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Thread: TR7 carb seeping fuel

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    Jedi Hopeful johnrip's Avatar
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    TR7 carb seeping fuel

    I have some gas seeping on the bottom of my bowl in the front carburetor. When I took off the K/N airfilter, there was gasoline in the carb, as shown in picture. This is while the car has sat 24hrs. Is this normal? The car runs very well, but I have smelled gas for a while....
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    Jedi Trainee Merlin63Tr4's Avatar
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    Re: TR7 carb seeping fuel

    Not normal.
    Either the fuel level is set too high on that carb or you have a leaking/stuck float valve.

    M.
    Merlin the '63 Tr4.... it grows younger !

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    I agree with "Merlin"...but between the 2, I'd be betting on the float chamber needle valve.
    And it would be a good idea to check your oil...if you smell gas on the dipstick, don't start the engine until you fix the problem and have changed oil and filter.
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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I now have noticed the second carb is doing the same thing. I had previously checked the float valve, they seemed to be working. What would make both carbs do that?

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    You say the needle valves seemed to be working...does that mean they shut off the flow of fuel when the floats rise ?
    It sure doesn't sound like it.
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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I am not sure. It just seems odd that it would be both carbs. It seemed to refill after I cleaaned it out overnight, with out running the car. Could there be some sort of a siphoning effect going on? Perhaps with the emissions hosing?

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    It could be the siphoning effect...I had that problem when the gas tank was full and the car sat overnight..I put a fuel cut off valve after that happened a couple of times.
    Have you checked for the smell of gas on your dipstick oil ?
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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I don't smell gas now....but it had plenty in the oil after it sat for the winter. I was thinking of the shutoff valve. Do you close yours whenever the car sits ..all the time?

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    Not for short periods of time, but if it's a few hours or overnight then yes, I use the valve...But the only time I've experienced flooding in the past was if the gas tank was full or nearly so.... I don't trust needle valves (Grose jets even less) so to avoid breaking the habit, I raise the bonnet and close the valve regardless of the fuel level... I attribute all that to hydraulic head, once the head is lower, the pressure is reduced
    I think you can see the valve here with a red knob:
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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    What do you mean "the head is lower". I noticed that when I turned off my car, my fuel filter was 2/3 full. Then I clamped the line, above the filter, and the filter was full 5 hrs later. It seems there is syphon pressure, even with a clamp on the line. Is this a gas tank issue (I am 2/3 full), or could it be from the EGR or emissions stuff? PS thanks for your help!

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    Hydraulic Head "The force exerted by a column of liquid expressed by the height of the liquid above the point at which the pressure is measured. Although head refers to a distance or height, it is used to express pressure, since the force of the liquid column is directly proportional to its height. Also called head or hydrostatic head."
    It's my theory, John, and maybe that's all it is but I can't think of any thing else that would cause my carbs float chambers to over flow when the gas level in the fuel tank is about maximum..that plus I don't believe that the float chamber needle valves are real good at acting as cut off valves and I don't trust them to. They function more like a valve that regulates the fuel level within the float chamber.
    Anyway, regardless of the underlying cause of my carbs flooding under certain conditions, a cut off valve (and using it) put an end to it.
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    Jedi Knight AltaKnight's Avatar
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    Re: TR7 carb seeping fuel

    Both my carbs on my TR6 suddenly sprung leaks from the large plug in the bottom of the fuel bowl, put new seal rings in and it now doesn't smell of gas.
    Cheers, Graham
    '73 TR6; Olive Oyl;
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    If I had it to do all over again .... I probably would .... if I could remember how!

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    Re: TR7 carb seeping fuel

    Are you running a stock fuel pump? or electric aftermarket? If you have a electric, you may have to put in a pressure regulator.

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    Re: TR7 carb seeping fuel

    The above question is important. Your carbs can not handle more than 2 PSI. Over 2 PSI will cause flooding, from a little to a lot depending on pressure. Most electric pumps are set at 4 PSI, so are not suitable for your carbs, without a regulator.


