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Technical Why is my SBC running on?

Discussion in 'The Hokey Ass Message Board' started by Jay Tyrrell, Jun 18, 2014.

  1. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Ok this is where I am at. I have a thumper cam in a SBC with a 600 CFM Edelbrock Carb. The car has a automatic 350 transmission.

    I installed the distributor and fired the car right away. I set the air fuel screws 2 turns out and set the idle. I then timed the car at 10 degrees and took the car for a run. It ran great. Came home and shut down the car and it ran on. I retarded the timing again and it did improve the run on but it still runs on for a split second when the car is shut off. Timing now is at 6 degrees with a very very very small run on when the car is shut down and now the car has seemed to lose its "umph" that it did have for a 350 sbc. I turned down the idle to get rid of the run on but when you put it into great it dies or needs the accelorator pedal to be pushed slightly. Turn the idle up and then of course we got run on going on again. Any ideas guys? If you need more info I can provide.
    Thanks
    Jay
     
    Last edited: Jun 18, 2014
  2. oldolds
    Joined: Oct 18, 2010
    Posts: 3,153

    oldolds
    Member

    What are the RPM's at idle, in gear and in park? What is the stall speed on the converter?
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 48,790

    squirrel
    Member

    Turn off the key, then put it in Park.

    I noticed that mine had more tendency to run on with 87 octane, vs 91 octane.

    Or put on a "fast idle solenoid", they're usually used with air conditioning to give it a faster idle. But you can use it to prevent run on, set the idle speed with the solenoid for normal driving, and the normal carb idle speed low enough that it will not run on when you turn off the ignition.

    edit: http://www.summitracing.com/parts/edl-8059

    kind of pricy.
     
    JohnEvans likes this.
  4. walker
    Joined: Dec 29, 2008
    Posts: 217

    walker
    Member

    I fought this for a long time on my '67 pickup. When I recently had the motor apart, i put an indicator on the number one piston and found the balancer, which was new, and I put indicator marks on so I could tell if it spun, did not agree with the timing tab, by about a third of the circumfrence. I was never even in the right ballpark while hunting for the right advance.
     

  5. I'd say increase the octane as well....its a cheap fix if it works
     
  6. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    In park it sits at 1000RPM in gear 600-650RPM 1800 stall converter
     
  7. bulletproof1
    Joined: Feb 23, 2004
    Posts: 2,080

    bulletproof1
    Member
    from tulsa okla

    mine ford did it with 87 octane as well..
     
  8. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Running a mid grade. 89 Octane. We can get 91 at most pumps in Canada.
    Jay
     
  9. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Interesting and good idea Jim but what is the real reason it is doing this???
    Jay
     
  10. VoodooTwin
    Joined: Jul 13, 2011
    Posts: 3,455

    VoodooTwin
    Member
    from Noo Yawk

    I've opened a bunch of engines that 'ran on' and usually found them to have heavy carbon deposits which would stay hot enough to ignite the compressed residual fuel at shutdown. Are your combustion chambers clean?
     
  11. squirrel
    Joined: Sep 23, 2004
    Posts: 48,790

    squirrel
    Member

    Probably could use more stall speed....so you don't have to have the idle adjusted so high.

    For doing what? the fast idle solenoid? it's so you can set the idle speed on the carb low, so it won't run on.
     
  12. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Good point man! However the heads on this car have been rebuilt and cleaned before they were installed on the car which was only fired a week ago. Hmmmmm?
    Jay
     
  13. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Can you school me on this Jim? Everything I have done Hot rod wise has been with manual transmission. Never run into this. I thought torque converters were more related to power bands and such. By installing a higher RPM torque converter how would that help in this situation? I honestly can say I don't know much about them.
    Jay
     
  14. VoodooTwin
    Joined: Jul 13, 2011
    Posts: 3,455

    VoodooTwin
    Member
    from Noo Yawk

    Scratch that cause then! Squirrel makes a good point. I was going to also suggest checking your carb floats to make sure the fuel levels arent too high.
     
  15. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Another good idea VT I did the drill bit thing when I rebuilt it. Everything was good there. Good point about the torque converter? Hmm? How would that effect it though? I guess this is new territory for me?
    Jay
     
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 48,790

    squirrel
    Member

    The stall speed is the rpm where where the converter becomes fully engaged, at full throttle. The amount of slippage at idle is related to the stall speed, although it's different for different converter designs. In general, you want a converter that will not fully engage until the engine is well into the power band. If the cam is so lumpy that you have to set the idle (in Neutral) to 1000, and the converter bogs it down to 600 (Drive), it's a sign that the converter is not quite high enough stall for the cam.
     
    lothiandon1940 likes this.
  17. Jim P
    Joined: Apr 27, 2005
    Posts: 239

    Jim P
    Member
    from Tyler, TX

    I would try to idle it down a bit more with it in park. Try bringing it down to 900 and see what it does. 1000 sounds a bit high. Slower rpm might allow it to lug easier and die without the "diesel" effect as my old man would call it. Hopefully that makes sense.
    Jim
     
  18. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Ok now it is starting to make sense Jim. So by putting lets say a 2200 RPM converter in I will be able to back off on the idle because the converter will not engage as much as it was with a 1800RPM converter? Am I right?
    Jay
     
  19. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    This is something that I did try and it seemed that like it was seriously making micro adjustments on the idle screw. I just could get it. a little lower that 1000RPM stalled it when it went into gear. Too high run on occurs?
    Jay
     
  20. cac646
    Joined: Feb 7, 2007
    Posts: 176

    cac646
    Member

    I had a run on issue on my wagon. My idle was set too high. Reducing the idle 100% fixed my issue!!


