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Little Trucks, Big World

Hey everybody!  I have a 81' king cab 4x4 with a 32/36 carb on it.  Everything is running just fine and I am very pleased overall.  However, being who I am, I always have to mess with stuff. 


I am tired of the way my truck acts when the weather is cold.  It starts fine, but since weber only has ONE step on their fast idle cam, when I hit the gas and the choke opens the engine rpms are so low that sometimes it dies.  There is about a 300 - 350 rpm difference in the idle when it is cold and when it is warm.  I've tried turning the idle up a little (900 rpm when warm) but I tend to get run-on when I cut it off.  I was wondering if anybody has installed some kind of idle switch to compensate.  I know that my old ford had a switch on it that would push the throttle a little when the AC was cut on.  If I could wire a switch up like that, I could just turn that on when the engine is cold.  Any thoughts?  Ideas?





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cool, I didn't know you could add those idle cut off solenoids.  I was thinking this whole time that they were on a different model carb.  I'm definitely getting one of those.  I think that will satisfy me for now because the run on is a little embarrassing sometimes.



I had the same problem, I was trying to deal with too-low idle running headlights or A/C.  When I set idle to 900ish I'd get run-on if the engine was warm.  It blew some of the vacuum caps off the IM so I was about set to buy a solenoid, but did some more research about run-on in general.  Seems it is often caused by carbon buildup.  At 177K and the DPO driving with a stuck-open thermostat and half-working carb, I felt this could be my problem :-)

So, I ran Sea Foam through the engine this morning.  I don't believe, as some seem to, that Sea Foam will cure whatever ails ya, but I bought a can for this particular issue.  And with idle set to 1000 now, no run-on at all the last three trips.  Engine was quite warm.  YMMV, solenoid is probably a better solution, if the issue resurfaces I'll go that route myself...but I like the simplicity with no solenoid.  And the smokey drive is kinda fun!

I would change the fast idle step for something higher revving. This way you don't have to run a higher idle to begin with. Fast idle is usually 1,800 to 2,300 rpms.

Got the switch, still have occasional run on.  I slightly bumped the timing down and that didn't work.  I even tried tuning the idle circuit again.  Nothing seems to get rid of it.  My idle is set at 850-900rpm.  If the switch was defective it wouldn't run at all would it?

So, you have the idle cut-off solenoid wired so that when you turn off the motor, there's no voltage to it correct?  This switch fails closed, needs 12V to turn it on.  So, it should have 12v both with the ignition on, and while the engine is running.  You really shouldn't get any run-on if it's wired up correctly.    Also, from my experience "best lean idle" is too lean, and can cause run on on these engines.  You can try fattening up the idle mix just a bit and see if your run on problem goes away...

It appears to me in your photo, that you have the fuel cut-off solenoid wired to the same power as the auto-choke heater, and the auto choke heater is only powered on while you're cranking the motor, before the alternator starts generating voltage.  Once the alternators starts putting out voltage, the auto-choke relay turns off the auto-choke heater.  So, if you have the cut-off solenoid wired to that relay, your solenoid is cutting off fuel while the engine is still running...

oops, sorry, saw that wasn't your photo, but you should check with a voltmeter to make sure when you do and don't have voltage to the cut-off solenoid...

It may very well be, but then your choke heater must be on all the time.  It's not a big deal, the choke will open faster than it should, and it will fatigue more quickly if it's heated continuously, but it will work just fine.

Is there something I should change?

I wired mine up with two connections...red wire that is hot with the key in the "on" position to idle cut solenoid (which I did wind up installing) and the blue wire controlled by the choke relay to the choke.  My truck is missing that plastic connector piece so I can't really compare to yours, I had to go back with a multimeter.

My run-on came back big time when I retuned to "best lean" idle.  The flat spot went away though, and the solenoid fixed the run-on.

Well, I hesitate to tell you to change something if you're happy with the way it works now lol... But, if the original carb had both auto-choke and anti-dieseling connections, it would be best to hook each one up to those original wires.  That way your choke heater would only be on while you're cranking the engine, and the anti-dieseling solenoid would be on with the engine.

The thing is if you're heating that bi-metal choke spring all year long, winter and summer, eventually it's going to fatigue.  It's not hard to replace, or that expensive, but it just would be needless to have it heated all the time.

If it's working fine, I wouldn't worry too much about it, but if you get a chance and have a voltmeter handy, just check to see if you have 12V to the choke heater when the engine is running.  If so, you might want to change it sometime just to make it last longer...

Sorry again, that was supposed to be a reply to Thomas lol, I'm all F'd up today...

ag, I've found the same, best lean idle is too lean and causes pre-ignition and results in run-on, but the cut-off solved it in my case as well.  On a carb without the cut-off, I just fatten up the idle mixture so the throttle plate can close further at the same idle speed, and that usually solves it too.

I'm not getting audible pinging, but the plugs are maybe a little too clean.  When I pulled the main jets out they were leaner than I expected in the first place, 120 IIRC.  The run-on is indisputable however.  Before I retuned I had the mixture screw out way more than I do now, maybe I need to back it out just a bit and see how it drives.

But unlike Jared, my run-on vanished completely with the solenoid.  :-(


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