1978 FJ40 desmog (1 Viewer)

Chestcutter

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Have a couple of questions for the experts. I have a 1978 model FJ40 which has a factory replacement block in it
Date code on the block is 7 10 06 so likely Oct 77
Carb appears to look like 78 model year. Carb date code is 7J5 so 12/5/77 likely original?
Previous owner did a frame off restoration and started the desmog.

Removed:
Air injection and smog pump
EGR gone and capped
Vacuum lines are a mess - charcoal canister hooked up backwards etc.

I’ve corrected most of the issues following Jim C’s desmog diagrams and associate threads. Kudos to all who have contributed.

Questions
1: how to plumb the throttle positioner circuit which currently goes through the VSV ( not sure if this or the computer are functional though present)
2: is a blue VTv needed on the TP circuit whether or not the vacuum switching valve is used?
3: what to do with the port at the front top of the carb which originates next to where the pipe from the OVC valve connects to the float bowl see photos

Any help from the experts here is appreciated!

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1. The TP functions to keep the idle speed at about 800 during engine braking to prevent popping in the exhaust. The VSV and smog computer drive this using the speed to switch it on and off below 10 MPH. If your VSV isn’t working there is no reason to keep the TP Because it won’t work without its connection to the speed sensor.

2. Don’t confuse the TP with the choke breaker CB. The CB runs off manifold vacuum as does the power valve which is the brass tube connection #8 Facing forward on the carb top cover.

3. If your VSV isn’t working, you can connect the power valve and CB directly to manifold vacuum.
 

Chestcutter

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Thanks! I'm going to test the computer based on the emissions manual and see if this function still works. This carb doesn't have a choke breaker. Either was removed or never had one.

Appreciate the insight. Your Youtube videos are a great help preparing to rebuild the carb.
 
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The #1 problem with the smog computer and VSV is that the plug solder connections on the computer board break due to vibration. If you can wiggle the plug and hear the VSV clicking, then you need to resolder and then “pot” the plug with silicon caulk or epoxy to keep it from cracking again.
 

Chestcutter

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Thanks for the tip. No clicking at the board plug and the VSV tests OK with a battery according to the emissions manual.

So will plumb TP via the VSV connections to TP port. Power valve to manifold vacuum. If I understand right, no need for the EGR port.

WIth an unmodified distributor- advance is working but a little stuck - distributor advance should be to the advancer port or manifold? I've read multiple recommendations here.

Hoping @FJ40Jim will weigh in re the benefit of recurving and if that changes things. From what this newbie has read I should be able to function with a rebuilt stock carb for the moment, albeit less power / efficiency than a Jim C carb rebuilt specifically with desmog in mind?
 

FJ40Jim

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Lots going on here.
The date code on the service engine block is probably 1987, not 1977 for a 78 truck.

Carb date 7 J 5 is 1977 Oct 5th.
1978 federal 2F carb.

The CB is also the TP on this carb. It can be connected to the manifold vacuum fitting just in front of the carb. No VTV is needed or desired.

The brass tube on the air horn is the breather for the (missing) HIC valve. Cap it to prevent dirt entry. The PV vacuum circuit is internal on the 77&78 carbs.

Dissy advance port connects to one of the ported vac fittings ( either of the lower 2).
Nothing connects to retard port on dissy.

HTH.
 

Chestcutter

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Jim thanks much for your help. Based on posts on some other threads re the Dissy in a desmogged scenario - sounds like timing should be set for 10-12 BTDC rather than 7.

What is your current lead time for a recurve? Am I right this will mostly affect power / acceleration and not idle? Still struggling to get the truck to idle with choke off. Idle solenoid checks out and grounds to carb body on this model. I suspect I need to tear it down and clean everything out, and get on your list for an expert rebuild.
 

FJ40Jim

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Won't idle: could be a gunky idle circuit in the carb.
Or it could be a warped manifold & leaky gasket.
Or all of the above.

Lwad time for dissy is 2 weeks.
5-6weeks for carb & dissy.
 

Chestcutter

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OK thanks Jim. I am going to work out a temp carb and dissy arrangement so I can send you both over the next few days.
 

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