    Posted using the Full Custom H.A.M.B. App!
     
  21. High combustion chamber temps, and higher engine temps in general, can contribute to "dieseling" and run-on after ignition is shut off. I've had cars that tended to do this if they were shut off too soon after a long stretch of highway driving. But after a couple of minutes of slower driving or idling they'd shut off with no issues. Cooler engine temps may help.

    With a warmed up camshaft it's common to have to open the throttle plates a bit more to get a liveable idle, especially with an automatic trans. The lumpy cam also makes less manifold vacuum and as a result your power valves or power pistons tend to richen up the mixture earlier and more frequently. You can often see a bit of fuel dripping from the main nozzles and wetting the throttle plates at idle.

    As a result you can shut off the ignition but the throttle plates are open that little extra amount and there's now enough airflow to continue pulling a slight amount of fuel into the combustion chamber and compress it. There may be no spark to ignite the mixture but if there's enough heat in the combustion chamber, or a little glowing hot chunk of carbon, it will fire the mixture. This is where adding an idle stop solenoid will help, as previously mentioned. You set your running idle speed with the solenoid energized thru the ignition switch. Turn off the ignition and the solenoid de-energizes and allows the throttle plates to close fully, or more nearly so.

    The engine needs air, fuel, compression and spark to run. This way you're not just removing spark from the equation, but air and fuel as well. Also putting a fine tune on the carb to work properly with the new cam could go a long way to curing the run-on problem.
     
  22. KrisKustomPaint
    Joined: Apr 20, 2007
    Posts: 1,107

    KrisKustomPaint
    Member

    Are you running electric fans? I ran into this once, the fans were back feeding the coil after you shut it down and it would run on until the fans slowed down enough not to generate 12 volts. Put a diode inline, shut off just fine. Just a thought. Good Luck.
     
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  23. Larry T
    Joined: Nov 24, 2004
    Posts: 7,661

    Larry T
    Member

    What kind of pistons and what cc combustion chamber does your engine have? Simpler question, what compression ratio are you running?

    I've seen the runon with electric fans too. ^^^^ Another good reason to run relays with them. It will take care of the problem.
     
    55gasserkid likes this.
  24. squirrel
    Joined: Sep 23, 2004
    Posts: 48,790

    squirrel
    Member

    That's right. What are the cam specs? or just the part number? And what rearend gearing do you have, what will you use the car for mostly,etc? trying to get a better idea of what the "right" converter would be for the car
     
    55gasserkid likes this.
  25. Jay Tyrrell
    Joined: Dec 9, 2007
    Posts: 1,631

    Jay Tyrrell
    Member

    Got to go to work right now but I will respond to all of these great possibles later tonight. Thanks for the ideas and tips.
    Jay
     
  26. Very educational thread. Great advice from squirrel (as always) and others.
     
    55gasserkid likes this.
  27. kuhn1941
    Joined: Feb 15, 2013
    Posts: 194

    kuhn1941
    Member

    I ran into this also . It was the distributer mechanicle advance under the rotor ( one of springs were broke )
    . Shutting the engine off in gear will always solve the problem , but fixing solving the problem is a good idea . Good luck
    Posted using the Full Custom H.A.M.B. App!
     
  28. Weasel
    Joined: Dec 30, 2007
    Posts: 6,693

    Weasel
    Member

    Electric fuel pump? Alternator backfeeding? GM one wire alternators can have a tendency to do this. Painless Wiring offers a kit to fix the problem - just a thought.
     
  29. 5window
    Joined: Jan 29, 2005
    Posts: 8,128

    5window
    Member

    Why is my SBC running on?
    It's female?

    OOh, early morning sexist joke-just getting ready for work (3 men 28 women)
     
  30. walker
    Joined: Dec 29, 2008
    Posts: 217

    walker
    Member

    I forgot about this, i had this happen as well. Also, you need to adjust your idle mixture, the ol' 2 turns out rule is okay, but just a guess. Look in your manual for setting lean best idle.
    So, by now you have figured out that run on is caused by too hot of temps in the CC. Two chief causes are running too lean, and too much advance at idle. Try retarding it another degree or two and see if that helps. Then set your idle mixture by the book, or at least make it richer by a half turn or so and see if that helps. Changing your stall speed isn't going to do squat for you idle speed. The tranny and pump are what is really affecting it.
    Another thing you can do is install a bung in the exhaust, and put in an o2 sensor and a cheap A/F meter. That will tell you for sure where you are at as far as the mixture goes.
     

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