'85 GL Hatch vacuum timing

Discussion in 'General Motoring' started by Guest, Sep 15, 2005.

  1. Guest

    Guest Guest

    Hey all. I've got an '85 GL Hatchback 4WD with a carb'd EA-81. It seems to
    be losing what little power it's got, so I checked the timing. I've had the car for a
    few years, but never checked it before.

    The book says it's supposed to be at 8 BTDC with the vacuum removed (and
    plugged)... mine's set to about 16! The centrifugal weights on the distributor seem
    to advance it some (I didn't run it all the way up when I checked that). The "vacuum"
    line on the diaphram doesn't seem to have vacuum on it, either. Apparently it's not
    manifold vacuum, but rather ported to the carb? The diaphram also appears to leak
    down a bit.

    My current theory is that someone (before me) didn't figure out that the
    diaphram was broken, but had the gutless performance of the engine running at 8 BDTC.
    So, they cranked the distributor up to about the limit of it's adjustment (16 BTDC)
    and left it alone. How much advance does the diaphgram really give? I've heard that
    it's supposed to run at up to 35 BTDC at high RPM... sound reasonable? Will replacing
    the diaphram likely do all I need, or should I be looking at the flyweights as well?

    Thanks,
    -Cory

    --

    *************************************************************************
    * Cory Papenfuss *
    * Electrical Engineering candidate Ph.D. graduate student *
    * Virginia Polytechnic Institute and State University *
    *************************************************************************
     
    Guest, Sep 15, 2005
    #1
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  2. Guest

    S Guest

    Hi Cory!

    On Thu, 15 Sep 2005 11:36:52 +0000 (UTC),
    You're in VA, basically at sea-level? Set it to 8-10 BTDC @
    800-1000RPM. Vac advance _is_ a port on the carb; venturi vacuum just
    above the butterfly IIRC. Won't do anything 'til you get some flow
    happening. There are some thermo-vacuum valves in this circuit as
    well, but I think they simply block the "advance" signal until the
    engine/intake manifold warm up. The mechanical advance does most of
    the work anyway, and I wouldn't think that ~35 degrees of advance at
    4000-4500RPM is unreasonable.

    None of this is likely to get much of your "lost" power back, the
    EA-81 is pretty much a stone axe, and not very sensitive to trimming
    ignition timing, or even FA ratio when you get right down to it.
    Be sure that you have a clean air filter, and good ignition parts;
    plugs, wires, cap, and rotor.
    Check engine compression. You'll want to see at least 120PSI all
    around; 140+ is better. Should be consistent between cylinders, too,
    not more than a 5PSI variance. Check manifold vacuum at idle; should
    be fairly high (25in or so) and steady. Set the timing as above. If
    your engine doesn't have hydraulic lifters, adjust the valves. (Some
    got 'em, some didn't; it will say "do not adjust valves" or something
    to that effect on the valve cover(s) of a motor with hydraulic
    lifters.) The CAT on these cars is at the junction of the Y pipe, and
    for various reasons can become plugged. I can't recall if it's
    possible to see the element on these, but if you can it should be
    whitish, definitely not dark colored. You might just swap in another Y
    pipe if you can find one. If this results in a dramatic improvement,
    do _not_ keep driving the car. Something caused the CAT to plug, and
    unless you fix it you will shortly plug up another. Check your air
    suction valves, and the "duty valve" indicator on the ECU. (The LED
    should flicker if the engine is held at a constant 2500-3000RPM;
    steady-state on or off is bad.) Rebuilding the carb may be necessary,
    but it's a pain, and the kit costs $65 or so, so I'd hold off on that
    until you've checked everything else.
    Probably have to replace the entire thing. Unless it's leaking oil
    (oil fouling inside of dist cap), or really rusty to the point where
    the weights/springs are frozen, I wouldn't worry about it.

    I have a fair bit of experience with these motors. Feel free to
    contact me directly if the above doesn't help.

    ByeBye! S.


    Steve Jernigan KG0MB
    Laboratory Manager
    Microelectronics Research
    University of Colorado
    (719) 262-3101
     
    S, Sep 20, 2005
    #2
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  3. Guest

    Guest Guest

    Thanks for responding. I was beginnging to think that only
    bling-bling kids wanting to know what polish was best on their WRX wheels
    were the only ones on the list... :)

    : You're in VA, basically at sea-level? Set it to 8-10 BTDC @
    : 800-1000RPM. Vac advance _is_ a port on the carb; venturi vacuum just
    : above the butterfly IIRC. Won't do anything 'til you get some flow
    : happening. There are some thermo-vacuum valves in this circuit as
    : well, but I think they simply block the "advance" signal until the
    : engine/intake manifold warm up. The mechanical advance does most of
    : the work anyway, and I wouldn't think that ~35 degrees of advance at
    : 4000-4500RPM is unreasonable.

