only take two pumps of the accelerator before starting, and it should start quickly. The diaphragm for the main accelerator pump is on the lower passenger side of the carb and can be replaced on the truck fairly easily, but if the ports are clogged then the cap of the carb will need to come off. It might also just be stuck or the linkage could be worn.
The carb adjusts for being cold mainly by using the choke, plus other things are on the Aisin carb, such as the auxiliary acceleration pump, cold mixture heater, and hot air intake. Carbs typically do not adjust for elevation by themselves, but some of the smog controls on the Aisin carb can make adjustments. I am not sure what year your truck is, but in the late 80's they had ports on the carb that appear to take pressure off some of the fuel circuits to allow smog devices to make small adjustments to the mixture. In California trucks the computer uses an oxygen sensor and electronically controls a valve that changes pressure through a vacuum hose attached to the carb. Federal smog trucks have an optional high altitude compensation valve that basically opens the ports at a certain atmospheric pressure, which roughly corresponds to higher elevations. None of these are as accurate as modern fuel injection, but seem to work well if adjusted properly. I also like the fact that the carb can still operate fine without the computer, so you don't have to worry about expensive electronics leaving you stranded on the trail.
The acceleration pump needs to be fixed to get a fast cold startup. Without that being fixed, it is going to be difficult to fine tune the choke because it will always be hard to start when cold. When everything is adjusted right, it should only take two pumps of the accelerator before starting, and it should start quickly. The diaphragm for the main accelerator pump is on the lower passenger side of the carb and can be replaced on the truck fairly easily, but if the ports are clogged then the cap of the carb will need to come off. It might also just be stuck or the linkage could be worn.
The carb adjusts for being cold mainly by using the choke, plus other things are on the Aisin carb, such as the auxiliary acceleration pump, cold mixture heater, and hot air intake. Carbs typically do not adjust for elevation by themselves, but some of the smog controls on the Aisin carb can make adjustments. I am not sure what year your truck is, but in the late 80's they had ports on the carb that appear to take pressure off some of the fuel circuits to allow smog devices to make small adjustments to the mixture. In California trucks the computer uses an oxygen sensor and electronically controls a valve that changes pressure through a vacuum hose attached to the carb. Federal smog trucks have an optional high altitude compensation valve that basically opens the ports at a certain atmospheric pressure, which roughly corresponds to higher elevations. None of these are as accurate as modern fuel injection, but seem to work well if adjusted properly. I also like the fact that the carb can still operate fine without the computer, so you don't have to worry about expensive electronics leaving you stranded on the trail.
