Tuning the carb on an l16

[mcdatsun]

Hey there, I'm not much of a wrench. I used to do a lot of tinkering but since I got newer cars I haven't touched anything but a oil filter , drain plug or changed a set of brake pads in years. Anyhow, my 71 521 started running rough , intermittently, at idle, in fact almost stalling. I give in a rev and its fine for a while. I took a look at the carb and have a few questions.

 

1. What direction should I turn the mixture to screw (in or out) to lean the mixture? I've been assuming in but I'm not sure.

2. Is there some sort of baseline setting I should start at for the idle screw and the mixture screw?

3. Inside the air cleaner assembly (or the frying pan, as I've always known it) there are two vacuum valves. What are they for?

 

I got the truck running nicely (once warm), but I don't know if it's running rich or lean or what (I don't notice any black exhaust). Also when I first started fiddling, I got the truck running nicely, but as soon as I reattached the frying pan it stalled out. Then I found the vacuum valve that appears to be always closed. I reattached the hose and retuned it. But as far as I can see that "valve" just stays shut. I can't figure out what it's for.

 

 

[datzenmike]

 

1. What direction should I turn the mixture to screw (in or out) to lean the mixture? I've been assuming in but I'm not sure.

2. Is there some sort of baseline setting I should start at for the idle screw and the mixture screw?

3. Inside the air cleaner assembly (or the frying pan, as I've always known it) there are two vacuum valves. What are they for?

 

 

1/ In to lean out for rich

 

2/ Before any carb adjustments you should... check/set valve lash adjustment on hot motor. Have an ignition system in good orde ( wires, plugs, rotor and cap) then confirm timing is correct. If motor won't idle slow enough/well enough, set the carb adjustments then come back and set timing, then go back and re-set carb.

 

Turn idle speed up or down so it will remain idling but as low as you can. Turn idle mixture screw in or out. Idle may improve or get worse. If it gets worse, turn the opposite direction. Idle may improve then get worse. Note where idle is smoothest and set adjustment there for now. Idle likely increased in speed so turn down. Repeat the in/out adjustment to find the smoothest running position and then turn the idle speed down if needed. Continue until you cannot improve the idle quality and idle speed is low and normal.

 

Now check the ignition timing if you couldn't before and adjust as needed. If adjusted, re adjust the carb. Valve adjustment and timing are crucial to a smooth idle. Setting the carb adjustments without setting the valve lash or timing is a waste of your time.

 

I got the truck running nicely (once warm), but I don't know if it's running rich or lean or what (I don't notice any black exhaust). Also when I first started fiddling, I got the truck running nicely, but as soon as I reattached the frying pan it stalled out. Then I found the vacuum valve that appears to be always closed. I reattached the hose and retuned it. But as far as I can see that "valve" just stays shut. I can't figure out what it's for.

 

The valve closest to the valve cover is a hot idle compensator. It's there to compensate for a rich idle condition when idling in high temperatures. This is likely why you haven't noticed that it is doing much of anything. A faulty one can open when it should not and cause an erratic idle. Pull the hose off and plug the end to disable.

 

The other sensor is to open and close a vacuum signal to the valve on the air filter snorkel to draw more or less warmed air from the exhaust manifold hear shield. The warmed air is drawn up through the 'stove pipe' connecting the manifold to the snorkel. Winter is coming so be sure to keep this on and working.