1985 720 King Cab 4X4 Carb/tune issues

[spddm0n]

 

I just purchased this awesome truck, but boy has it been a bear to get running correctly.  The previous owner could not get it to pass emissions here in CO.

 

I found the wrong carburetor on it.  I waited a while and found the correct carburetor on a vehicle in a salvage yard.  It had a bad air fuel ratio solenoid on it, but I was able to get a company in Florida to rebuild the solenoid.

 

I purchased a carb rebuild kit and starting cleaning and assembling.  I had to use the dashpot from the carburetor the truck originally came with.  I still think it's not working correctly though, and adjustments are not working.  It still takes a long time for the engine to idle down, and I have the bleed screw at the top all the way out.

 

I also have a problem with the auto-choke.  It is not seeing any voltage from the relay.  The relay is doing something really weird too.  It's BUZZING constantly.  I've never seen that before.

 

I believe the throttle valve switch and the anti-dieseling solenoids are working correctly on the carb (wow there is a lot of electrical devices on the carb!).

 

I think I found another issue...the AB valve may not be working.  The carb had been adjusted in the past, such that I think it was compensating for the constant flow-through of the AB valve passing air from the air cleaner to the manifold.  I had to plug off the small vacuum line to the AB valve to keep it running.  I may need to remove the whole valve and plug the lines for further testing of other issues.

 

Lastly, this truck appears to have the altitude compensation equipment on it.  I have an altitude "switch" under the dash on the passenger side, near that auto-choke relay.  I have the altitude "relay" on the passenger-side inner fender apron, and I have the altitude "solenoid" mounted to the apron next to the altitude relay.  I could not find any reference on how to correctly run the two vacuum lines from the solenoid, but I found one reference suggesting they go to the two nipples on hanging off the top section of the carb on the driver side.  I didn't have these nipple on my other carb.

 

I hooked the vacuum lines up two the two nipples, in no particular order, and it would not run.  I switched them around and it ran better.  However, shortly after, the engine started to hesitate and stumble, so I disconnected the lines from the altitude solenoid, and found gasoline in the line!  How is this possible?

 

When I bypass the AB valve, there appears to not be any more fuel in the line to the altitude solenoid.

 

I made sure the timing was correct (I disconnected the vacuum line from the distributor and plugged it).

 

I have some trouble getting the idle steady.  The AB valve is causing some issues of high idle. The idle speed was adjusted as low as it would go, and it would not drop below 1000 RPM. Now that the AB valve is bypassed, I am able to achieve a much lower RPM.

 

I need to check operation of all the EGR equipment too, but I'm, wading through a mess of previous owners not being able to tune this carb.  Wow, it's complicated with crazy emission controls!  Too bad I need emissions here.

 

I'll check the plugs, wires, cap and rotor today too.

 

I was not able to find a service manual that speaks in any detail about the complexity of this carb or the altitude compensation equipment.

 

Any thoughts would be appreciated!!!

 

[datzenmike]

Buzzing auto choke heater relay is common. One of the inputs that keeps the relay on is a charge signal from the alternator. Usually it a badly charging alternator or one with even one bad diode. Not enough for the red light to be fully on. 

 

Gas in the line? Are you sure it's connected to the correct carburetor pipe and not the fuel bowl vent?

 

I highly recommend getting a factory service manual (FSM) produced by Nissan. NOT a Haynes or Chilton's. Look on line at e-Bay. Be sure to get your year. The other books cover all 7 1/2 years of 720 and isn't very precise about your year.  Pys for itself the first 1/2 hour you use it.

 

[spddm0n]

Thanks Mike!  I'll look for the FSM.

 

I figured out the buzzing relay issue.  Yeah, it was a blown fuse in the fuse block.  I don't have the cover for the fuse block, so I don't know what the fuse is for, but I found a wiring diagram online showing the relay connected to the fuse block (one of the four wires on the relay).  It's the 2nd fuse in (closer to the driver and further from the firewall).  a 10 amp fuse.  Any idea which fuse this is?  I replaced it and no more buzzing. 🙂

 

Yeah, gasoline in one of the two lines connected to the ports on the driver-side of the carb.  I don't even know if I have these lines correct though.  I also can't find any info on the vacuum routing of the lines to the "altitude solenoid".  See image attached.

