Choke stuck wide open, causing starting issues when cold

[rambo]

Hey all!

 

I have had recent issue with a 720 during cold starts, outside of that runs perfectly once warm, and it will always start but not without attempting to crank several times.

 

 

Looked at fuel delivery since the pump was exhibiting malfunctioning behavior i.e. got increasingly louder over the last few weeks, and eventually stayed on and ran continuously until power was cut off. I replaced the pump and filter (placed the filter after the pump fram g3 type, adding this for context as I recently got delivery of different filter similar to what was on before that i will install pre-pump).

 

Now, after i did this, pump behavior went back to normal, carburetor site bowl at half full. But now i started hearing a buzzing sound underneath passenger dash, i removed the plate holding the relays, and by the look of it, its the fuel pump relay buzzing, not all the time, but frequent enough. I cleaned the chassis and block ground terminals, and battery posts at this point. Buzzing went away, but after 24 hrs, its back. Now interestingly, when i pull the choke relay, the fuel pump relay never buzzes, tried a bunch of times, soon as i plug the choke relay it starts buzzing again, same thing when i tested with another choke relay.

 

During all this, I also noticed that the choke is wide open all the time, even when cold, pressing the pedal does not close the choke plate, curious what would cause this? Furthermore, looks like the blue wire at the back of the choke heater got slightly burned at some point, I tried to pull it but it seems stuck on there and didn't want to break anything.

 

Long story short, trying to figure out why the choke wont close, and eventually figure out the buzzing issue of the fuel pump relay and/or whether any of the above context provided are related.

 

Thanks!

 

[datzenmike]

Hold throttle at least half open and manually try to close the choke flap. Don't force it. What happens???

 

 

The auto choke relay has often been associated with the alternator beginning to fail. When buzzing it's not sending full power to the choke heater.... sort of an on off square wave. Most apparent at idle and low speeds. Not suggesting you go out and replace. Swap your other alternator in and see if it goes away.

[rambo]

Thanks for quick response Mike, I have been reading a bunch of your posts since i started picking up nissan 720s last year!

 

During my test I couldn't even manually close the choke plate. It was binding against another moving part on the arm(i forget now, but it was binding/ stuck to another arm of some sort that possibly coming from the choke heater?). Nonetheless, once I managed to unbind it, I was able to manually close choke, however it wasnt spring back to closed position, it would stay where you moved it (engine cold). At this point, I decided to pull the choke heater apart to re-seat heating element, ensuring its capturing the rotating choke plate arm in closed position (engine cold). Fired up immediately as it used to. 

 

I have confirmed 14.3 charge from alternator, and I'm getting about 13v to the choke heater. I was also able to see the choke slowly open while running. I am letting it cool down right now to see what position the choke plate will be in, but after dislodging and resetting the choke heater, im able to easily push the choke open, and it quickly springs back to MOSTLY closed, not fully.

 

Now, I read on other posts that when you step on the accelerator in the off position, it should force the choke plate back to closed position, and this functionality is not working. I tried to visually identify the relationship between throttle movement and choke plate arm, but I cant possibly see how  throttle movement would move the rotating arm into closed position, is this because it might be an aftermarket carb ? i.e. for this to work, I was expecting to see a spring or any moving part attached to the choke arm that would force it into closed position throttle / pedal is depressed. Its an autoline c273 carb.

 

I wonder if its worth noting here that since dislodging the choke from the stuck open, the fuel pump is no longer buzzing again ( i will continue to monitor).

 

[rambo]

Good news, my choke returned to fully closed after dealing with issues. I let the engine cool after the below changes and found that my choke plate returned to closed position once engine was closed, which it wasnt doing before.

