This system has been discussed before, but I want to delve deeper with this thread. Fair warning, it goes a bit out in the weeds...
I'm about to install a CityRacer/Fuji desmog carb (CR/Fuji) on my desmogged (still have Decel Fuel Cut, PCV, Distr Vent, & EVAP) '87 FJ60. Could keep the HIC/HAI systems if I want to. The CR/Fuji carb has only the one vacuum advance port. It's a pretty straight-forward install, except for the Fuel Cut Solenoid (FCS). I'm new to TLCs, but have dealt with stuff like this for decades. Looks to me like the CR/Fuji carb with 1-wire FCS to be installed on my truck cannot correctly perform the Decel Fuel Cut system job intended by the origional OEM 2-wire FCS/carb setup in an OEM FJ60 truck.
The OEM FCS system consists of a Fuel Cut Port, FCS, Vacuum Switch (VS), computer, and associated wiring & plumbing (see Components Diagram). I'd like to keep my truck from popping and banging on high-rpm throttle back-off, but don't see how it can be replicated with a 1-wire FCS setup. It's my understanding that's the only thing the FCS is meant to do--stop popping & banging in the exhaust during high-rpm deceleration (see Operation Table). It has nothing to do with engine idle when functioning normally. Problems with it could affect engine at idle or other revs if the FCS cuts the Slow Circuit when it shouldn't.
First, there is no Fuel Cut Port on a CR/Fuji carb to correctly drive the FJ60 VS. Trying to 'T' off the CR/Fuji Vacuum Advance Port will drive the VS in a way exactly opposite to how it should be driven by an OEM carb Fuel Cut Port. An Advance Port is low vacuum at idle, increasing vacuum as throttle butterfly opens; a Fuel Cut Port is high vacuum at idle, decreasing vacuum as throttle butterfly opens. A direct comparison of these two ports can be made by reviewing the bottom diagram on pg. 3-3 of the really excellent 2F Emissions FSM. The 2-wire FCS is switched on & off by the OEM 60 computer, which needs accurate input information from a correctly-ported VS and Engine RPM to operate nominally.
Second, the fact that the CR/Fuji FCS is a one-wire solenoid (grounded through carb body) cuts off the ability of an FJ60 computer to control the FCS, even if a correct Fuel Cut Port was available for the VS.
In an OEM 2-wire FJ60-controlled FCS system, one wire is switched 12v, the other is ground--but through the computer, then through the vacuum switch to ground (see FJ/BJ 60 Factory Wiring Diagram & 2F Decel System Diagram). This allows the computer to keep the OEM carb Slow Circuit open most of the time, only closing during higher (>1,800) revs, high vacuum deceleration (see Table). I.e., it cures the popping (afterburning) & banging in the exhaust only on the overrun. Nice to have.
So hopefully we can deal with the second problem by swapping in an origional OEM FJ60 2-wire FCS to the CR/Fuji carb, with a regular OEM 2-wire FCS hookup to the truck. If the 2-wire FCS Swap works, that leaves the first problem of correctly porting the VS. Looking for ideas on that from this very capable Forum. Where do you CR/Fuji folks port your VS? To the aircleaner, to the gas filter? Other?
Using the Gas Filter fitting might partially work, as it is higher vacuum at idle, decreasing vacuum as throttle butterfly opens, high vacuum on overrun. Though it probably won't be as sensitive and quick-responding as a proper OEM carb Fuel Cut Port. On the overrun, venting VS to ambient (air cleaner) leaves the Slow Circuit open (VS is always on, opening slow circuit, but with the OEM setup rpm is a factor here too). So the symptom of this scenario would be exhaust banging/popping on overrun? But what would the computer do under a high rpm, no vacuum to VS (remember, VS is now vented to ambient) scenario? Really in the weeds here, beginning to smell smoke....;}
Another proposal is to have the 1-wire FCS connected to switched 12v, which means the CR/Fuji carb will function normally, though you'll have afterburning in the exhaust on overrun. Thinking about it, folks who report having to run their CR/Fuji carb with the choke on a bit all the time would have to do so if their FCS isn't energized (Slow Circuit is closed), which leans the carb a bit through the rev range.
So, my first try will be with the CR/Fuji carb with a 2-wire FCS (if it swaps) hooked up normally, but with VS vacuum line going to Gas Filter manifold fitting (due to lack of a Fuel Cut Port). The CR/Fuji Advance Port (the only port) will be 'T'ed to the distributor advance diaphragm and the EVAP VCV through the existing single BVSV. The remaining emissions systems don't require carb vacuum ports.
Can't experiment until I solve a 2-minute unbelievably, horrendously loud steady whistling sound during initial warmup of my truck. Plus I just work slowly, I'm retired :}. Constructive criticism/error analysis most welcome.
