Odd Carburetor Issue

[jkthomas94]

Hi there!

 

I am a new member from the Upstate of South Carolina. I have a 2 owner 1984 Nissan/Datsun 720 Pickup. What a great truck! It has lasted a long time, and with a little TLC should last even longer. I am trying to get it back on the road at this time but am having some issues getting it to run well. Right now, it will start easily from cold and rev up to around 2000rpms immediately. It runs fine when cold, but when it warms is a different story. As soon as it gets warm, it immediately shuts off and is almost impossible to get running again until it cools off. Any ideas as to what causes this? I'm thinking either some kind of temperature sensor or a clogged jet in the carburetor? Thanks. It's a 2.4l engine.

 

 

[datzenmike]

Welcome to Ratsun.

 

Most guys are familliar with EFI, carbs are much different. It's normal to step on the gas before starting to set the choke which also engages the fast idle. The fast idle is normal when cold and helps speed up the warm up. The choke is required to force a rich condition when cold because the fuel does not evaporate efficiently when all the internal passages and combustion chamber are cold. The engine will act like it's running lean and have poor performance.

 

The choke on can cover two things. Like you said a clogged main jet (very good) This would affect anything above idle when driving. Also the idle cut solenoid which shuts off fuel to the idle circuit to prevent run on or dieseling when the key is shut off. If it won't idle then I would start with this.

 

Listen near the back of the carb and have someone turn the key to the on position several times (not start) and you should hear a light clicking sound. See what happens.

[ggzilla]

Haha this is a very common carburetor issue.

 

It runs fine when cold, but when it warms is a different story. As soon as it gets warm, it immediately shuts off and is almost impossible to get running again until it cools off.

if the idle-cut solenoid is not working it won't idle when warm (when choke comes off), but will still run at 2000 rpm or higher.

 

So listen for the click of the idle-cut on the carburetor, or the click of the fuel-pump relay on the inner fender.

[MikeRL411]

Odd chance but vapor lock if all other trouble shooting fails.  Does the fuel line to the Carbs run very near a hot water or exhaust line?

[jkthomas94]

I checked it out today, and I wasn't able to replicate what happened in the past. It started fine and ran smoothly despite the fact that it won't idle down. Also, it appears that the choke was operating correctly as well. I tried spraying carburetor cleaner with no improvement. No vacuum leaks as far as I can tell, because I individually checked all the lines for cracks and tears. It kept running when warm, but still around 2000-2200rpm. I let it run for probably 30 minutes or more and did some yard driving. It never idled down to normal or died, which I find strange because it used to die a lot. Someone said earlier that this idle-cut solenoid prevents dieseling as well. Well, after I turned the key off, the engine began dieseling slightly for several fires. I just wanted to get y'all's opinions one more time before I go buy a $60-$70 carburetor part. Thanks for your help again!

[ggzilla]

Idling at 2000 rpm will cause dieseling.

 

Anti-diesel valve won't cure the idling speed.

[jkthomas94]

Ggzilla, got any ideas on how to fix the high idle? This isn't something that has gotten worse over time. One day, it was fine, the next, it wasn't.

[ggzilla]

It is caused by too much air. The engine cannot run that fast unless it gets enough air.

 

* throttle open too far. Idle speed screw turned in to far. Or the dashpot came loose. Or the fast idle cam has slipped.

 

* air jet is loose. Check the top of the carb and make sure nothing is loose.

[datzenmike]

Fast idle...

 

Idle speeds screw needs adjusting

 

Choke is NOT fully off. Take top off air filter and look down inside. Is the choke plate closed or fully open??? The fast idle is set on by the choke and may be stuck when the choke turns off normally.

 

Throttle cable too tight. Loosen and let it drop and then re tighten it.

 

Throttle plate worn into carb body and jammed, not letting it close properly.

 

Secondary throttle plate stuck, or jammed with foreign object. Engine off look down secondary with flashlight. The plate at the bottom should be fully closed.

 

Vacuum advance  or mechanical advance stuck forcing an advance ignition which will raise the idle speed.

[jkthomas94]

Ok, it idles down now! Turns out the fast isle can was getting into a bind. Cleaned and lived it and now isles down find. The only thing now is that it's still a little too fast for my taste- around 1100rpm. Any advice on how to get it lower, or just leave it?

[datzenmike]

Go through the rest of the list...

 

Fast idle...

 

Idle speeds screw needs adjusting

 

Choke is NOT fully off. Take top off air filter and look down inside. Is the choke plate closed or fully open??? The fast idle is set on by the choke and may be stuck when the choke turns off normally.

 

Throttle cable too tight. Loosen and let it drop and then re tighten it.

 

Throttle plate worn into carb body and jammed, not letting it close properly.

 

Secondary throttle plate stuck, or jammed with foreign object. Engine off look down secondary with flashlight. The plate at the bottom should be fully closed.

 

Vacuum advance  or mechanical advance stuck forcing an advance ignition which will raise the idle speed.