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    From =?UTF-8?B?Q2FybCBJamFtZXM=?=@21:1/5 to All on Mon Jul 8 19:12:17 2024
    On Mon Jul 8 11:12:12 2024 KevinJ93 wrote:
    On 7/8/24 1:52 AM, Jeff Layman wrote:
    On 07/07/2024 23:51, KevinJ93 wrote:
    On 7/7/24 9:57 AM, Don Y wrote:

    In The West, it is not uncommon to find yourself driving through varying >>> elevations (mountains). Relying solely on the engine for braking often >>> won't allow you to comply with the posted speed limits *or* "safe
    driving
    conditions".

    [OTOH, being overly reliant on the brake can lead to brake
    overheating and
    failure]

    My recent cars automatically increase engine RPM by changing gear to
    maximize engine braking, in addition to using brakes if necessary.

    Some people find that disturbing to have the engine spinning at
    4000-5000RPM down steep hills.

    A few years after I learned to drive (in 1972!) I was in England's Lake District which has some long and pretty steep hills. Coming down one in
    my manually-geared car I decided I could save petrol by turning the
    engine off and leaving the car in second gear, using the aid of engine braking to save heavy use of the brakes. What I'd forgotten is that the fuel pump wasn't electric, but was driven off the engine shaft. So as
    the engine braking was rotating the shaft all the way down, at the end
    of the run not only hadn't I saved any fuel but I'd flooded the
    cylinders as well and the engine wouldn't restart until all the excess
    fuel had been expelled!


    That shouldn't have happened - the float needle valve in the carburetor should cut off any flow from the fuel pump.

    The fuel pump typically operated off a cam on the camshaft. It was
    spring operated so that if the flow is restricted by the float valve it
    would reduce the stroke of the fuel pump - potentially down to zero.

    With modern cars going downhill it is non-obvious whether it is better
    to put the car into neutral to cause the engine to go to idle where it
    stays consuming some fuel or leave it in gear such that engine goes into fuel-cutoff but adds some braking resistance.

    kw

    When the engine is not running and not spinning over, there is no suction on the carb and you are correct that once the bowl fills to the point that the buoyancy of the float applies enough force to the needle valve there will be no fuel flow. However,
    in this case the car with manual transmission and clutch out is in gear and moving so the the engine is continuously spinning over, causing air to flow through the carb and sucking fuel through at least the idle circuits and most likely also through the
    main jet circuits. This continues until the engine stops spinning. Do it for a few seconds and you can get a bit of raw fuel into the exhaust manifold which then ignites from the hot exhaust when you turn the key back on and engine start running again.
    It also rinses off a bit of the oil on the cylinder wall and rings so you don't want to keep doing it or you eventually cause some wear, and also get raw fuel flowing past the rings into the oil pan. Do it for a few seconds and you get a little explosion
    and bang out of the tailpipe. Do it for many seconds with an old, wimpy, rusty exhaust system and you can get a big bang and literally split an exhaust pipe or blow the muffler off the pipe. Do it for even longer and you flood everything with so much
    fuel that it is so rich the engine won't restart when you turn the key back on, and you have to change the oil and wait for everything to dry out. Ahh, the fond memories of teenagerhood :-).

    --
    Regards,
    Carl

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