I'm about to install a CityRacer/Fuji desmog carb (CR/Fuji) on my desmogged (still have Decel Fuel Cut, PCV, Distr Vent, & EVAP) '87 FJ60. Could keep the HIC/HAI systems if I want to. The CR/Fuji carb has only the one vacuum advance port. It's a pretty straight-forward install, except for the Fuel Cut Solenoid (FCS). I'm new to TLCs, but have dealt with stuff like this for decades. Looks to me like the CR/Fuji carb with 1-wire FCS to be installed on my truck cannot correctly perform the Decel Fuel Cut system job intended by the origional OEM 2-wire FCS/carb setup in an OEM FJ60 truck.
The OEM FCS system consists of a Fuel Cut Port, FCS, Vacuum Switch (VS), computer, and associated wiring & plumbing (see Components Diagram). I'd like to keep my truck from popping and banging on high-rpm throttle back-off, but don't see how it can be replicated with a 1-wire FCS setup. It's my understanding that's the only thing the FCS is meant to do--stop popping & banging in the exhaust during high-rpm deceleration (see Operation Table). It has nothing to do with engine idle when functioning normally. Problems with it could affect engine at idle or other revs if the FCS cuts the Slow Circuit when it shouldn't.
First, there is no Fuel Cut Port on a CR/Fuji carb to correctly drive the FJ60 VS. Trying to 'T' off the CR/Fuji Vacuum Advance Port will drive the VS in a way exactly opposite to how it should be driven by an OEM carb Fuel Cut Port. An Advance Port is low vacuum at idle, increasing vacuum as throttle butterfly opens; a Fuel Cut Port is high vacuum at idle, decreasing vacuum as throttle butterfly opens. A direct comparison of these two ports can be made by reviewing the bottom diagram on pg. 3-3 of the really excellent 2F Emissions FSM. The 2-wire FCS is switched on & off by the OEM 60 computer, which needs accurate input information from a correctly-ported VS and Engine RPM to operate nominally.
Second, the fact that the CR/Fuji FCS is a one-wire solenoid (grounded through carb body) cuts off the ability of an FJ60 computer to control the FCS, even if a correct Fuel Cut Port was available for the VS.
In an OEM 2-wire FJ60-controlled FCS system, one wire is switched 12v, the other is ground--but through the computer, then through the vacuum switch to ground (see FJ/BJ 60 Factory Wiring Diagram & 2F Decel System Diagram). This allows the computer to keep the OEM carb Slow Circuit open most of the time, only closing during higher (>1,800) revs, high vacuum deceleration (see Table). I.e., it cures the popping (afterburning) & banging in the exhaust only on the overrun. Nice to have.
So hopefully we can deal with the second problem by swapping in an origional OEM FJ60 2-wire FCS to the CR/Fuji carb, with a regular OEM 2-wire FCS hookup to the truck. If the 2-wire FCS Swap works, that leaves the first problem of correctly porting the VS. Looking for ideas on that from this very capable Forum. Where do you CR/Fuji folks port your VS? To the aircleaner, to the gas filter? Other?
Using the Gas Filter fitting might partially work, as it is higher vacuum at idle, decreasing vacuum as throttle butterfly opens, high vacuum on overrun. Though it probably won't be as sensitive and quick-responding as a proper OEM carb Fuel Cut Port. On the overrun, venting VS to ambient (air cleaner) leaves the Slow Circuit open (VS is always on, opening slow circuit, but with the OEM setup rpm is a factor here too). So the symptom of this scenario would be exhaust banging/popping on overrun? But what would the computer do under a high rpm, no vacuum to VS (remember, VS is now vented to ambient) scenario? Really in the weeds here, beginning to smell smoke....;}
Another proposal is to have the 1-wire FCS connected to switched 12v, which means the CR/Fuji carb will function normally, though you'll have afterburning in the exhaust on overrun. Thinking about it, folks who report having to run their CR/Fuji carb with the choke on a bit all the time would have to do so if their FCS isn't energized (Slow Circuit is closed), which leans the carb a bit through the rev range.
So, my first try will be with the CR/Fuji carb with a 2-wire FCS (if it swaps) hooked up normally, but with VS vacuum line going to Gas Filter manifold fitting (due to lack of a Fuel Cut Port). The CR/Fuji Advance Port (the only port) will be 'T'ed to the distributor advance diaphragm and the EVAP VCV through the existing single BVSV. The remaining emissions systems don't require carb vacuum ports.
Can't experiment until I solve a 2-minute unbelievably, horrendously loud steady whistling sound during initial warmup of my truck. Plus I just work slowly, I'm retired :}. Constructive criticism/error analysis most welcome.
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