afterfires after ignition turned off

tigermoth
tigermoth Expert Adviser
edited November -1 in HUDSON
hello, after a drive, with the engine up to operating temperature. my '35 six afterfires after the key is turned off. the car has only 1000 miles since i put the newly coated exhaust manifold on and the ss exhaust system. any thoughts? regards, tom

Comments

  • RL Chilton
    RL Chilton Administrator, Member
    Could be several things, but most likely the timing is too far advanced. Worse yet, if you don't fix that soon, you'll very likely be blowing holes out the side of your new exhaust.
  • oldhudsons
    oldhudsons Senior Contributor
    forget the "new exhaust", if advanced too far you'll burn holes in top of the pistons!
  • Geoff
    Geoff Senior Contributor
    What do you mean backfire? If you mean it runs on, then you may have a spot of carbon, or gasket edge getting red hot, causing it to self-ignite after the ignition is switched off. More information please!

    Geoff.
  • tigermoth
    tigermoth Expert Adviser
    hello to all. thank you for your replies. i originally posted more information, but i decided to delete most. it was starting to look more like me asking a question then supplying an answer. i wanted to hear what you folks had to say. the engine is timed to TDC per the manual. i believe i could advance a few degrees, but it starts so easy at TDC. it is not run on. the after fire is soft and is delayed. i turn off the ignition and as i am just getting out of the car it does the soft pop. it does not do it every time, but it does do it. any other thoughts or questions that may help you? i corrected the title of the post..it is not a back fire through the carb, but an after fire out the exhaust. i should have been more clear from the beginning. thanks again. regards, tom
  • Matt1950
    Matt1950 Senior Contributor
    I had a similar problem on my old Dodge and found out that the carb was spilling fuel out after it was shut off. It only happened whan I shut the vehicle off when it was at full operating temp. A little hot carbon in the cumbustion chamber and that extra fuel ignited and made a little poof. I found that my gas cap was not venting and it was pressurizing the fuel line and forcing the fuel into the carb and overfilling it on a hot soak.

    My recommendation is to check your gas cap and check your carb for fuel leaks (High float setting).
  • Park_W
    Park_W Senior Contributor
    Common cause for high float level when the carb's not been messed with is a cracked float. That allows some fuel in, thus it rides lower, resulting in higher float level. Pull the float and shake it to see if there's gas in it.
  • Geoff
    Geoff Senior Contributor
    Tom, it could be ahot spot in a combustion chamber, cauing it, with just enough unburnt gas left to cause one firing, but it is difficult to diagnose from a distance! Or it could be a build-up of unburnt gases in the exhaust system, but what would cause it to ignite? Things to try as already suggested, plus check that the anti-percolating valve is opening at throttle shut-off. If not it could cause gas to pressurise into the venturi. High float level as mentioned caould also cause overflow, but it is difficult to envisage how this could cause it to fire in the exhaust. I had a 1942 Dodge , many years ago, which used to run on after turning the key off. Sometimes had to put in gear with handbrake on, and stall it to stop it. Was minimised by running at a very slow idle, slightly rich mixture, and wait about 20 seconds before turning off. Good luck,

    Geoff.
  • tigermoth
    tigermoth Expert Adviser
    thanks geoff and all, the car is not experiencing run on. it is definitely in the exhaust manifold and is delayed enough that i am half way out the door after turning the key off..and i am not as quick as i once was..so it is a moment or two after the engine stops turning, and definitely out the tail pipe. any other thoughts? regards, tom
  • tigermoth
    tigermoth Expert Adviser
    hello hudson308, i assume you found it with a compression test? i will do one tomorrow. regards, tp
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