Please help me get my truck running right

[mas84_720]

Hello, I recently bought an 84 720 and have been fighting for a couple weeks to get it running. It has a brand new head and rebuilt motor, new distributor, plugs, and wires, and new alternator when i bought it. It would not idle at all though, I was told that it was the original carb and needed a rebuild. Heres where i started...

I replaced every vacuum line and connector on the truck and put a new air filter on. No change. I then completely rebuilt the carb (it didnt look that dirty for being 30 years old and 180000+ miles on it but did it anyway). No change. I then took everything vacuum and emission control related off (i left the vacuum advance to the dizzy on) and picked up a new air filter assembly with adapter. That got it running, but very rough. HORRIBLE gas mileage (like 10-14mpg), no power under load (can barely make it to 65 after a 2 mile straight, and if I see a hill im screwed). Checked cat, its got a straight pipe through it. Replaced fuel filter. Checked timing. Did compression check, 180-185psi on all 4.

Heres my new discovery...
I have a 6 pin wire harness on the carb like in this picture.


But the wire harness on the truck is a 3 pin like you can see (barely) in this picture.


Am I screwed or can i make this work? Is this still required to be plugged in without all the emission control stuff (I am assuming yes because my next thought of what is bad was the air/fuel mixture solenoid, which is attached to the wire harness). How should this be wired or what can i do to fix this. Do i need a new carb?

[mas84_720]

Here is a picture of the wire harness on my truck.

[Telkwa]

I don't know a lot about this but can at least get you started. Follow the wiring back on the truck. If you see no sign of wires being cut, then it's safe to say that your truck is a non-feedback carb truck and that is NOT the original carb. A carb with that big round plug like you have is a feedback carb. The feedback carbs are newer than the non-feedback. The wires on a feedback carb are for choke heater, air/fuel solenoid, a little throttle position sensor, and idle cut solenoid.

Here's a link to a thread I started, asking basic questions about our carb. It might help you.

http://community.ratsun.net/topic/52789 ... le-switch/

I don't know what happens when you have a feedback (FB) carb on a non-FB truck. There are people that do, so maybe someone will pipe up.

So here are some guesses. My guess is that your ECU (the little computer under the seat) is looking for signals that aren't arriving. datzenmike mentioned in my thread that if the ECU doesn't see the signal from the 02 sensor it defaults to a run-rich "limp home" mode. Maybe your ECU is doing the same thing? I'm trying to think of non-destructive testing you can do. Maybe the first step would be to figure out what goes on in the old style 3 pin plug like you have. My guess is that it's just the idle-cut solenoid and the choke heater. Let's say that it is, just for sake of argument. You could jumper those connections (temporarily connect the correct wires from the carb to the wires coming from the truck) and see if the truck runs.

Without more knowledge on this subject, I'm tempted to say that getting the proper carb would be the best solution.

[mas84_720]

OK, did some more research. Apparently 84 had 2 wires for the carb (like my truck) and in 85 and newer they went to 6 wires (like my carb), so not original carb from what i have found. So my question is does anyone have a wire diagram for a newer one with 6 wires? I even bought a Haynes manual and it only has the "typical" 2 wire. Am I going to be able to make this work? Do i need a new ECU or new Carb?

[Telkwa]

Does your truck have a metal pipe coming off the exhaust manifold that runs over to the air cleaner?

Does it have an ECU under the driver's seat?

Does it have an 02 sensor on the exhaust manifold?

Those are some clues as to whether the truck was feedback or non-feedback. From this thread.

http://720world.com/forum/topics/z24-ba ... 1#comments

There's a picture of the wiring diagram in the following thread - I believe the photos I took of our truck's wiring matches the diagram

http://community.ratsun.net/topic/48285 ... -problems/

I think you have an ECU under the seat, but I'm pretty sure it's much simpler than the later ECU's. If your ECU and associated wiring is non-feedback, it isn't capable of running the air/fuel solenoid on your feedback carb. If that air/fuel solenoid on your feedback carb doesn't get a signal, it's setting in it's default position (open or closed, I don't know which) and that is not what it's supposed to do.

Also guessing your truck doesn't have an O2 sensor, and your ECU isn't looking for that signal. In a feedback truck, the ECU is looking for a signal from the O2 sensor, and it uses that info to tweak the air/fuel solenoid. So your carb is waiting for a signal that never comes.

I've googled around a bit, expecting to find some stories from people who are in the same predicament as you. Haven't had much luck yet.

[mas84_720]

I definitely have a non-feedback truck and a feedback carb... with that being said i have found a couple posts on various forums about this situation. Here are the choices I have found; 1: get a new ECU with new wiring harness, and new exhaust manifold with O2 sensor. (not gonna happen) 2: get a new carb. So i ordered a new Weber, should be here in 2-3 days.

[mas84_720]

OK so i got the Weber carb and installed it last night. Still a couple small issues, main thing is electical problem. I hooked up the electric choke to the fuel cutoff solenoid wire, the problem is it started right up but was idleing weird so i did trouble shooting... there was no power to the wire. Why did it start?? It even says to check the fuel cutoff solenoid remove the wire and the engine should die. Doesnt this need power to work??

Well anyway, I traced the wire back and spliced it into a portion of the same wire that has power... still no difference whatsoever. I can sit there with engine running and attach and remove the wire and absolutely no change, no click, no nothing.

I messed around with some things and got the idle fixed by adjusting the throttle cable and timing. It runs perfect when warm, but was stumbling this morning when cold (choke prob not working??). It also seems to be running a bit rich still, though not as bad as before. Did i get a carb with a defective electric choke, fuel cutoff solenoid??

I plan on actually getting a timing light and dialing that in exactly and doing some more trouble shooting / tuning this weekend but any ideas on what to check or look for would be nice. Thanks.