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Thread: Popping backfire.....?

  1. #1

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    Popping backfire.....?

    One day my 150 idles smooth and quiet. The next day, for no apparent reason, it is backfiring and sitting at idle with a pop, pop, pop.....when it is run up it's fine and in the air, no problem. Then after it has run for a while, it doesn't do it either at idle. Any thoughts?

  2. #2
    Steve's Aircraft (Brian)'s Avatar
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    Check your exhaust system for leaks. Could be after it warms up it expands enough to close any leaks that it might have when cold.

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    Thanks! I will check that.

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    Floatin_around's Avatar
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    check the primer, make sure it is locked...

  5. #5
    Tim's Avatar
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    One of your plug wires could be breaking down, that happened to me last year.
    Tim

  6. #6
    Hyrdflyr
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    Could be a sticking valve also..

  7. #7

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    My C-85 did that when one exhaust gasket had gone slightly bad.

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    My cub did that last month. Turned out to be an intake gasket. Mark

  9. #9
    Crash's Avatar
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    Could be sticking valves. Might try 16oz Marvel Mistry Oil in each tank and crank case.

    After the first 100 hours on my C-180 with only 130 hours TT since rebuild, three cylinders checked out in the low 40's. Three hours on the above diet and they were all back in the mid to upper 60's. 100LL is a very "dry" fuel. Made a beliver out of me and my wallet ($10K to top an IO-520 vs $13.00 at WalMart for MM oil). Crash

  10. #10

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    Thanks for all the responses. Found a blown exhaust gasket on the right side. Fixed that; fixed the problem!

  11. #11

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    whooyoooo!! Iwas right. Where do I pick up my prize?

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    You will be amply rewarded.......all in good time! Thanks again!

  13. #13

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    Somebody help me out here. How does an exhaust leak cause backfire?

  14. #14

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    By letting cooler dense air mix with expanded hot gases with enough fuel available (like cool dust) to reignite on the next cycle

  15. #15
    Steve's Aircraft (Brian)'s Avatar
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    When the engine is at or just above idle, cold air (oxygen) can enter the exhaust system and cause any unspent fuel to burn at the area where the exhaust is leaking. Thus causing a backfire. Also small intake leeks can cause irregular fire because of improper fuel/air mixtures. Once the engine is running at high RPM, the area of the leak is bypassed due to the amount of air mass at higher pawer settings.
    I have also seen the same thing on a car equipped with headers. So, when checking for backfire, check both your exhaust and intake systems for leaks before digging to far into your engine.

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