 

I'm wondering if:

1 - I have a crack in the carburetor body somewhere?

2 - I did something wrong in the recent carb rebuild (maybe I missed a check-ball or somethimg??).

3 - The bowl is too full (I don't have a sight glass in mine.  It was missing on the carb I found in the salvage yard and the carb that was on the engine when I bought it, is just a metal plate - no glass).

4 - I simply have the lines wrong and this would be normal behavior for how I connected them??

 

Does your '85 manual have a diagram that offers info on the altitude solenoid vacuum line routing?

 

Also, most of the carbs I find pics of don't have these two ports at all.  If they do have them, they are simply capped off.

I did try simply connecting the two ports with a vacuum line, and it seemed to run ok, but I'm concerned something is not correct yet. 🙂

 

Thanks!!

[spddm0n]

Also, I can't seem to find the 1985 manual on ebay, except one place selling it for $179! 🙂  I wonder if the 1986 book would contain the info I need.  I know a lot of these went to TBI in 1986.

[datzenmike]

After April '85 they were Z24i or throttle body injection. I think new they are $75-$80 so don't pay more.

 

#2 fuse is auto choke heater and idle cut solenoid.

I opnly have an '84 FSM. Not much on the high altitude and nothing that looks like yours.

[spddm0n]

Ok, thanks.  I'll keep looking for the 85 manual.  Thanks for the fuse info.  Makes sense now.  I'll have to find a diagram of all the fuses until I can find a cover in the salvage yard. :)

 

I just ran another test...I capped off that lower port, and ran the engine for a couple minutes.  It still filled up with fuel.  Appears to have nothing to do with the altitude solenoid at all.  I wonder if the bowl was too full, if fuel could get to that port.  Or, perhaps the carb body casting is cracked.  I did find some kind of evidence of epoxy or something around the base of the port, upon closer review.  Check this out...

 

[spddm0n]

Here is the image I found of the carburetor having these two "altitude ports" on the carb.

 

[spddm0n]

Turns out...I don't think that port with the epoxy-looking stuff on it is leaking.

 

I also took the carb off and checked the bowl float and re-calibrated it slightly.   I am also getting fuel in the bowl vent line, so I thought it would help.  I put it all back together and it still runs fine until it heats up and fuel makes it's way into the lower of the two ports on the driver-side of the carb.

 

I'm wondering if I might have the wrong gasket between the top and middle section of the carb.  I matched the new gasket with the old one that came off, but the old one was a replacement one from a previous rebuild (it looked as though it had been rebuilt once before anyway).

 

Perhaps, the way the gaskets are design, I could somehow be mixing air channels with fuel channels.  

 

Does anyone know how you would go about ensuring the right gasket is used, without using the old one as a model?

[Charlie69]

Here is a cheaper one;

 

85 720 FSM

[spddm0n]

45 minutes ago, Charlie69 said:

Here is a cheaper one;

 

85 720 FSM

Thanks!!  After filtering through what appears to be some creative premium ebay sellers, I managed to find a manual that is "well used" for $25.  

 

I can't wait to get it!

[spddm0n]

I received my FSM and have been reading it for days!  Several things made simpler with it. 🙂  Ironically, it contained the single image I could not find anywhere, indicating the correct routing of the two vacuum lines to/from the altitude solenoid (I'll post it later for reference).  More ironically, once I hooked it up correctly, I learned the solenoid doesn't seem to actuate anyway.  After applying voltage directly to the solenoid, I found it didn't click  - so I think it's dead.  I decided to bypass it for now (I know I'm at altitude - above 4200 feet, as indicated by the FSM), so I simply connected the two hoses together and moved on to see how it ran.  It seems a little better.

