Tinker

So, after the 3rd episode of dead battery blues (yes, my fault for leaving the headlight switch on...), I decided to wire the headlights to come on with the key. Not a difficult mod, just requires a little hacking, soldering, and screwing 🙂.

Looking at the fuse panel on my 78 620, all the fuses on the right are hot while the fuses on the left are tied to the ignition. 

1. Remove fuse panel and disconnect wiring harnesses. Remove the fuses too. Note the view from the backside means the hot fuses are now on the left.

2. Remove the HOT bus from the plastic housing with needle nose pliers. Just squeeze each fuse terminal and it pops right out. Use your preferred cutting tool (dremel for me) to hack the bus above the bottom fuse location.

3. Solder a 12V relay in place. I used this one from Radioshack for $5. Solder the relay outputs to the buss pieces that were cut before reinserting into the housing. Otherwise you'll melt the plastic housing 🙂. This is pins 87 and 30 for this relay - polarity doesn't matter. I used black for the output wires, and white for the control wires. Reinstall buss in housing. It should look like below

4. Screw the fuse panel back into the truck and connect the control wires (pins 85 and 86). Again, polarity doesn't matter. One end goes to ground, the other to a switched 12V source. I screwed the GND wire to the mounting bracket for the fuse panel as shown below:

5. Connect the second lead to a switched 12V source. I spliced into the radio wire because it was easy to find. It's the blue one  just to the right of the image above. 

6. Add a cable tie to tidy things up, and you're done. 

Voila, now the headlights only come on when the key is on. No more dead batteries!

I leave my headlight switch on all the time now. Might as well have them on during the day for visibility. For those concerned about the additional current draw when starting, the radio wire is actually off when the key is in the "start" position, so the headlights aren't drawing juice when the starter is going. The turn on as soon as the key springs back to "run". 

Why use a relay instead of simply running a wire from the switched (left) side of the fuse panel to the headlight/parking bus you ask? This would put more load on the wire powering the switched side of the fuse panel, and I was worried about the wire being too small to handle the additional current. Using a relay means the current for the lights is still on the original power wire, so no worries about frying any wires.

Thanks for all the help, and sorry for the delay in updating.

I rebuilt the carb last weekend. Lots of crud on the bottom of the secondary valve. I also went through and capped all the lines that weren't used anymore (anti-backfire, thermal switches on air cleaner, etc.). I had already removed the air pump because it wasn't working, so all the air injection stuff is now a paperweight.

Result:

The truck runs better and seems to have a little more oomph to it. It does idle well - but I still can't do the "typical" idle adjustment of turning the mixture screw in/out until it has the highest revs. With the throttle screw backed out completely (throttle closed), the idle speed simply keeps increasing as I turn the mixture screw out. That said, it does idle well, I just have an interesting time setting the idle. I can either have the throttle screw completely closed and use the mixture screw to attain ~800rpm, OR I can open the throttle a little, then lean the mixture to attain ~800rpm. Both cases seem to idle the same. Maybe it's time to just drive it...

Here are things I noticed during the rebuild:

- The choke pull-off was leaking vacuum and not actuating. Ordered a new one and will install this weekend. Covering the vacuum leak with a finger didn't change the idle.

- The throttle shaft is very loose where it enters the carb. I can wiggle it quite a bit. As I understand, there is nothing to do about this except save for a new carb. This one has 40years and 250,000 miles on it.

- I'd like to replace the secondary diaphragm, but can't find one. Anyone know where to get a secondary diaphragm?

- I set the fast idle per the carb kit instructions, and I think it is much too high now. What is the fast idle speed supposed to be?

Again, thanks for all the help!

Great, I'll try that. Didn't rev it after turning the screw before. I'll check the fuel level as suggested too.

Thanks

Just to confirm - I don't see a BCDD solenoid on this engine. The Autozone repair guide indicated there is not one on the 78 year - is that correct? https://www.autozone.com/repairguides/Nissan-Pick-ups-and-Pathfinder-1970-1988/EMISSION-CONTROLS/Boost-Control-Deceleration-Device-BCDD/_/P-0900c1528004f465

I tried turning the BCDD screw 2 turns CCW - no change in idle

I then went back to original, and went 2 turns CW - The idle got very rough. So, I turned it back to the original position.

I measured the timing at 11-12° (at ~900rmp), so it's close.

Choke works - visibly full open after warm up

The 2nd barrel looks closed. I tried to gently press it with a screwdriver to see if it would open or close any more - and didn't see see it move.

I also confirmed the fast idle cam is not engaged - when warm it's rotated past the set screw as it should be.

I'll check the vacuum advance line.

One more thing: when playing with the carb yesterday without the engine running, I noticed that fuel comes out from the throttle rod when I move the throttle by hand. I don't think it happens when the motor is running because it gets sucked in. But, when I move the throttle not running it gets wet where the rod goes into the carb.

Is there another way air could be getting in to mix with the idle fuel? BCDD system? AB? Other spot?

No $$ currently for a new carb - are there any red flags to look for before I rebuild this one?

Just got my first Datsun, a 1978 KC. It runs pretty well considering its age and being a farm truck for 40 years. 

Going through the tune up procedures, I've been unable to get the idle down to where it should be. Here are the symptoms: I have the throttle adjusting screw out far enough it's not engaging; when I adjust the idle mixture screw out I never get to a point where the idle slows down (indicating a mixture too rich). In fact, the RPM increases a lot in conjunction with backing out the mixture screw. The ONLY way I've been able to make the idle drop from ~1500rpm (initial) to ~900rpm (current) was to turn the mixture screw in until the idle speed went down. I'm at a loss for what else to try at this point. Here's what I've gone through:

- Confirmed the throttle screw is not touching - throttle valve is completely closed as far as I can see/feel. I can feel it close and hit the stop (either inside or the one on the outside)

- Disconnected the cable from the pedal to confirm that was not holding it open

- Disconnected the Air Pump belt (which eliminated the backfiring ? ). This was unrelated.

- Turned the BCDD adjuster out 2 full turns to confirm it was not stuck on at idle. No change. Returned to original position

- Checked AB valve, which does not appear to be working. I can't feel any vacuum from it when revving the engine and snapping the throttle shut.

- All vacuum hoses I can see appear to be in good shape and connected. 

It seems like there is air getting into the intake somewhere which is defeating the throttle being closed. I assume that's why I can't make the RPM reduce by turning the mixture screw out.

Any ideas?