I have a Luscombe with a Continental O-200 engine. The #4 cylinder is running cold at idle, does anyone have an idea why? The other three cylinders are running fine.

Some extra information:

  • The engine has only 300hrs SMOH but it hasn't run for two years and it's just had some maintenance work done (carb, filters, plugs)
  • Cylinder #4 is cool to the touch, the others are too hot to touch
  • A mag check seems normal (expected RPM drop on both mags)
  • A compression check seems normal (76/80 on #4 cylinder)

1 Answer 1


A: Idle circuit is set too lean and the cold jug is not firing.

B: Intake leak, causing same condition as A.

C: A little of both.

I know that a lot of people will lean out their idle mixture, by adjusting the screw on the carb, to reduce plug fouling with 100LL fuel (since leaning with the cockpit mixture doesn't do anything in the idle range). You can tell when this has been done because the engine doesn't have the little RPM bump when pulling idle cutoff, and also it may require extra prime to keep running when cold.

Also, please write your questions with a bit of structure and grammar. That stream of consciousness no-grammar style (that a lot of people use in emails out of laziness) is a total pain to read and interpret.

  • $\begingroup$ Idle adjustment set perfect, intake manifold to carburetor gasket brand new, and as improperly noted in my question I'm replacing all rubber's for intake tube's. $\endgroup$ Commented Jun 17, 2018 at 20:34
  • $\begingroup$ Do apologize for the sloppy grammar been very sick and loopy on pain medication $\endgroup$ Commented Jun 17, 2018 at 20:38
  • $\begingroup$ No worries. Not to say you need to write arduous essays, and brevity is much appreciated, just use enough caps and punctuation and structure it to make it easy to follow. Hope you are feeling better. Anyway, it runs fine at higher rpms? Also does it misfire the same with carb heat on (slightly richer mixture)?. $\endgroup$
    – John K
    Commented Jun 17, 2018 at 22:13
  • $\begingroup$ Yes ran up to 1800 rpm and did a mag drop sounded perfect. I did not do carb heat because cowl is off and so is the ducting. $\endgroup$ Commented Jun 18, 2018 at 17:51
  • $\begingroup$ Sure sounds like an intake leak that only happens at low MP where the suction is highest, like from a crack in the rubber intake tube. $\endgroup$
    – John K
    Commented Jun 18, 2018 at 20:00

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