Mechanics

720 RUNNING RICH

1986 Nissan Truck • 4 cylinder 4WD Manual • 273 miles

I have a 1986 Nissan 720, non-hardbody 4wd 2.4L z24 motor with 273xxx miles on it. It's running really rich and I'm trying to get it to pass smog in California. The carb is newly rebuilt (less then 50 miles), new o2 sensor, new spark plugs and wires, fuel filter was changed maybe 15000 miles ago. The cat is the original from 86 but is passing the cat test involved with the smog. The mechanic that I talked to after it was smoged says its either my computer or a vacuum solenoid that controls fuel. I don't know what this solenoid is or where its located. Everything else is passing, just running rich. I don't know what else it could be. I have found 2 vacuum switches on the passenger side of the engine compartment and one it not actuating like it should be (hooked up my meter and reads 000.0 oms with engine both off and running). I don't know what the switches are for but I doubt these are the switches/solenoids the tech was talking about. For a picture of the switches and there location follow link below. Switches are part code 22360 (bottom left of picture). please help.

http://www.nissanpartszone.com/components/1986-nissan-720-pickup_vacuum-piping-223-411_2.html
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Hoffert88
May 13, 2011.



Those parts look like they are open air valves since they have only 1 hose connection.

Tiny
Fixitmr
May 13, 2011.
What kind of cold start enrichment does the carb use. Does it have an electric or vacuum butterfly choke?

Tiny
Fixitmr
May 13, 2011.
Also some carbs have a power valve to add extra fuel for hard throttle if this is stuck/faulty it will cause richness. If it's vacuum operated it would explain the vacuum angle. I had a 88 sentra with carb that was a nightmare of vacuum controls! Real easy to get them mixed up if removed.

Tiny
Fixitmr
May 13, 2011.
It has an electric choke. Everything on the carb itself is working great. Nothing sticking or anything like I said just rebuilt less then 50 miles ago. And I guess my next question is what do these open air valves do and control? And could it have even the slightest bit to do with fuel control?

Tiny
Hoffert88
May 13, 2011.
So if carb has no variable jetting controls then the only other way to control mixture is adding air? Mine actually had a vacuum controlled siphon jet for cold mixture control. Adding air to siphon circuit controlled mixture.

Tiny
Fixitmr
May 13, 2011.
The only cold air regulator I know of is on the intake. Its a vacuum controled valve that closes when the engine is cold and takes the air from off the exhaust manifold to heat the engine faster. Outside of that I dont know of any air mixture valve. The carb dose have an air fule mixture screw (so the instructions said that came with the carb when I first baught it) but I havent messed with that because the same instructions said it was pre set and should not need adjustment.

Tiny
Hoffert88
May 14, 2011.
Any louck yet? That adjustment screw sounds like an air for idle control mixture screw.

Tiny
Fixitmr
May 26, 2011.
So the problem was resolved, the problem was the air filter being cloged. I never relized it because while doing maintance on the carb I had always had the top cover off so it was getting plenty of air to run right. I had cleaned the filter not long before all of this but I guess I had grabbed the wrong oil for the filter (a K&N filter) and not realized it so it caused all the dirt and grime to just gum up and clog the filter. So with the lid on, the carb was starving for air and more or less had the choke on all the time. R2 the filter and all is well now. Thanks for all the help.

Tiny
Hoffert88
Aug 28, 2011.
On an 1986 Nissan there is a mixture control solenoid valve located in the upper part of the carburetor. You must remove the airhorn to gain access to it. This can be done with the carb still mounted to the engine but it is easier if it is done with the carb off of vehicle. The Nissan part number is 16102-31M10. If this solenoid malfunctions it can cause very high levels of HC and CO (rich running condition) This solenoid is quite expensive but should fix your problem after you have eliminated everything else.

Tiny
CAB
Jan 7, 2012.
CAB, he already said his air filter was overly clogged.

Rivermikerat
Jan 22, 2012.