Key on, engine off, something near the carb is ticking like a time bomb

[Telkwa]

Anyone know what makes that ticking noise?  Our '86 720's done it since I can remember.  This is with the key in the "run" position, but the engine off.

 

The ticking is fairly quiet.  When I try to google it I keep getting Ratsun and other hits but they're always about engine noise when running or testing the idle-cut solenoid.

 

I always thought it had something to do with the electric fuel pump.  Maybe some sort of relief valve.  Now I'm wonderin' if it's something else.

[TENDRIL]

is it a electronic tick?..

 

or a mech. tick? 

[Telkwa]

It sounds like a mechanical ticking.  Ticketyticketytickety as soon as the key's turned to 'run'.  I figured it's something all these old Datuns do but maybe not?

[MitchellGB]

ive got an 85 and it does the same thing, i have no clue though. i just bought it last week

[datzenmike]

Could be the electric fuel pump.... but the sound would be coming from in front of the right rear tire.

[Telkwa]

Well, yeah, mike, that's what I thought.  Our truck has a dump box on it so I can run the box up and have complete access to the fuel pump.  The ticking is definitely at the carb, not the fuel pump.

[Telkwa]

Well, thanks to you guys I've been learning about our old '86 720 with its feedback carb. 

 

The time bomb ticking is definitely the air/fuel ratio solenoid.  I removed the the white wire from the round plug (post #13 of this thread).  Put the round plug back together.  Turned the key to "run".  No tickeytickeytickey.  Went ahead and started the truck.  It seemed to run OK, didn't seem worse than before anyway.  I peeked into the ECU's peephole.  The green LED was not on. 

 

Turned the truck back off.  Reinserted the white wire back into the round plug.  Turned the key to "run".  Tickeytickeytickey.  Started the truck.  It idled OK, about the same as with the fuel/air ratio solenoid de-energized.  The ECU's green LED was blinking.  The blinking wasn't rhythmic.  It was kind of random.  It was on roughly as much as it was off, but sometimes it was on longer, sometimes off longer.

 

So I'm left with questions: is the ticking normal?  Is the air/fuel solenoid supposed to tick tick tick with the key turned to "run" but the engine not running?  If not, does that mean that the air/fuel solenoid in the Hitachi carb is broken?  If so, can the solenoid be replaced?

[kickstart]

My 85 does the same thing! It runs good ,gets good mileage so i figured it not broke so dont fix

[Telkwa]

Well, I'm glad to hear that somebody has the same symptom and it doesn't seem to be hurting anything!

[wivrrat]

Mine ticks without the key turned on.  Continuous ticking coming from the carb.  Does sound like a fuel pump but I know otherwise.  When I have the ticking, I have idle problems.  See my post, idle cut solenoid thread. 

[Telkwa]

I didn't realize you meant that the carb was ticking with the key off.  That's different!

 

During my troubleshooting, I'd already disabled our idle cut solenoid, and still had the ticking.  I verified by pulling wires that in our case the ticking was coming from the fuel/air solenoid. 

 

As far as I know the idle cut solenoid could make a similar sound if it was receiving a continuous on/off signal.  I think the simplest thing for you to do at this point would be to open the round plug and carefully pry back the barbs on either the red wire going to the idle cut, or the white wire going to the fuel/air solenoid, then slip the wire out of the plug.  Put it back together and verify which solenoid is ticking.

 

Do you know how to use a piece of metal as a listening device?  You might be able to verify which is ticking by listening, but I doubt it.  Both are attached to the carb, and they're only a few inches from each other.  The idle cut is on the outside of the carb.  The fuel/air is inside the float bowl area somewhere, so maybe it could be done. 

 

At this point in time I know enough to be dangerous, so please take my advice with a grain of salt.  But from what I've gathered so far the air/fuel solenoid won't stop the truck from running, although it may not run as economically as it should.  A malfunctioning idle cut will definitely ruin your idle.  But once you're off the idle circuit the truck should run OK even with a malfunctioning idle cut solenoid.

 

Solenoids ticking with the key off, choke plate doing crazy things - I wonder if the ECU is going gunny sack.