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Thread: Vacuum leak

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    Vacuum leak

    Still searching for a vacuum leak on my SU H6 carb.
    Question...Is there supposed to be a gasket on the piston chamber to the top of the carburetor. I don't have one but the literature also doesn't show one.
    I'm also missing the washer on the damper cap but I doubt that would cause a vacuum leak.

    Steve

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    Luke Skywalker
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    Re: Vacuum leak

    No gasket on piston chamber. I'd put a copper washer on the damper cap, probably available locally, but that is probably not the problem as you say. There are very places for vacuum to leak on a TR3. The throttle shaft is one but if I remember correctly, that was ruled out.
    Tom
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    Re: Vacuum leak

    Yeah I sprayed the throttle shaft with little or no effect. Float bowls are filling correctly. Haven't checked for manifold leak, god forbid.
    Car runs ok with the left carb. richened until the nut falls off, minus one turn. I noticed that the damper in the right carb. slides up about 1/4" before the oil slows it down. The left only slides up about 1/8" before slowing.
    Don't know what that means but they should be the same.

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    Great Pumpkin TR3driver's Avatar
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    Re: Vacuum leak

    You're still ttrying to stop the lope?
    If you can set the mixture by the book, ie idle rpm rises then falls back if you lift the piston a bit; then IMO you don't have a big enough leak to matter.

    You also said IIRC that the car had been raced before. Unless someone deliberately detuned it back to stock, you almost certainly have a racing cam. I won't argue over the distinction between hunt and lope, but a wild cam can very definitely produce a condition where idle rpm varies up and down about once a second.

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    Re: Vacuum leak

    Problem is I can't quite set the mixture by the book because I run out of threads. If I choke it a bit it runs better.
    Maybe I could put a little spacer washer on the jet to get me another 1/32 of an inch.
    That said, the two carbs. should be the same so something is off.

    Thanks

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    Re: Vacuum leak

    I found that my connection between the vacuum line for the vacuum advance and the front carb was leaking. Put a vacuum pump on the other end (or a rubber hose and suck on it ) and cover the hole under the butterfly valve with your finger. Then see if it holds suction. While you're at it, switch it around and check that the vacuum advance itself is holding air. Maybe not enough of a potential leak to affect the mixture noticeably though.
    Last edited by Snowkilts; 12-03-2018 at 06:28 PM. Reason: clarity
    60 TR3A TS66043

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    Re: Vacuum leak

    Have you considered trying different needles since the car was raced and you cannot make it rich enough with jet adjustments? Do you know that the two carb springs are the same? Do the pistons fall at the same rate when you remove the dampener pistons from them?
    Doug L.
    '64 Morris Mini Cooper-S 1275
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    Re: Vacuum leak

    You have probably already done all of this:

    but I use a piece of hose as a stethoscope to listen for leaks. Go around the manifold, carb bases, etc and listen. Leaks are easy to hear.

    In setting the carbs there is a special tool for getting them in harmony and I have one, but the same hose is better in my opinion. Stick it in the opening of each carb and compare the sound.

    But first, you have to solve the issue with the jet and needle on that one carb. Running out of thread just isn't right. Replacing the jet and needle costs next to nothing. Then you need to get them centered. This takes patience and a beer. Once you have the pistons falling with a nice clunk, you are ready to start tuning. Loosen the coupling between the carbs and set the idle speed so the car will run. Then set the mixtures. Then set up the idle speeds to give the same hissing sound with the hose and at the correct RPM. Then recheck the mixture. Reconnect the coupling and you should be good.
    My hovercraft is full of eels.
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    Re: Vacuum leak

    Are your needles at the same depth in the piston? Are your needles the same?
    Bob

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    Re: Vacuum leak

    Thanks for all the advise. Especially the beer one. Vacuum advance is good. I've never checked if the needles are the same or if they are both seated correctly. (all the way in).
    I tried spraying the carbs to listen for leaks but not the stethoscope yet. Will do. I have a synchronizer and can get the carbs. to match air flow but left side is still lean.

    The car came with bags of old parts which were replaced. I bet I have the old needles etc. Might try an experiment with them.

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    Re: Vacuum leak

    Could a bugger (Varnish from old gas) be stuck in the jet ? Just a thought.

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    Re: Vacuum leak

    I don't recall the depth criterion, but there is one in the manual. I think it is the shoulder of the base of the needle flush with the piston bottom.
    Bob

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    Re: Vacuum leak

    What kind of beer works best for this ???

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    Re: Vacuum leak

    For me a good IPA would work well.

    David
    TR3A TS75524L

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    Re: Vacuum leak

    Thanks. I was thinking Guinness and might have really bollixed thing up.

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    Re: Vacuum leak

    The adult beverage should probable be enjoyed while you sit back and go through all the possible solutions in your mind rather than in hand while working.

    David
    TR3A TS75524L

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    Re: Vacuum leak

    You're no fun at all Dave...
    Art
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    Re: Vacuum leak

    The shoulder of the needle needs to be flat with the bottom of the piston. If you don't have the centering tool, using the needle will work (but it takes pure luck to actually get the jet centered - and one beer of your choice per carb is highly recommended).
    My hovercraft is full of eels.
    1959 TR3 SN42940

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    Re: Vacuum leak

    Quote Originally Posted by mt10flyer View Post
    If you don't have the centering tool, using the needle will work (but it takes pure luck to actually get the jet centered
    I have no experience with centering jets on H series SUs but on HS carbs it is no problem to center the jets using just the needle. I have never used the jet centering tool.
    Doug L.
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    Re: Vacuum leak

    Perhaps there is more than one tool? The one I got in the "SU tool kit" many years ago both didn't work for me, and appears to have a fundamental flaw in that it locates the jet to the main carb body. If the hole in the body could be that accurate, there would be no need to center the jet!

    I've also found that my domes are actually loose enough on the body to affect the centering. I now twist them clockwise while snugging the bolts down, so they wind up in the same place every time. My point being that the jet has to be centered to the piston, not to the carb body.
    Randall
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