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Thread: Hot se-starts rethought

  1. #1

    Hot re-starts rethought

    I witnessed fuel dribbling out of the tips of the accelerator squirter nozzles of my Webers on my car after running on a warm day, my air cleaners were off since I was tweaking jetting so my attention was directed to carb issues. Since then, I have been thinking about fuel percolation due to heat soaking beyond what has already been discussed such as:
    • insulator plates
    • return-to-tank plumbing of the fuel lines
    • non-ethanol gas
    • modifying top covers of Webers using PMO drilling device

    These considerations are all good save for the PMO drilling, I found that this directed percolation fuel in the float bowls to be passed over the air correction jets for the slow speed circuit which floods it making hot re-starts difficult. I've found another method for venting the float bowls that avoids this issue, previously posted some years ago. The PMO solution (developed by Ferrari for the 512 BB) also causes flooding of the slow speed circuit during racing turns where fuel climbs the wall of the float chamber and floods one bank of the slow speed circuit rendering exit throttle to be hindered until the rich condition is rectified.

    My current thinking regarding hot re-starts addresses the fuel escaping from the accelerator squirter nozzles which occurs prior to float bowl percolation.

    My contribution to this issue (subject to the sage comments surely to follow this posting) is to use a "Throttle Stick" to hold the throttles Wide Open after shutdown. The purpose is to allow fuel that enters the carbs from the accelerator squirters to evaporate instead of ponding on top of the closed throttle valves and eventually seeping into the throttle bores where it is unable to evaporate thereby creating a rich combustion tract during re-starting efforts. Previously, hot re-starts are effected by holding the throttle at wide open while cranking to clear the rich mixture, unfortunately, opening the throttles will potentially more fuel down the bores making the re-start even more difficult.

    So, here is my suggested procedure for mitigating hot re-start issues, make a "Throttle Stick" that holds throttle wide open at shutdown to allow fuel evaporation:
    1. Come to a stop and set handbrake
    2. Rev engine to 3000+ RPM
    3. Immediately kill ignition and simultaneously depress pedal to the floor
    4. Insert "Throttle Stick"

    Re-starting procedure:
    1. Depress pedal, remove "Throttle Stick" while maintaining WOT
    2. Crank engine a few revs, drop throttle to closed position and re-open by about 10% pedal position and continue to crank
    3. Depending upon engine response, adjust pedal judo to re-start

    Theory behind the above:

    By revving engine to 3000+ RPM you will have discharged some of the available fuel from the accelerator pump and it will be burned by the running engine. Then, by shutting engine down and flooring the pedal you will completely discharge what remaining fuel there is to be delivered from the accelerator pump and that fuel will be quickly atomized by the spinning of the engine while at WOT.

    Placing the stick will maintain WOT which will help atomize & evaporate any fuel that percolates out of the accelerator pump galleries during heat soaking of the carbs.

    When re-starting, the WOT will not activate any slow speed or high-speed fuel contribution and no additional accelerator squirt will be added to the hot start issue so clearing of fuel from the cylinders and intake tract will be assured. Once a few revs at WOT are completed the throttle may be closed and a more normal starting procedure may be attempted.

    Note that heat soaking may purge fuel from the accelerator galleries so a couple of pedal depressions will be required to re-charge them before they become effective.

    Name:  P1140654.jpg
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    Picture shows "Throttle Stick" in place on my car, approximate length is 17".

    Name:  P1140655.jpg
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    Picture shows the fuel gallery feeding accelerator squirter nozzle which is subject to heat soaking and purging after shutdown.
    Last edited by 1QuickS; 09-08-2023 at 02:21 PM.
    Paul Abbott
    Early S Member #18
    Weber service specialist

  2. #2
    Your tips are much appreciated, thanks you.

  3. #3
    Quote Originally Posted by Rick Cabell View Post
    Your tips are much appreciated, thanks you.
    I second the motion!
    Peter Kane

    '72 911S Targa
    Message Board Co-Moderator - Early 911S Registry #100

  4. #4
    Senior Member
    Join Date
    May 2009
    Baton Rouge, Louisiana
    I am thankful you did not retire. Many thanks.

    Gary Cox
    Baton Rouge, Louisiana USA

    1958 356A Speedster, Silver/Black
    1967 912, Aga Blue/Black

  5. #5
    Moderator Chuck Miller's Avatar
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    Oct 2002
    Reseda, CA.

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    Paul is still 'on the job' .... Good for us !!! ...
    Chuck Miller
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