 

Nutshell:

 

- Choke was permanently stuck open cold or warm engine.  Arm circled RED that rotates the choke plate from open to close was stuck behind arm in BLUE that i dont know the purpose of yet, at the elbow (imagine both arms as a V shaped, binding at where the two diagonal lines meet of both arms, hoping someone technical can explain better). I made sure that the RED was not stuck and able to meet freely i.e. RED was un-tightened by loosening screw and slightly moving towards the choke heater (thermostat) and then re-tightened once there was sufficient gap (maybe an extra 2mm or so), thereby providing enough space so that RED and BLUE wouldn't bind/stuck (please note, i did this just by visually inspecting mechanical movement, i do not know the implications yet of what i did yet, outside of making sure the plate can rotate freely without getting stuck/bound).

 

- Another thing that i made sure of is that the heating element of the choke heater/thermostat was fixed into position that ensured that the element held the choke arm at fixed position (watch any video on youtube regarding choke heater installation to understand this better). I had to unscrew the choke heater from the carburetor to do this.

 

- made sure my grounds were clean i.e. chassis, block and battery terminals. Confirmed that alternator was working and that power was getting to the choke heater. 

 

I'm hoping someone here can chime in with explaining this better than I did, and highlighting if i made any incorrect assumptions with my fix.

 

 

 

 

[datzenmike]

The arm going down is connected to the fast idle cam. Then the choke closes that arm falls of it's own weight and the fast idle cam drops into position blocking the throttle from fully closing and the idle runs higher letting in more air for the richer mixture provided by the choke itself. As the choke slowly returns to full open that linkage is lifted upwards and the cam rotates through stepped positions slowly decreasing the idle till it is fully out of the way.

 

Once the choke has been opened and then allowed to cool it's can't close and set the fast idle unless you first step on the gas to 'set it'. The cam can only drop into place it the throttle is first opened slightly to let it drop into place. On a cold engine the choke may appear partly closed but as soon as you step on the throttle it snaps closed.

 

 

The choke relay has only one function. It is only on if the engine is running and here's why. Assume it is a cold morning and you start your truck normally and the choke heater is warming the choke in preparation of eventually turning it off as the engine warms. Now imagine you go back in the house and keep warm and finish your coffee as the truck war,s up, but for some reason the truck stalls. Now without the relay the heater would continue to heat the choke and turn it off. You come out and the engine is dead cold and without the choke (now open) restarting is very difficult. With the relay the heater is turned off with the engine and the choke does not continue to open. This is my theory of operation.

[rambo]

Thanks for that explanation! I may have moved that fast idle cam when i was trying to figure things out, is it supposed to be set a certain way/direction or does it set itself even if id moved it?  Nonetheless, we had our first frost this morning woke up to 1°C, the 720 fired up perfectly, choke was closed, and i could hear it slowly go from fast idle to normal operation.

[datzenmike]

You can bend that downward linkage to change the fast idle cam slightly. There is a set screw under it, but you can't get at it with the carburetor on the manifold. It's set at the factory and doesn't need to be adjusted. Fast idle should be about 1,800 - 2,200 RPMs with choke full on. If it revs up with choke on... good enough.

 

It will run perfectly fine without the choke relay, if you wire a switched power source to it. You just won't have the heater shut off if it stalls. 

[DwayneOxford]

I figger whoever rebuilt the carb didn't check for binding linkage.

[rambo]

So unfortunately, my choke got stuck open again... so after a second look, i noticed that the plate is getting stuck behind this notch. Plate opens up enough that with the little bit of left and right play on the arm, it just locks behind the notch... thereby, telling me that my above root cause was incorrect. The notch looks like its been eaten away somewhat... any ideas? Is the minor left and right play on the arm normal ?( we are talking maybe 1mm play) But that 1mm, if that, gets the plate stuck behind that notch, ultimately leaving it wide open.

 

[DwayneOxford]

I'd take it off and emery cloth it good side to side to smooth out the notch till it don't hang. The side play shouldn't be a prob.

[datzenmike]

 

The shaft that the butterfly is on has side to side play. I don't know if that can be reduced but you might loosen the screws holding the butterfly and see if it will shift over slightly right. If not, mark where contacting and Dremel the carb body for clearance.

[rambo]

Just to update, I put the dremel to the notch and this fixed things.