1982 720 carburetor

[nlmb14k]

 

i have a 1982 datsun 720 2wd king cab

  i got sum questions about the carb, i’ve had the truck for about 2 months i’ve been learning the motor.

  my problem is my rpms stay at 2500 when my truck is fully warmed up. if i take fuse #2 out my rpms drop to 2000. i believe my fast idle cam isn’t working properly, and i say that because when i take fuse #2 out my rpms go down so something is working. also when fuse #2 is in these two pipes that go into my air filter that come from the exhaust just roar when the fuse is in, vice versa take the fuse out and it gets quiet.

and the guy before me installed a fuel pump kill switch so idk if he didn’t wire the electric carb correctly or accidentally tapped into those wires.

i’ve tried to mess around with the mixture screws to change my rpms but my truck is so keen on running at 2000-2500 that any little change in the mixture screws my truck runs like shit, and is there a third screw that i’m not seeing? 

 

one more question 

i kept having a short in my radio and i was running through fusses, one day i heard sparks under my dash and this wire that i’m holding was touching my gas lever. i cut it and it solved that problem can anyone lmk what this thing i’m holding does thanks.

 

these three pics are where i’m missing vacuum lines

 

 

 

 

and this guy right here is connected to fuse #2 and is a blue and white wire i’ve looked at my om and the diagram labels it as (gasoline/diesel) i also tried to see if my choked moved while it warmed up fully but i didn’t notice anything or i didn’t look properly is this the mechanism that controls the fast idle if anyone can help thanks.

[datzenmike]

First... that is not the correct Z22 carburetor. Not that it won't work just that it's from something else. How is the air filter secured to it. The Z22 carburetor has a threaded rod in the center. Can't really see....

 

 

This is the idle cut solenoid and allows gas to the idle circuit with the key ON. It's job is to shut the fuel off and stall the engine in the OFF position. Also it should be connected to a Red wire. The Blue wire should go to the choke heater.  Looks like someone used the Red to power the electric fuel pump which begs the question what is the choke heater doing?????. The choke heater warms the choke at a known rate and turns it off. In turning the choke off the fast idle also decreases. The idle cut is never on the side of the carburetor but on the rear above the throttle cable and beside the choke heater. It must be on with the ignition if you ever hope to lower the idle speed and adjust the mixture.

 

I can't see the choke at all is it a manual one? Maybe post a better picture of the top and rear of the carburetor????

 

 

 

This is the mechanical fuel pump. I don't know why someone would go to the bother of putting an electric one in. The two metal pipes will make noise as these just connect to the exhaust. They use exhaust pulses to draw air in from the air filter through one way reed valves. The oxygen rich air helps burn off emissions in the catalytic converter. They are working normally.

 

 

The hose pairing off to the right is the fuel return to tank. To work properly there should be a restriction in it so the pump builds enough pressure to push into the carburetor fuel bowl. If it runs we'll leave this for now.

 

 

 

This is the idle speed screw. turn it down to lower the idle speed. The other screw is for setting the idle mixture, but until the idle is 700 or so this won't be needed.

 

 

 

Looks like a noise suppression filter for a radio or something electronic. Not stock, someone added it. Just remove or tape out of the way.

 

Missing hoses won't affect much. We can deal with them later.

 

 

 

Fast idle

Turn the idle speed screw down that I mentioned what happens???

[nlmb14k]

well damn that explains a lot, i knew the carburetor was replaced but i thought with the right fitment. this air filter fits really good. but it’s the wrong one. where can i find a z22 air filter??

 

 I turned the idle screw down and half a turn in and the rpms got higher. i went back to its position and turned it half out and it started bogging so the engine stays around  1900 before it’s dying. i took pics of the carb and the choke but idk how to insert them it only gives me a options to insert a link

 

ur right the solenoid is connected to a red wire but i guess since it’s the wrong carb it’s a black one but it’s spliced into a red wire. it’s connected to a bunch of blue wires on the fire wall, above the fender well.

the reason there is an electric fuel pump is cause there’s a fuel pump kill switch. and that noise suppression thing is true too cause i took a mustang equalizer/alarm out of it and i think i left that live wire open.

 

 

i also was looking under the truck and noticed this blue wire and red wire coming from the side of the transmission on the right side what are those??