    A few things. I'm actually in southwest virginia. Field
    elevation here is 2132' (I'm also a pilot) and it gets warm... routinely
    over 4000 effective altitude (density altitude) in the summer. It's not
    the southwest though, to be sure.

    I've done a bunch of reading on ported vs. manifold vacuum. I
    know now that my car has ported, but I've found conflicing definitions of
    that. One school of thought is that the port is right above the
    butterfly, so it's basically manifold vacuum except at an idle, where
    there's very little. The other information I've found is that the ported
    vacuum is taken from the venturi area. If that's the case, then the
    vacuum it produces would be more a function of mass air flow. Subtle
    difference, but it would also mean more advance at WOT, high RPM.

    My particular setup has a thermo in the circuit, but not for the distributor
    diaphram. It's connected directly to the ported vacuum. I have tested the diaphram
    and verified that it does move, but it leaks down. I have been unsuccessful finding a
    replacement shy of buying a whole rebuilt distributor.

    : None of this is likely to get much of your "lost" power back, the
    : EA-81 is pretty much a stone axe, and not very sensitive to trimming
    : ignition timing, or even FA ratio when you get right down to it.
    : Be sure that you have a clean air filter, and good ignition parts;
    : plugs, wires, cap, and rotor.

    That's why I'm trying to get it more or less right. At 16 BTDC with the
    vacuum ineffective it was gutless. When I timed it back to 8 BTDC as per the book, it
    was extra-gutless. I've redone the rest of the ignition awhile back (to try to get a
    little more poop back).

    : Check engine compression. You'll want to see at least 120PSI all
    : around; 140+ is better. Should be consistent between cylinders, too,
    : not more than a 5PSI variance. Check manifold vacuum at idle; should
    : be fairly high (25in or so) and steady. Set the timing as above.
    I'll try that when a get a chance. The boob who had the car before me
    partially cross-threaded a plug. It took me 1/2 a day to get it threaded back in
    correctly when I replaced plugs awhile back. Maybe I'll get 3/4 of the compression
    checks.

    If
    : your engine doesn't have hydraulic lifters, adjust the valves. (Some
    : got 'em, some didn't; it will say "do not adjust valves" or something
    : to that effect on the valve cover(s) of a motor with hydraulic
    : lifters.)
    Lifters are allegedly hydraulic... has the "don't adjust me" sign on the
    covers. Yay... :)

    The CAT on these cars is at the junction of the Y pipe, and
    : for various reasons can become plugged. I can't recall if it's
    : possible to see the element on these, but if you can it should be
    : whitish, definitely not dark colored. You might just swap in another Y
    : pipe if you can find one. If this results in a dramatic improvement,
    : do _not_ keep driving the car. Something caused the CAT to plug, and
    : unless you fix it you will shortly plug up another. Check your air
    : suction valves, and the "duty valve" indicator on the ECU. (The LED
    : should flicker if the engine is held at a constant 2500-3000RPM;
    : steady-state on or off is bad.) Rebuilding the carb may be necessary,
    : but it's a pain, and the kit costs $65 or so, so I'd hold off on that
    : until you've checked everything else.

    I just looked inside the CAT... seemed fine. The pipe right in front of it
    sheared off (sounded like a go-kart for a week or two). The rest of the exhaust is in
    good shape, so I just repaired the rusted spot. I was tempted to remove the CAT to
    try to get a bit more poop, but it was easier (and more legal) to leave it on.

    Mine doesn't have an ECM.

    : >Will replacing
    : >the diaphram likely do all I need, or should I be looking at the flyweights as well?

    : Probably have to replace the entire thing. Unless it's leaking oil
    : (oil fouling inside of dist cap), or really rusty to the point where
    : the weights/springs are frozen, I wouldn't worry about it.

    : I have a fair bit of experience with these motors. Feel free to
    : contact me directly if the above doesn't help.

    My current plan is to rig a digphram from a comodity engine onto the
    distributor. Something like a smallblock Chevy. I know that the force/distance may
    be a bit off, but as long as the mechanical/centrifugal timing is correct the
    high-power timing should be correct. I wouldn't think I'd have to worry too much
    about detonation even if the vacuum comes in a bit early/late.

    For the time-being, I've connected the leaky stock diaphram to manifold
    vacuum through a long restrictive hose. It's got a lot more flow capacity than the
    port so it can tolerate the leak better. I've also got my vacuum gauge inside the car
    rigged in to read the diaphram vacuum while driving and set the base timing to 10
    BDTC. Much more responsive, so I'll probably try to rig in a non-broken diaphram this
    weekend.

    Sound reasonable (albeit longwinded)? How much timing could I get away with
    running 87 AKI fuel?

    -Cory

    --

    *************************************************************************
    * Cory Papenfuss *
    * Electrical Engineering candidate Ph.D. graduate student *
    * Virginia Polytechnic Institute and State University *
    *************************************************************************
     
    Guest, Sep 21, 2005
    #3
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