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Thread: Naughty SBC

  1. #1
    Join Date
    Jun 2003
    Location
    Brackley, Northants, England.
    Posts
    821

    Naughty SBC

    Hi Guys.

    It's not often I need help with my GD but I have an engine problem that I/we can't fathom out.

    I'm sure I'm not the only one to have ever had this problem so I'm looking for ideas as to the solution.
    Fortunately I have a very helpful and knowledgeable friend who has lots of Chevvy Experience from owning a Dragster.

    I was out in the car last year, the engine had been running fine and put it in the garage when I got home.
    A few days later I tried to start it early on a Sunday morning, September 2017 and trying to be quiet, and it started and ran for about 5 seconds then died.
    Thinking that the engine was cold I restarted it and the same thing happened, ran a few seconds then died.
    I restarted it a few more times then it stopped and wouldn't go again with a strong smell of fuel. At first I thought it was a stuck float needle.
    Rounded up a few neighbours to help me push it back home.

    I spent a few days in odd moments trying to get it started but other, more pressing, things got in the way.
    Fast forward to a few weeks ago and a Silverstone Infield pass I thought it was time I got it going.

    The symptoms are as exactly as before. Thinking it was a fueling problem the Holly 4150 (80670-1) Street Avenger was stripped, cleaned and reassembled with new parts and gaskets. It was just the same.
    I had a new unused MSD HEI Dizzy so I replaced both the Cap and coil and then the Ignition trigger module. Still the same problem as we expected.

    My friend then took the Carburetor off of his fully working Southern Roadcraft Cob and we put it on mine. Exactly same result so both Carb and all ignition components have now been replaced and still it won't continue to run.

    Float levels are correct, checked both before and after the engine dies. Fuel pressure is 7PSI, a little high for the Holley but it's not overfuelling or been a problem.

    As it stands now 3 pumps on the throttle it starts immediately and will run for about 5 seconds and then it's just as if I've turned off the ignition. Whilst it's dying A Timing light still generates flashes so there are still sparks.

    I can run it up to 3,000RPM and intake vacuum is about 20in/hg, If I start it and let it idle for the 5 seconds or so I get about 15in/hg. Timing at idle about 20degs BTDC

    So any ideas as to the problem bearing in mind the ancillaries have all been replaced and whilst it's running it sounds absolutely as normal with no misfiring or rough running it just keeps stopping.

    My friend has told me what he thinks is the problem but we thought it a good idea to see what others, more knowledgeable than me, think before taking more drastic action.

    Thanks

    Denis

    If you take something to bits often enough, eventually you'll have two of them......


    GD427E039

  2. #2
    Join Date
    Apr 2004
    Location
    on the edge...almost falling off
    Age
    56
    Posts
    8,514
    If it's not ignition then it could be fuel, so clogged fuel filter? Vacuum lock from tank?

  3. #3
    Join Date
    Jan 2005
    Location
    leatherhead,surrey
    Posts
    979
    if youre running an electric pump is it still getting an electrc ifeed after the starter feed cuts and it runs off its own supply- ie igntion switch/fuse etc
    sounds like its running on fuel supplied off the starter phase and running out of juice once its 'on its own'
    neil

  4. #4
    Join Date
    Jun 2003
    Location
    Brackley, Northants, England.
    Posts
    821
    Thanks for the replies.
    The float chambers are full when the engine dies.
    It does have a mechanical fuel pump and there is a fuel filter but there's no problem with fuel flow. That's what makes it so odd

  5. #5
    Join Date
    Sep 2007
    Location
    Hilperton,Wiltshire
    Posts
    2,562
    Have you tried putting in a separate independent power feed to the dizzy?


    Andy

  6. #6
    Join Date
    Nov 2008
    Location
    Guildford
    Posts
    311
    Do you have an immobiliser fitted?

  7. #7
    Join Date
    Jun 2003
    Location
    Brackley, Northants, England.
    Posts
    821
    Funny you should suggest trying a separate power supply for the Dizzy. I knew the wiring was up to scratch but in desperation I tried that this very afternoon before I put the post up.

    Nooo I don't have an immobiliser. Electric immobilisers are a complete and utter waste of time on a cob like mine. Put 12V on the Dizzy, give it a push in gear and it'll run. Well, it used too!

    Denis

  8. #8
    Join Date
    Sep 2007
    Location
    Hilperton,Wiltshire
    Posts
    2,562
    If you are sure that the fuel supply is good it can only be a ignition fault. Only needs fuel and spark. Poor earth?

    Does it start up straight away after it cuts out?
    Andy


    Dax De Dion 350 sbc

  9. #9
    Join Date
    Jun 2003
    Location
    Brackley, Northants, England.
    Posts
    821
    Thanks Andy.
    When it dies the float chambers are still full. Because the engine hasn't had time to get warm it needs a prod on the throttle to get some fuel in. It then fires up immediately and runs for 5 seconds or so. It does this with both the original and borrowed carbs. Fuel pressure on external gauge on the fuel rail is 7psi.

    As you probably know, the HE I distributor has only two components the ignition coil in the Dizzy and a electronic trigger module both of which have been replaced with new items. I powered the Dizzy this afternoon from a separate power supply and it's just the same running only for 4 or 5 seconds.

    I still design Electronics professionally, at one time for Motorsport and I know the electrics are all OK as far as the Dizzy. It has to be something simple as it was running fine and then a few days later this problem. The borrowed carb has gone back now so I'll have to put my Carb back together before I can do any more checks. As you say it only needs fuel and a spark.
    It's all a bit frustrating but I really am very grateful for the suggestions so far.

    Denis

  10. #10
    Join Date
    Feb 2007
    Location
    West Sussex
    Posts
    8,527
    Faulty ignition module?
    Crendon Chassis No.49
    Huddart FE428 + toploader

    Not listed in the Shelby Register.

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