Aisan Decel Fuel Cut Circuit (1 Viewer)

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The 12V routes through the 10A Engine fuse. When desmogged, I believe the only thing running off this circuit is the ICS. With the fuse pulled, the vehicle will still run but the idle circuit is disrupted. I can keep mine running by feathering the throttle and using the accelerator pump to supply fuel. Once RPMs are around 2000, it smooths out as the primary circuit is taking over.
 
If you are tuned fairly well, an easy opcheck of the system is as follows. While sitting in Neutral, bring RPMs up to about 2200 steady, slowly reduce RPMs down and around 1800 the motor should cut out a little, dropping your RPMs and then catching and running smooth again.
...and/or...at idle, disconnect the connector and it should sputter and die.
 
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If you are desmogged, think about this when troubleshooting this system. The 10Amp engine fuse supplies the 12VDC for the emissions valves and such that were probably removed and the connectors may be lopped off. If one of those 12V wires is shorting, it is going to effect your 12V going to the solenoid. That and as we have all seen when troubleshooting electrical...proper grounds.
 
One thing I found while testing the Vacuum switch..if you push the vacuum line onto the port too far, you get a vacuum leak. You can see in the pic that there is a slot that allows air to leak when pushed too far on.
Something that's easy and quick to check.

IMG_7295.JPG


IMG_7294.JPG
 
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Lately I have been putting a 't' in various vacuum systems with a line going to a gauge in the cab and logging vacuum readings at different driving conditions.

I wanted to see if the FCS closed during times other than on decel and at shutoff. The vacuum is manifold equivalent at idle. Under all acceleration types, I can't get the vacuum above 5" no matter how light or heavy I am with the throttle. Makes sense if you look at where the port is in relation to the butterfly. In 1st and 2nd gear cruising at 1500 RPMs, I can get the vacuum up to a level where it would trigger the FCS ..all other gears and RPMs the vacuum is too low.

The only time I see where it would close besides decel and shutoff is during a shift, just that brief moment while your foot is off the gas and RPMs are still above 1800.

So....
If you want to remove the FCS system entirely from the equation for testing purposes, you can remove the FCS from the carb and replace it with a 8mmx12mm fine thread 1.0 pitch bolt, use the washer from the FCS.

You'll have to stall it out when you shut it off and your idle will be rich and rough for a few moments after decel but at least you will know if there's a problem with the FCS system components or not...and you don't have to hack up your wiring.

IMG_7214.JPG
 
Great diagnostic videos! Thanks very much for posting !

Now, the real test would be with an Air/Fuel meter in conjunction with what you've done above....

I happen to have an A/F meter.... :idea:

How did you hook up the noid light ? (you may have said in 1st vijayo but I didn't catch...)
 
FWIW, there's also a crude VSS (speed sensor) in the speedo that inputs to the 'Computer' so that's why the ICS does not cut out until you're above 35 mph even though you're above 1800 rpm earlier. At around 1.09 in 1st vid.

Also, I had done some crude testing of the decel circuit with A/F meter and with all hooked up, mix goes way lean on decel, without goes slightly rich. So people who desmog have nothing really to worry about other than spewing a few more Al Gore chokers into the air, providing engine is otherwise tuned and running well.

Great tech thread.... in duh FAQ ?

@Mace @Rigger

image-jpeg.1398577
 
With the green connector disconnected (FCS) closed, it was very rough and surged/stumbled/surged/stumbled through the RPMs till it got to 2000. Over 2000 RPMs it smoothed out.
 
Can a failing or not perfectly running FCS cause deceleration back fires? It's one a annoyance I would like to fix on my de-smogged '85. When letting go of the gas (from highway or near highway speeds), I sometimes get a backfire out the exhaust. Of course it's not consistent enough for me to spot a pattern, so I figured it may be a component that is sometimes acting a bit wonky - Jim C rebuilt carb and re-curved dizzyy, HAC deleted (primary vac advance port is not working though - won't hold a vacuum), I'm at proper timing and believe my idle mixture is correct - I have 20+ inches of vac at idle (near sea level). Any insight would be appreciated - thanks...
 
If the throttle is open just a little more than idle, like if you are slowly reducing speed with your foot barely on the throttle, that can cause some fuel to buildup and pop out the exhaust (not enough vacuum on the decel diaghragm to cause it to go open). If I have my choke pulled a little after it's warmed up and I decelerate, I get some good popping. If you're getting it with throttle fully closed to an idle setting, a check would be to put a meter and mighty vac on the diaghragm and make sure it goes open at 14 in of vacuum applied, and that it holds vacuum.
 
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chichiflys said:
Can a failing or not perfectly running FCS cause deceleration back fires? It's one a annoyance I would like to fix on my de-smogged '85. When letting go of the gas (from highway or near highway speeds), I sometimes get a backfire out the exhaust. Of course it's not consistent enough for me to spot a pattern, so I figured it may be a component that is sometimes acting a bit wonky - Jim C rebuilt carb and re-curved dizzyy, HAC deleted (primary vac advance port is not working though - won't hold a vacuum), I'm at proper timing and believe my idle mixture is correct - I have 20+ inches of vac at idle (near sea level). Any insight would be appreciated - thanks...
Click to expand...

Yes... that is the main purpose of the FCS
 
If your Emissions computer is removed or failing, or the Vacuum switch for the ICS is removed, or the ICS is grounded to the carb, then it won't make any difference. Backfire is from unburned fuel heading down the exhaust and igniting. ICS helps lean the mix on decel. Decel operation of the ICS is defeated by any of the above modifications.

Installing an Air/Fuel meter really helps in dialing in the carb.
 
Ok copy all folks. I will do some testing of that circuit/vacuum. Any suggestions for a good Air/Fuel meter?

I also have a new Troll Holl carb I was thinking of installing and rebuilding my OEM carb for backup/$hits and giggles....but I'll test the ICS first before going that drastic yet.
 
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