 

It appears there is still something that is not functioning correctly.  It seems to run better when it's cold and warming up.  It doesn't idle smoothly at the recommended 800 RPM, and seems to run best when I have the carb mixture screw out a long way.  I can increase idle speed and it will idle just fine, but it still isn't great.  Seems to indicate a vacuum leak somewhere.

 

There doesn't seem to be much throttle response either, and it really lacks in power.  I've never driven one of these before, so I'm wondering if this engine simply has very little power.  I don't know how people put big tires on these trucks. 🙂

 

I've checked the following things...

(I'm wondering if it could be the exhaust gas sensor.  I have not tested that yet).

 

1.  Anti-backfire valve.  I blocked this off previously, since it seemed to be a very large vacuum leak.  I'll start a new thread on the interesting things I found on this.  It actually could be working properly.

2.  Removed and tested the B.P.T. valve.  It seems fine.

3.  Removed and tested the EGR control valve.  It seems fine.

4.  Throttle switch on the carb is working.  I tested the circuit as per the FSM with pins on the ECU harness.

5.  Air/Fuel solenoid in the carb is working.  Clicks consistently while the key is in ON position.

6.  Choke on the carb is working.  Heats up and moves the butterfly valve accordingly.

7. Anti-dieseling solenoid on the carb clicks and appears to work (no dieseling after shut-off).

8.  Coolant temp sensor.  It does NOT return the correct resistance results, as per the FSM.  Could that affect the idling?  Does that temp sensor also control the dash temp gauge, or is there another sensor for that?

9.  I checked all spark plugs, gaps, wires (broke most of them trying to get them all out.  I had to re-crimp the ends.  I've checked resistance and all seem in the appropriate resistance, relative to the length).

10.  Checked for spark on each of the coils.  Seems strong.  

11.  I've traced all the vacuum lines for leaks (all the way to the distributor advance and over to the charcoal canister).  I think I'm good.

 

Things I have NOT checked yet...

1.  Thermal vacuum valve.  I took so many other components off that I can't start and run the engine to warm it up to check this device.  🙂  I also can't easily control the temperature of water, so it might be hard to remove and test.  I hope I can make a reasonable assumption with the engine "cold" and the engine "hot".

2.  Valve lash.  I have not done this.  I have compression checked the engine, and all cylinders are about 157.5-165 psi.  

3.  ECU.  I assume it works, since the various solenoids are working, but I noticed the indicator "lamp" doesn't behave as the FSM states.  In the testing procedure for the Air/Fuel mixture, it suggests the lamp should blink 4 times in 10 seconds.  Mine turns on when I have the key in ON position, and then goes off and never comes back on.  Is this normal?

4.  Exhaust gas sensor.  The factory wiring to this sensor was cut and modified, as a replacement sensor was installed by the PO.  I don't know for sure if it's working.  Is there a way to determine if this sensor is working correctly?

5.  B.C.D.D on the carb.  I have not checked this yet.  The testing procedure is a bit odd, with three levels of vacuum indicated on a graph.  There are no actual values of the vacuum levels on the graph.  I'll read this again and see if I can figure it out one the engine is actually back together and running. 🙂

6.  The two vacuum "switches" on the passenger-side inner fender apron (next to the windshield washer fluid reservoir and altitude solenoid).  I think I read they only actuate at full throttle or something, so I prioritized them for later.

 

I'll keep working to test these additional things.  

 

Any ideas on why the engine would have difficulty with stable idle, poor throttle response, and low power output?  I will go have an emission test performed and see if that indicates anything more (the whole purpose of keeping this complex carburetor in the first place). 🙂

 

[spddm0n]

UPDATE:  

I also just learned by idle compensator in the carb was not secured properly.  The screw was tight, but it tightened down before it was actually securely holding the device into the top of the carb.  The threads were not perfect.  I fixed it and secured it properly (other than I don't know which direction the little metal tabs are supposed to go yet).

 

I'll see if this makes any difference with the idle.