    The mechanical pump on the 7 can produce more than 2 PSI if the pump with the long arm is fitted, & the 3/16" spacer is missing from between the pump & the engine block. The closer to the block the pump, the higher the pressure.


    Old needle & seats in the float bowl are not good as shut off valves. When the engine is stopped there will be some residual pressure in the hoses from the pump to the carbs. This will slowly leak into the float chamber causing an overhigh float level. The excess fuel will run just by gravity through the carb jet, & lie in the carb throat, as in your photo. This clears when the car is started, & the carb will not flood while the engine is running. It uses more fuel than will leak past the needle& seat.


    The fuel pump should not pass fuel when stopped. However if there is dirt or silt in the valves a very full fuel tank can cause some fuel to pass them to the carbs, & cause the same flooding. This is not common.


    To test any of this, next time you run the car, immediately you stop the engine, remove a fuel hose from one carb. A small amount of fuel will squirt out as you do, reliving the residual fuel pressure. Refit the hose, & check a few hours later. You should not find any fuel in the carb throat. If so, you now know why it's been getting there. If this is all it is, it really doesn't matter, unless it makes the car hard to start. It will clear as soon as you start the engine.

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    Quote Originally Posted by AltaKnight View Post
    Both my carbs on my TR6 suddenly sprung leaks from the large plug in the bottom of the fuel bowl, put new seal rings in and it now doesn't smell of gas.
    That's a simple gravity leak thru the O-ring seal, but John is experiencing something different in that a full float chamber is overflowing into the carbs' mixing chamber...when there is no more room in the float chamber the incoming gas is using the 0.10 opening in the fuel jet to escape the float chamber.
    Residual fuel pressure, or hydrostatic head when the engine is not running...a cut off valve puts an end the overflow.
    Too much fuel pressure from the pump should have shown up when the engine is IDLING...overflow, flooding or fouled spark plugs at the very least.
    Last edited by poolboy; 06-15-2017 at 01:12 PM.
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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I have a mechanical fuel pump. I see fuel after the car sits for a few hours. I am going to reset float level, but I am thinking there is a certain syphon issue. I will install a fuel **** off, but still would like to know why there is the syphon. again, thanks for all the help offered!

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    Yoda poolboy's Avatar
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    Re: TR7 carb seeping fuel

    If it turns out that you have Grose jets instead of needle valves, get rid of them and put in the original type needle valves.
    Grose jet for ZS 175 carbs on far right:
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    Jedi Knight AltaKnight's Avatar
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    Re: TR7 carb seeping fuel

    Seems to me that fuel pooling in the carb when parked requires two things, first a fuel bowl jet that is "passing" (leaking), second it needs residual pressure in the fuel line to the carbs. on the TR6 the fuel line between the mech fuel pump and the carbs is routed across the top of the engine (above the water pump and higher than the fuel level in the gas tank) which would probably break any "siphon" effect but I'm not sure how this is set up on the TR7.
    Cheers, Graham
    '73 TR6; Olive Oyl;
    '07 Mercedes SL550 Roadster

    If I had it to do all over again .... I probably would .... if I could remember how!

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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I think I have original needle jet...I will check. I have an appointment with my mechanic in ten days. I clamped the fuel line, so that I don't flood the engine while it sits, and over the last 12 hours, it is still drawing fuel slowly. All with the line clamped! I assume it will stop soon...
    As per the fuel lines are routed under the carbs on the tr7.
    I tried with gas cap off, and it made no difference.
    I am going to get the float level adjusted, and fuel turn off installed, but I am shocked that it continues with line clamped (at inlet to fuel pump).

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    Jedi Hopeful johnrip's Avatar
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    Re: TR7 carb seeping fuel

    I clamped the line after the fuel pump, before the carbs, and it stopped an hour later...

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