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Thread: engine over heating with new su carbs

  1. #1
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    Default engine over heating with new su carbs

    I swapped out my new su carbs from z therapy for the 73 flat tops.
    A little confused on the heating hose which comes out of the engine and runs thru the carbs for warming.
    1- Is there a sensor placed in this in this line? ( pic shows blue hose from engine to carb metal line).
    2-Also , I see engines without the heat shield, do I need to replace?
    3- the engine has been overheating in about 15 minutes and the mechanic can't find anything for cause.
    Thanks,
    Brian

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  2. #2
    Registered User olzed's Avatar
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    Default

    Did you drain the radiator?
    You could have air trapped in the block. Try loosening the engine side hose clamp at the front, and with the engine running give it a few wriggles, then retighten. Try this on other hose clamps. Worked for me.

  3. #3
    Nova Scotia,Canada,Earth Blue's Avatar
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    check plugs... are you running lean?
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  4. #4
    Boat Anchor Repairman Captain Obvious's Avatar
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    Referencing page ET-4 in the 73 FSM...

    In 73 and 74, there were actually three parallel paths for water to flow through the carb and intake system. All three of them originated from the lower portion of the thermostat housing (the upstream side of the thermostat). From there, they split to three different routes:

    A) Through the balance tube.
    B) Through the intake manifolds.
    C) Through the carbs themselves.

    All three of those different routes then rejoined in the plumbing back by the firewall before wrapping around the rear of the engine above the bell housing and then heading back to the inlet side of the water pump.

    The blue hose appears to be the one that goes through the intake manifolds.

    Of those three routes, only the route trough the carbs themselves is thermostatically controlled, and that route has been removed completely when you swapped to the round tops. The other two routes are (supposed to be) as free flowing as the small diameter lines will allow. And from what I've seen, they are often so plugged with crud that they might not be passing any fluid at all.

    I think it's important to have a small amount of flow through the thermostat housing, even when the engine is cold. If that area is completely stagnant, then you run the risk of overheating because the thermostat won't be subjected to the hot water and will instead be sitting in a stagnant cold pool.

    So a far fetched theory might go like this:

    If the coolant paths through the balance tube and intake manifolds are plugged with crud...
    And the path through your flat top carbs was not...
    And your thermostat does not have a small bypass hole through it...

    Then you might have caused yourself a problem when you swapped carbs by causing a stagnation in the area below your thermostat that did not exist with your flat tops.

  5. #5
    Registered User Hunter260Z's Avatar
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    Captain Obvious

    You hit the nail on the head.I'm wanting to go with a different balance tube and plug the holes in the intake. I also have a long black hose that carries water from the thermostat to the back of the engine. This attaches to the lines coming from the intake manifold and balance tube. I have 170 degree thermostat (factory) in mine now. So does the factory thermostat have this bypass hole through them.
    Ray
    1974 Datson 260Z
    RLS30-27748 Matching #'s
    L26 Stock w/72 Round Tops
    Interpart Front Spoiler
    Addco Front & Rear Sway Bars
    My Very First Car
    Purchased 5/23/1974

  6. #6
    Registered User Hunter260Z's Avatar
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    Default

    I also have the early 72 round tops,sorry I should have added to the other post.
    Ray
    1974 Datson 260Z
    RLS30-27748 Matching #'s
    L26 Stock w/72 Round Tops
    Interpart Front Spoiler
    Addco Front & Rear Sway Bars
    My Very First Car
    Purchased 5/23/1974

  7. #7
    Registered User Stanley's Avatar
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    Yes there's a sensor for the carb heater line, it's a hot=closed thermostat that attaches to the rear of the intake manifold, as described in the FSM. If the stock one isn't available, there are substitutes, described in posts on this site. In Alaska, it might be a good thing to have if you drive in cold weather.

    That blue hose looks like a loop from the hose barb where the short hose is supposed to go from the thermostat to the tube through the intake manifold. Don't understand purpose of the loop. There's a plug in the thermostat housing where the hose barb is attached for the manifold heater hose. It's a BSPT thread, 1/4" pretty sure.

    Maybe use a header blanket if the stock heat shield doesn't work with the headers. Or use some header wrap on the area near the carbs.

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    Thanks for all the ideas, I have the round tops on now, and tossed the flat tops, sorry if I confused anyone.
    I'll check with the mechanic and see if something works.

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