1985 nissan PU carb problem

[neverhappen]

1985 Nissan PU 720
Z24 Engine fed emmission
4 speed M/T
no air
2 bbl feedback carb
150,000 plus on the odometer
This year Nissan was the start of Nissan getting into the computer manage carburetor. The ECM doesn't hand out codes and the diagnositic procedure is to check for each switch to go to ground under certain conditions i.e. you check the clutch circuit by ground the ecm terminal, pressing the clutch, you either have continuity to ground, or you don't, end of test.
I found a bad water temp switch, which caused the "sensor" light to glow in the dash. Replacement didn't fix my problem however so he's the deal
First, this little truck passed the AZ emmissions test about 30 days ago without a problem. The number weres were well below the acceptable ranges. A year ago I overhauled the carb, replaced all the tune up parts (plugs, wires, cap rotor, fuel filter). I also rebuilt the carb as I was having an idle problem I thought to be a plugged idle circuit. The carb looked like it had water inside of it for some time as there was a lot of corrosion, and pitting. Turns out the idle problem was a bent spade in the connector attaching the idle solinoid, fuel solinoid, choke heater, and TPS. Once the connection was repaired the idle was perfect, the truck ran perfect, and as I said, it passed emmissions.
A week ago while cruising down the freeway I lost speed. I limped home, thought maybe I'd pick up a little dirt in the primary jet. Revved the engine closed the choke to clear it up, amd for a day it ran good again. But....
So I took off ther carb, did a visual inspection, No rust, or dirt. I can force air and carb cleaner through the primary jet, Put the carb back on and no change. In stead of blaming the ECM, Nissan puts a disconnect for the solinoid in line to the carb. When disconnected it runs full rich. I disconnected it, No Change
I can open the secondary plate and watch the fuel flow smoothly over the venturi. When watching the fuel run over the primary, it flows eractically and surges. Not a 500 RPM surge, but a 50 rpm surge. It idles perfectly, and when enriching the idle screw I can change the rpm , same if I lean it out. I realize that other conditions can cause a surge, but why doesn't it surge with the secondary's open?
Today, if you feather the gas and clutch you can get down the road, sometimes it spits farts and coughs. Sometimes it stalls. Starts right up, idles fine. everything "off idle" is an adventure.
Exhaust plug? again, not a problem with the secondary.
My conclusion is a leak internally in the carb causing it to be unable to suck the fuel up the fuel circuit for the primary venturi. But at $300.00 I want to be sure.
My brain hurts, can you suggest a fix
Thanks

[re-tired]

You obviously have more knowledge than our average poster. Based on your description of the internal pitting etc , I would agree with the internal bypassing. Could you find a salvagable used unit to rebuild or use as a donor?

[neverhappen]

I worked in the automotive repair field for 15 years or so. Still like to get dirty and fixing is a sickness only mechanics can understand and appreciate. I pulled the carb again and did a little futher inspection. Nissan placed o-rings between the venturi and carb body (primary and secondary). (not really shown on any exploded veiws) Two small screws which they "peened over" to keep tight hold the venturi tight against the ring. With a little persuasion ( I actually had to drill one out and retap it) I found the o-ring to be hard and brittle. If this is the problem, the vacuum created to pull the fuel up the tube was lost at the ring. I'll have it back on later today and test, For the readers, I post my findings. I may well have to find another rebuildable unit ....Thanks

[neverhappen]

The O-Rings made a difference.... but the difference wasn't enough to call it fixed. A question for you Dan,
The solinoid which can be disconnect so the ECM doesn't monitor fuel/air ratio allows the carb to go "full rich". If this solinoid were clogged, (there is a small jet in the bottom, but I'm not sure if it controls fuel or air(there is also an o-ring to seal it from fuel in the bowl, if it didn't seal, would it run rich or lean)) would I get a lean condition anyway? When disconnecting the solinoid I don't get much change in drivablity. Please reply

[DanD]

Sorry neverhappen but I don’t have much experience with the internals of a Nissan carb.
I did look through Mitchell and they may as well have not bothered to print what they did on the system.
Sorry.
Dan.

[neverhappen]

Isn't it fun to run into a roadblock like this. I found my info in a 1985 Mitchell and did the line by line test of the circuits, after everything checks out it doesn't tell you to "replace the ecm, or repair the carb". I've search the net for a Nissan Factorty manual without any luck. I can send the carb to an overhauler out of state and they will put it on a flow checker for me and tell me if it's got a problem. How they check the air/fuel solemoid without an ECM signal is beyond me and my fear is even if it's perfect they'll want to overhaul it for the $299.00. Then upon return I'll find out the ECM is the problem and spend another $150.00 for a used one. I think the thing to do here is buy a 1984 non feedback carb, install it and be done. I'll continue to post for the value of the readers on my progress, you never know when someone might have the same problem do the road. By the way, I visit Canada and fish once a year. Beautiful place.

[DanD]

Good luck with your project, wish I was of more help.
Where do you fish when up come up here?
Dan.

[neverhappen]

We fly out of Kenora Ontario up to Maynard Lake Lodge about 100-120 miles north. Fishing is mainly walleye and northern, and occasional small mouth bass. I tracked down an inconsitant ground signal from the neutral switch. When I ground the connection (Back probe the wire at the ECM) The engine RPM picks up substancially. Drivability improves also, but not perfect. Thing is when the vehicle is in a gear, and the clutch dis-engaged, the ground the circuit is open as per the wire diagram and test chart. My guess is the ground is then supplied through the trottle position switch which grounds at 1300 RPM telling the ECM we're under motion. No matter what I never get a voltage change at the O-2 sensor which tells me it never makes closed loop. I think I have a combination of little problems. The water temp sensor seems out of range also, so I spent the $16.99 today and will replace it later. The new one has completely different ohm reading than the one in the vehicle. I'm running out of options and if this water temp sensor is the fix, I'll be pretty happy. more on this later.

[neverhappen]

FYI..... bought an OEM manual off of Ebay (lucky me) and did every test in the book. The ECM doesn't respond, and as I expected the little LED light is suppose to flash when in closed loop. Still could not assume that even without the ECM this thing could run this bad and be so unpredictable. Decided to spent the big bucks and went to the locale Import Parts dealer to buy a new rebuilt carb. All they had in stock was the "Canadian" carb which didn't incorporate the ECM system in 1984 or 85. Bought it, Installed it, plugged the choke heater and decel valve into the proper places and bid-a-bang, runs like a new truck. I don't recommend this to everyone, but at the cost of a rebuilt carb $350.00 and the cost of the ECM, $668.00 (dealer only) I figured since all I have to do is pass emissions in 2 years, this was the most efficent fix.

[EllRift]

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