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  1. #1

    Pressure testing 657x Motor


    Just want to confirm I have this correct. 657x motor has labyrinth sleeve which means can pressure-test whole motor at one time. I could not find anything in manual on pressure testing this motor as in later manual versions like 717/787....

    Last time i did 657x motor I blocked off carbs, exhaust manifold and pulse line, plus one spark plug - have a pressure fitting that screws into other spark plug hole and pumped up to 10 psi and checked for leaks.

    After reading thru a couple threads where they talk about cross overs and fully blocking off exhst manifold - I am wondering if I need to make a block off plate for the whole exhst manifold and/crossover at intake inlet?? Last time I just used a round plug in the center section of exhst /same with carb inlets on intake manifold.

    thanks in advance for assistance/clarification.

  2. #2
    wkuadam07's Avatar
    Join Date
    Feb 2013
    Bowling Green, KY
    Leaks occur most frequently at the cylinder base gaskets, case half line, and the rotary shaft oil bath seals. If you are checking these areas for leaks with your method i would say that you are doing just fine. People block off the entire manifold to check the water circuit for leaks. I think air leaks in the motor are much more important to check for, if air can't get out, water can't get it. I would be totally comfortable with your method... post pics if you don't mind.. i'd like to see the plugs that you use.

  3. #3
    Yes looking for possible air leak into carbs as I have a lean condition after rebuilding carbs (cleaned and flushed all jets etc x3) and fuel system and have isolated it to the carbs forward (Ran system off fuel bottle and still had hesitation off idle).

    Just wanted to make sure there was not a front/rear seal leaking...

    Here are a couple of pics of the plugs and modified spark plug inlet tool and a couple installed. I have a Mity Vac that I use to check pressure/vacuum - btw the case tested good.

    Plugs I got from Pep Boys but know that Orielly, Auto Zone carry them also.

    R/ Mark

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  4. #4
    wkuadam07's Avatar
    Join Date
    Feb 2013
    Bowling Green, KY
    Did you pop off test and leak test the carbs? BTW, what is that grey fuel line in there? Is that the feed to the front carb? Cause grey fuel lines are bad... your carb filters can get full in a quick minute from those grey fuel lines. Did you trim your spark plug wires back? I have seen spark plug wires that need trimming cause hesitation as well...

    I like your leak test method... I may have to go buy a set of those plugs next time i decide to mess with a motor tear down. Usually i leave the internal motor work to the pros because they give me a warranty and leak test it for me...and partly because i didn't want to make or buy costly leak test plates. Thanks for the pics!!

  5. #5
    The lone grey line is the pulse line - for some reason they did not replace that single one whenever they replaced the rest of the lines... It is clear thou.

    Yep spark plug wires trimmed and rechecked for good spark. I pulled the carbs out and disassembled once again - reverified that the pilot jet and circuits were clear via carb cleaner and compressed air... look good flowing thru all the holes ...

    Leak test was good - I replaced the needle valves in both sets with new 1.5 OEMs after first time thru as one seemed to be dribbling a bit. Pop Off was running high on both carbs ( should be 16-21 Psi per manual) I was getting 27 psi consistently for both w/ 65 gram spring. And when I trailer tested PWC got the same hesitation ... hence the reason I went back and did a pressure test on the case - thinking air leak somewhere else than carbs/fuel system.

    I am thinking that a high Pop-off may be causing the lean problem in mid range.... I believe I read that either here or Group K info...

    would a 6 Psi + make enough difference??

    As I had a new set of OEM 2.0's - I switched out the needle valve sets to 2.0/65 gram from 1.5/65 gram - and got Pop off running 14 to 15 psi. (do not have any 80 gram springs on hand)

    Have not reinstalled carbs yet will do this w/e or early Monday and then bring back to lake for trailer test again.

    Am not sure what else I am missing in tracking down the hesitation/lean condition right off idle ....

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