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Thread: Petrol Drains

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    Petrol Drains

    I realized for the first time today, or found today. the petrol drains in the front and rear of the bottom of the inlet manifold of my 3000 BN7. I see them in the catalog, but not in manual.
    I tried to look at the previous threads to find an answer for my question ...... These let air in, like a vacuum leak, wouldn't that make run lean, so when you adj carb mixture you are adding extra fuel to compensate for it??. I see one of mine is plugged and the rear one is not.
    So thats my question, does't it make it run lean??? So you compensate for it when adjusting carbs at idle??? Isn't that like having a lot of throttle shaft clearance??? That causes a problem, why would this not??

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    Yoda tahoe healey's Avatar
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    Re: Petrol Drains

    If you are referring to the copper tubes, they let fuel out and should not let air in. The should be diverted away from the exhaust system through islets mounted to the oil pan.

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    Re: Petrol Drains

    Well, mine appears to open the manifold. I can blow air through it. Whats supposed to make it drain when needed and closed the rest of the time?

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    Yoda
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    Re: Petrol Drains

    Quote Originally Posted by F Schreiber View Post
    Well, mine appears to open the manifold. I can blow air through it. Whats supposed to make it drain when needed and closed the rest of the time?
    Nothing. The drain tubes are so small as to be insignificant, and accounted for when setting mixture. At the most, and this is a stretch, they'd be like having the throttles open just the smallest crack, which some of us who have HD8s do anyway (in defiance of most of the manuals and 'how-to' books).

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    Re: Petrol Drains

    You will find lots of opinions (like certain anatomical parts, everyone has one) but I've never read anything 'official' from the manufacturers as to why they are needed, or how important they were/are.

    For the Healey models, only the 100 intake manifolds and the 6 cylinder log manifolds show manifold drains.
    The early 100-6 '2 port head' had no such drains. The tri-carbs had no such drains. I just looked up TR6 on Moss site and they don't show manifold drains. I also looked up MGB and MGA on Moss site and they don't show manifold drains.

    Could this be something that, in the day, some engineers thought was needed and some didn't?

    I have always had the same concern about vacuum leaks so for the 20+ years that I ran log manifolds, I always plugged the drain ports with Allen set screws. I now have triple HD6 and no drains - never had a problem, but, again it's just an opinion...

    Dave

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    Re: Petrol Drains

    a

    The drains are there to bleed excess fuel out of the intake manifold if that occurs, additionally they must be bent [ as generally shown ] in smooth bends to
    also prevent air leakage into the manifold which would impact the fuel system just like other air leaks would

    Properly installed the drains prevent fuel from falling on the hot exhaust and igniting.
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    Re: Petrol Drains

    Quote Originally Posted by Keoke View Post
    a

    The drains are there to bleed excess fuel out of the intake manifold if that occurs, additionally they must be bent [ as generally shown ] in smooth bends to
    also prevent air leakage into the manifold which would impact the fuel system just like other air leaks would
    Would a leak at the manifold exhaust flange (or any leaks in exhaust system) impact the fuel system?
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    Yoda
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    Re: Petrol Drains

    Quote Originally Posted by AUSMHLY View Post
    Would a leak at the manifold exhaust flange (or any leaks in exhaust system) impact the fuel system?
    Theoretically, sure. Dump enough raw fuel on a hot exhaust and you could have problems (I suspect this is how most non-electrical fires start). But, I think you're asking if exhaust gas can cause any other problems--other than fire--and (my) answer is I doubt it. Exhaust systems are to some extent, esp. in high-performance cars, tuned to the powertrain and a really leaky exhaust could affect overall running of the car--not to mention the monoxide hazard--but directly affect the fuel system? I can't think of any scenario other than the fire and monoxide hazards.

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    Re: Petrol Drains

    When I first had my car, I found that the tubes were coiled up and generally mangled. I set about gently straightening them and bought the two small brackets to fit on the sump, then I realized that there should be small rubber grommets fitted to them, for the pipes to pass through. At the time they were unavailable, and may still be for all I know, so I cut up some insulation from some spare wire that I had kicking around and fitted that in, - these brackets are still in my parts box - I have not fitted them in 17 years, despite rebuilding the engine 6 years ago.

    I must get around to it.



    Bob

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    Re: Petrol Drains

    Quote Originally Posted by Bob Hughes View Post
    When I first had my car, I found that the tubes were coiled up and generally mangled. I set about gently straightening them and bought the two small brackets to fit on the sump, then I realized that there should be small rubber grommets fitted to them, for the pipes to pass through. At the time they were unavailable, and may still be for all I know ...
    Moss sells them now (I believe they have a distributor somewhere in England). They're almost $10 each(!), and the 'ferrules' that come with them are just small, round rubber grommets.

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    Re: Petrol Drains

    Back when I had a stock manifold, I made those two small straps and fitted them with grommets from the hardware store.
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    Re: Petrol Drains

    I agree with Keoke that the drains are there to drain excess fuel from the manifolds. These manifolds on these old cars were not that efficient and their walls and corners trap fuel. Their inefficiency causes us to sometimes tune richly or over choke upon start ups, etc. If the ignition is left on maybe when working on the car or just because of lack of attention for a few moments, the electric fuel pumps pressurize the system, a rich or dirty jet can let fuel past it then it accumulated in the manifold. That's why sometimes you will see these things weep and others may never weep. The engineers allowed for these circumstances. The area of these tubes is very small and the distance of their opening from the combustion chamber is pretty far away. Add a few bends in the tubes and the combustion chamber never notices these as a vacuum leak. Them old timers were pretty smart sometimes. The MGA does have these tubes and I believe at least the early "B" did also. I first encountered these type of drain tubes on my 1961 MGA about 50 yrs ago.
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