[datzenmike]

The kill switch is not the cause of an electric fuel pump. Rather the pump was put in there (for some reason only the previous owner can say) and a switch added to turn it on and off.

 

 How did you post the original pictures?????? do the same again. Post one of the air filter... it might be the correct one. Also a picture of the choke heater on the rear side of the carburetor.

 

Idle speed screw IN speed it up. Turning screw out reduces the idle speed. Reduce the idle speed and try turning the idle mixture screw out. If the idle stabilizes keep repeating....

[nlmb14k]

for the life of me i don’t know how to upload anymore pics but anyway

 

 

i got the idle down to 1500 if i try to turn it out anymore it bogs

[datzenmike]

Engine off, turn ignition on off on off on off without actually starting it. Listen near the carburetor you should hear a soft clicking sound when the ignition is switched. Is it making this sound??????

 

 

The hexagonal thing with the black tape on the wire is your idle cut solenoid. It has to have power with the ignition in the ON position to allow gas down into the idle circuit. If not working it won't idle below 1,000 or so. Clicking would indicate that it is working.

 

 

 

Put in your profile what year your 720 is. Saves looking back through your posts.

[ShutterMeShort]

On 11/6/2021 at 6:14 AM, nlmb14k said:

 

i got the idle down to 1500 if i try to turn it out anymore it bogs

Thats a start!  I just went through this with my friends truck I'm getting ready to pass CA emissions - carb had several issues (float level and idle cut no ground to start).   As Mike stated - make sure that solenoid is working. Should have 12v on red wire with key on AND a solid ground.  You can pop it out and remove the needle and spring and put it back in as a test - with the guts removed it will allow fuel to bypass and get into the idle circuit but it will probably diesel when you go to shut it off. Good way to check if it's operating - 12v, ground and clicking don't always mean it's working.   Once you know you have fuel in your idle circuit, try lowering your idle speed. Your underhood VECI sticker will tell you your proper idle speed.  Once you get it close, check your timing to make sure its not off too much as it can also effect idle speed.  


One trick I use for telling if it is rich or lean is bring it to the point of bogging and not running right and spray some brake or carb cleaner in. if it runs better then you are lacking fuel.  If it stumbles worse then you have too much fuel or not enough air.  It's not the best test but it can give you an idea of what direction to start looking in.

Edited November 11, 2021 by ShutterMeShort

[datzenmike]

Or turn the idle mixture screw in makes the mixture leaner. Turn it out makes it richer. There about 1/2 a turn where the idle mixture is good with it dropping off rich or lean on either side. Set roughly in the middle. A better mixture adjustment will almost always rev the engine slightly so turn the idle back down and repeat the in and out mixture adjustment looking for the middle of the best strongest idle. Repeat this adjustment and turning the idle speed down as many time as needed till you cannot improve the idle quality and the idle speed is correct for California emissions... what ever that is.

 

A vacuum gauge is invaluable for this and better than your ear. Adjust for the highest vacuum reading at the correct RPM.

[nlmb14k]

On 11/6/2021 at 8:23 AM, datzenmike said:

Engine off, turn ignition on off on off on off without actually starting it. Listen near the carburetor you should hear a soft clicking sound when the ignition is switched. Is it making this sound??????

 

 

The hexagonal thing with the black tape on the wire is your idle cut solenoid. It has to have power with the ignition in the ON position to allow gas down into the idle circuit. If not working it won't idle below 1,000 or so. Clicking would indicate that it is working.

 

 

 

Put in your profile what year your 720 is. Saves looking back through your posts.

 

 

I had my friend turn the ignition on multiple times and no click from the circuit, i got my rpms down to 1000 but around 900 it dies. And how do i edit my profile???

[datzenmike]

Upper right, click on your user name. Drop down has Account Settings, click on it. Find Other Settings and below it Edit Profile, and click on it.

 

 

 

Idle cut solenoid....

Check for 12v on the Red? wire to it. Looks like it was spliced and taped to something. Remove tape and see what's up. There are likely two wires in that small harness, a Red and a Blue for the choke heater.

 

The idle cut is an electric valve held open by battery power. In the open position gas is allowed down into the idle circuit. It's job is to cut off the fuel supply when the ignition is turned off and this helps stall the engine quickly without run on or dieseling. If not working it won't idle below 1,200-ish.