75 2F Desmog ?

Discussion in '40- & 55-Series Tech' started by petescoffee, Jul 20, 2005.

  1. petescoffee

    petescoffee

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    I have desmoged my75 2F and have a couple of questions.
    -Where should I hook up the TP Diaphram port to?

    -At the TVSV on the thermostat housing. There are 2 ports I used the inner one (to AAP) should I plug the outer one?

    -Where should I pull the Vacuum line to for the Distributor? Since this is a 75 (retard) can I just leave it off?

    -The venturi Port at the top of the carb, should I just plug this or?

    Thanks
    1975 Carb Diagram 2.jpg 1975 Carb 2.jpg
     
    Last edited: Jul 20, 2005
  2. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    Since it is desmogged, the TP is now the Choke Breaker.
    CB is connected to manifold vacuum.

    TVSV is a switch. It needs power in and power out to do anything. Vacuum in from manifold, vacuum out to AAP.

    No hose is connected to vac retard. determine total mechanical advance available (appx 26 degrees), then set base timing to achieve total timing of 34 degrees all in.

    The port formerly known as Venturi is now Power Valve. PV is connected to manifold vacuum.
     
  3. petescoffee

    petescoffee

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    Thanks Jim.
     
  4. Pin_Head

    Pin_Head

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    If it is working, I would leave the TP and Power valve hooked up normally. TP helps prevent backfire/detonaton during deceleration and with the PV open all the time it will run rich and get even worse fuel economy
     
  5. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    This is especially true on the 75-76 carbs w/ the external PV vac fitting. The PV jet is a 110. The typical main jet is 135-150. So if the PV is open, total jet flow is about 75% higher than normal! AF ratio would be 8:1 instead of 14:1. It is critical that the the PV get connected to manifold vac on these carbs!
     
  6. CruiserMax

    CruiserMax

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    This discussion illustrates why it is so important to know what the hell you are doing when doing a "desmog" job. So many people believe that all you have to is yank out all the vacuum hoses and the truck will be so much simpler and run like a dream with great power and 20mpg. I have a 78 desomoged 2F and I had some studying and education to go through to get it to work right. Of course working with a carb that was just rebuilt by Jim really helped! Thanks Jim...

    Cheers,
    Max
     
  7. Pin_Head

    Pin_Head

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    Yes, but IIRC it closes with about 2 inchs of H2O vacuum, so if the engine is running normally it is always closed. When you put your foot in it, it is supposed to open just when you need it. It is probably better to either run it off of the VSV as it is supposed to run or close it and use a richer main jet if it is running lean.

    It is really not so much smog control as it is first generation electronic carburetion. As long as it is working it is best to keep it.
     
    Last edited: Jul 22, 2005
  8. petescoffee

    petescoffee

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    I don't have the VSV anymore so I can not hook back into that.
    On the Manifold Vacuum where is the best place to pull this from?
    I have an early intake manifold on the car. There is a manifold plug on the driverside of the manifold and I think I could put a port on that.
    The following is picture of my original set up. I had a 1979 Carb on the car and was having problems with it cutting out/flooding under loads at Moab CM05 and Rubithon. Perhaps the problems were related to the desmog. I found a 1975 era carb and picked that up. I will need to have a port added later when I install my 1978 ignition system, but I will deal with that at the end of the summer.

    Thanks for the help.
    Dave
    79 carb on a 75 5 small.jpg
     
  9. Pin_Head

    Pin_Head

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    I went back and looked it up and the PV is normally open with no vacuum applied. Vacuum is applied to close the valve and it doesn't take much vacuum to close it. Hook it up to any manifold vacuum and the carb will always be in "lean burn" condition. Chances are that the manifold vacuum will never drop low enough to open it. Read the plugs to see if it is too lean. If so, bump the main jet up a notch.
     
  10. petescoffee

    petescoffee

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    Thanks for the help. I am hoping to get it finished this weekend. If I am confused I'll post up. Good chance I'll be posting back.
    Dave
     
  11. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    Pin, can you edit this post so it doesn't end up in the archives forever?
     
  12. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    A 75-76 era carb is not in lean condition ever. W/ a 144 main and a small primary air bleed, they are very fat in stock form. The PV spring is of typical rate, so it still operates as normal. The best thing that can be done for one of those carbs is to reduce the PV jet to something more reasonable, in the 70-90 range.
     
  13. Pin_Head

    Pin_Head

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    Done. I inverted the open/closed to the correct state.

    The Toyota smog manual for '76 describes the PV as part of a "lean burn" cruise cycle to reduce emisions and increase fuel econony and thereby the company's CAFE. Obviously, it can't be too lean without getting into other problems, so if combustion problems are the definition of lean, then it isn't lean. W/O the EGR gas it may be a little leaner than designed for.

    Still, it apparently isn't rich enough w/o the PV for high demand WOT situations or Toyota wouldn't have put one in. My impression of the spring rate is that it is very light for a vacuum operated device, so I am guessing that its operation by the VSV is more binary than modulated as in the mechanical versions of the PV. I still think it is more advisable to leave it alone and let the "smog computer" operate it as Toyota engineered it.
     
    Last edited: Jul 22, 2005
  14. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    The '76 manual lies. ;p

    Yes, the PV is a binary device. The PV itself is not designed to be partially open, and when the engine is driven under load, the exhaust sniffer shows when the PV "pops" open and the AF ratio swings to "stinky rich, off the gauge".
    :beer:
     
  15. Pin_Head

    Pin_Head

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    Just for the record, I looked it up in the Mitchell manual, which is the smog technician's bible as it describes the operation of the PV system in more detail.

    "With the accelerator pedal at full throttle, throttle position switch will be turned on, which will turn on the VSV. This will allow manfold vacuum to act on PV. Since at wide open throttle {and temp below 122F, my comments in braces} vacuum will be almost zero, valve will open due to spring tension. When coolant temperature rises above 122F, solenoid valve is turned off. This allows vacuum from the surge tank to act on PC. This will keep power valve closed, except at {extended periods} of WOT, since surge tank will store vacuum."

    So basically, the PV is used mainly when the engine is cold and when it is warmed up, the surge tank vacuum keeps it closed until its vacuum bleeds off. The normal bleed off rate is 0.4 inchs of Hg per minute according to Mitchel. So the surge tank significantly delays the opening of the PV when warm to reduce CO under high load.
     
  16. FJ40Jim

    FJ40Jim The Cruiser Whisperer SILVER Star

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    The toyota manual doesn't mention the function or vacuum rate of the surge tank. :confused:

    Thanks for the info! :beer: :beer:
     
  17. Pin_Head

    Pin_Head

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    Lots of good stuff in the Mitchell manual. It has the details on the operation of everything including flow charts for troubleshooting the various systems. It even has nice instructions for rebuilding the carb ;)

    It is still a little ambiguous about the operation of the PV when warm. The leak down rate of the surge tank is a maximum value with the PV disconnected. I didn't pay much atention to how long it takes the PV piston to leak down, but the fact that you have the surge tank means that the point was to delay the opening of the PV when the temp was above 122F. How long of a delay isn't clear and I don't necessarily want to do the experiment.

    I had a smog tech friend xerox some of the '76 Toyota section for me. I need to get my '76 smogged for ever in CA and I have found that some of the technicians need to be schooled about what is and isn't going on with this particular model.
     
    Last edited: Jul 23, 2005
  18. petescoffee

    petescoffee

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    You guys shot right over my head with that.
    Did not get a chance to get the vacuum hoses hooked up this weekend.
     
  19. bonzai

    bonzai

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    Clean engine bay I like
     
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