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  1. #1
    Member stevolwevol's Avatar
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    Default Vacuum advance issues, and questions.

    I timed my 77 L48, when doing so I noticed the vacuum line that comes from the distributor connects to a thermal switch (probably controlled by water temp) then a check valve, then to the port on the carb. Is it necessary to have a thermal switch, and a check valve? the check valve allows air to be drawn into the carb via vacuum, but won't allow air to move toward the distributor. I've seen other sbc engines of this era that don't have the check valve, but mine has one. Input would be helpful, and appreciated.

  2. #2
    Administrator Tom Bryant's Avatar
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    It’s an emissions thing that doesn’t allow vacuum advance until the engine reaches a specific temperature. Does it still work?

    Tom
    NCRS 1360.............SACC 2082.............C1 Registry..............L81 Registry
    1959 Chevrolet Corvette 1981 Chevrolet Corvette , 350 L81 automatic Frost Beige

  3. #3
    Member stevolwevol's Avatar
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    Quote Originally Posted by Tom Bryant View Post
    It’s an emissions thing that doesn’t allow vacuum advance until the engine reaches a specific temperature. Does it still work?

    Tom
    Hello Tom, yes it still works. Long story, but I noticed the vacuum advance was on the center port of the Q jet, and the driver side port was open and causing a vacuum leak. When I plugged the side port, the engine was running rough with an intermittent miss. I looked at a diagram which showed the driver side port as the correct port for the distributor. So I plugged the center port, and put the vacuum hose on the correct vacuum port, and it ran much better. The hose that was connected to the wrong port was "hardened" into place indicating that it's been that way for a long time. Puzzling to say the least. It was when I plugged the port is when it ran rough. I bet I wasn't getting any vacuum advance either.

    My 79 has a vacuum hose going directly to the correct port, but it has no check valve, and is not connected to a thermal switch, but it runs very well. I would like to do the same thing to get rid of excess vacuum lines.

  4. #4
    Administrator Tom Bryant's Avatar
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    That is odd..
    NCRS 1360.............SACC 2082.............C1 Registry..............L81 Registry
    1959 Chevrolet Corvette 1981 Chevrolet Corvette , 350 L81 automatic Frost Beige

  5. #5
    Member stevolwevol's Avatar
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    Quote Originally Posted by Tom Bryant View Post
    That is odd..
    Odd x 10 I don't know how I could have overlooked the unplugged unused port, but I did. I did run a vacuum hose from the distributor directly to the proper port, and It ran without a problem. The check valve thing is bugging me though. I don't know what use a check valve is supposed to provide, emissions, or performance. Maybe the guy who hooked up the thing wrong might have the answer.

  6. #6
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    1977-1978-corvette-corvette-engine-hose-routing-non-smog-jpg
    I trashed all that crap when I replaced the Q jet with a Holly direct fit pre smog carb.Blocked off the EGR etc etc.on my old 78 L82/4 Spd.
    Currently 93 Ruby 40th Anniversary Roadster/6 SPD

    Previous 58,68,70,76,78,85,90
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  7. #7
    Administrator Tom Bryant's Avatar
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    This diagram shows the dist vac to the drivers side carb pipe like your label shows. The Check valve/thermal switch (on the thermostat housing) hose off the center carb pipe control the EFE actuator which opens/closes the heat riser valve. The passenger side carb pipe runs to the lower thermal switch that controls the EGR valve. It looks like someone had your hoses mixed up. Plug thing in like the diagram and check operation of the EFE, EGR Check valve and the thermal switches to be sure they operate. Also check that EFE actuator to be sure the diaphram is good.

    Tom
    NCRS 1360.............SACC 2082.............C1 Registry..............L81 Registry
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    1959 Chevrolet Corvette 1981 Chevrolet Corvette , 350 L81 automatic Frost Beige

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    Member stevolwevol's Avatar
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    Are you saying the check valve was for EGR, and not for vacuum advance?

  9. #9
    Administrator Tom Bryant's Avatar
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    No. The diagram shows it in the EFE hose.
    NCRS 1360.............SACC 2082.............C1 Registry..............L81 Registry
    1959 Chevrolet Corvette 1981 Chevrolet Corvette , 350 L81 automatic Frost Beige

  10. #10
    Member stevolwevol's Avatar
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    Quote Originally Posted by Tom Bryant View Post
    No. The diagram shows it in the EFE hose.
    Ok, so the previous mechanic put the check valve meant for the EFE was on the vacuum advance hose. GEEZ! I know this seems like small stuff but timing and vacuum advance stuff is all about drivability which matters to me. Thank you for the tips Tom, and the diagram Wayback!
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